Active Record Query Interface

Nếu bạn đang thường sử dụng raw SQL để tìm database records, bạn sẽ thấy rằng có cách tốt hơn để làm điều đó trong Rails. Active Record giúp bạn không cần dùng đến các câu lệnh SQL trong hầu hết các trường hợp.

Các ví dụ trong hướng dẫn này sẽ tham chiếu tới một trong các models dưới đây:

Các models dưới đây dùng id là khoá chính, trừ khi có chỉ định các trường khác làm khoá chính.

class Client < ActiveRecord::Base
  has_one :address
  has_many :orders
  has_and_belongs_to_many :roles
end

class Address < ActiveRecord::Base
  belongs_to :client
end

class Order < ActiveRecord::Base
  belongs_to :client, counter_cache: true
end

class Role < ActiveRecord::Base
  has_and_belongs_to_many :clients
end

Active Record sẽ thực hiện các truy vấn vào database giúp bạn, và tương thích với hầu hết các hệ quản trị cơ sở dữ liệu (MySQL, PostgreSQL, SQLite ...). Bất kể là hệ quản trị cơ sở dữ liệu nào bạn đang dùng, các phương thức của Active luôn như nhau.

1 Lấy các đối tượng từ Database

Để lấy các đối tượng (objects) từ cơ sở dữ liệu, Active Record cung cấp mỗi số phương thức.Mỗi phương thức tìm kiếm chấp nhận các tham số được truyền vào bên trong để thực hiện các truy vấn tương ứng trong cơ sở dữ liệu thay vì viết các raw SQL.

Các phương thức bao gồm:

bind
create_with
distinct
eager_load
extending
from
group
having
includes
joins
limit
lock
none
offset
order
preload
readonly
references
reorder
reverse_order
select
uniq
where

Toàn bộ các phương thức trên trả về một thực thể (instance) của lớp ActiveRecord::Relation.

Các thao tác chính của Model.find(options) có thể được tóm tắt như :

Chuyển đổi các tuỳ chọn được cung cấp thành câu lệnh SQL query tương ứng.
Fire các truy vấn SQL và lấy kết quả tương ứng từ database.
Khởi tạo các đối tượng Ruby tương ứng với các model cho mỗi bản ghi kết quả từ database.
Chạy after_find và sau đó là after_initialize callbacks, nếu có.

1.1 Lấy một Object đơn từ Database

Active Record cung cấp một số cách khác nhau để lấy một object đơn.

1.1.1 `find`

Sử dụng phương thức find, bạn có thể lấy được object tương ứng với khoá chính ( đã được chỉ định) phù hợp với các tuỳ chọn được cung cấp. Ví dụ:

#  Tìm client với khoá chính (id) 10
client = Client.find(10)
# => #<Client id: 10, first_name: "Ryan">

Câu lệnh SQL tương ứng với lệnh ở trên:

SELECT * FROM clients WHERE (clients.id = 10) LIMIT 1

Phương thức find sẽ raise ActiveRecord::RecordNotFound exception nếu như không có object nào được tìm thấy.

Nếu bạn dùng phương thức này để tìm nhiều objects ( multiple objects) .Gọi phương thức find và truyền vào các một mảng các gía trị của khoá chính. Giá trị trả về sẽ là một mảng các bản ghi tương ứng với giá trị khoá chính được chỉ định. Ví dụ:

# Tìm các client với khoá chính là 1 và 10 
client = Client.find([1, 10]) # Or even Client.find(1, 10)
# => [#<Client id: 1, first_name: "Lifo">, #<Client id: 10, first_name: "Ryan">]

Câu lệnh SQL tương ứng với lệnh trên:

SELECT * FROM clients WHERE (clients.id IN (1,10))

Phương thức The find sẽ raise ActiveRecord::RecordNotFound exception trừ khi a matching record is found for all of the supplied primary keys.

1.1.2 `take`

Phương thức take sẽ lấy một bản ghi mà không theo bất cứ trật tự ẩn nào.Ví dụ:

client = Client.take
# => #<Client id: 1, first_name: "Lifo">

Câu lệnh SQL tương ứng với lệnh trên:

SELECT * FROM clients LIMIT 1

Phương thức take trả về giá trị nil nếu không có bản ghi nào được tìm thấy và không raise exception.

Bạn có thể truyền vào một tham số dạng số (numeric agrument )vào phương thức take để trả về các bản ghi được giới hạn với số lượng tương ứng.Ví dụ:

client = Client.take(2)
# => [
  #<Client id: 1, first_name: "Lifo">,
  #<Client id: 220, first_name: "Sara">
]

Câu lệnh SQL tương ứng với lệnh trên:

SELECT * FROM clients LIMIT 2

Phương thức take! cũng giống như take, điểm khác là phương thức take! sẽ raise ActiveRecord::RecordNotFound nếu không có bản ghi nào được tìm thấy.

Bản ghi trả về có thể sẽ khác nhau phụ thuộc vào database engine.

1.1.3 `first`

Phương thức first tìm bản ghi đầu tiên được sắp xếp theo khoá chính. Ví dụ:

client = Client.first
# => #<Client id: 1, first_name: "Lifo">

Câu lệnh SQL tương ứng với lệnh trên:

SELECT * FROM clients ORDER BY clients.id ASC LIMIT 1

Phương thức first trả về giá trị nil nếu không có bản ghi tương ứng nào và không có exception nào được raise.

Bạn có thể truyền tham số dạng số vào phương thức first để giới hạn số lượng bản ghi trả về. Ví dụ:

client = Client.first(3)
# => [
  #<Client id: 1, first_name: "Lifo">,
  #<Client id: 2, first_name: "Fifo">,
  #<Client id: 3, first_name: "Filo">
]

Câu lệnh SQL tương ứng với lệnh trên:

SELECT * FROM clients ORDER BY clients.id ASC LIMIT 3

Phương thức first! tương tự như phương thức first, khác ở chỗ first! sẽ raise ActiveRecord::RecordNotFound exception nếu không có bản ghi nào được tìm thấy.

1.1.4 `last`

Phương thức last sẽ tìm bản ghi cuối cùng được sắp xếp bởi khoá chính. Ví dụ:

client = Client.last
# => #<Client id: 221, first_name: "Russel">

câu lệnh SQL tương ứng với lệnh trên:

SELECT * FROM clients ORDER BY clients.id DESC LIMIT 1

Phương thức last trả về nil nếu không tìm thấy bản ghi nào và không có exception nào được raise.

Bạn có thể truyền tham số dạng số vào phương thức last để giới hạn số lượng bản ghi được trả về. Ví dụ:

client = Client.last(3)
# => [
  #<Client id: 219, first_name: "James">,
  #<Client id: 220, first_name: "Sara">,
  #<Client id: 221, first_name: "Russel">
]

Câu lệnh SQL tương ứng với lệnh trên:

SELECT * FROM clients ORDER BY clients.id DESC LIMIT 3

Phương thức last! giống như last, khác ở chỗ last! sẽ raise ActiveRecord::RecordNotFound nếu không bản ghi nào được tìm thấy.

1.1.5 `find_by`

Phương thức find_by tìm bản ghi đầu tiên tương ứng với một số điều kiện. Ví dụ:

Client.find_by first_name: 'Lifo'
# => #<Client id: 1, first_name: "Lifo">

Client.find_by first_name: 'Jon'
# => nil

bằng với câu lệnh:

Client.where(first_name: 'Lifo').take

Câu lệnh SQL tương ứng với 2 câu lệnh trên:

SELECT * FROM clients WHERE (clients.first_name = 'Lifo') LIMIT 1

Phương thức find_by! giống như phương thức find_by, ngoài ra find_by! raise ActiveRecord::RecordNotFound nếu không bản ghi nào được tìm thấy. Ví dụ:

Client.find_by! first_name: 'does not exist'
# => ActiveRecord::RecordNotFound

bằng với việc bạn viết như câu lệnh dưới:

Client.where(first_name: 'does not exist').take!

1.2 Retrieving Multiple Objects in Batches ( Lấy nhiều đối tượng)

Chúng ta thường phải lặp một tập lớn các bản ghi, ví dụ như khi chúng ta gửi newsletter cho một tập lớn người dùng, hoặc khi chúng ta export dữ liệu.

Cách đơn giản:

# Cách này rất hiệu quả khi bảng users có vài nghìn dòng.
User.all.each do |user|
  NewsMailer.weekly(user).deliver_now
end

Nhưng cách tiếp cận này sẽ thiếu tính thực tế khi kích cỡ của bảng tăng lên. Vì User.all.each điều khiển Active Record fetch the entire table (toàn bộ bản ghi của bảng ) trong 1 lần, tạo các đối tượng model trên từng dòng dữ liệu, và sau đó lưu giữ mảng các đối tượng model này trong memory. Thực tế, nếu chúng ta có một số lượng lớn các bản ghi, toàn bộ collection có thể sẽ vượt quá dung lượng của memory.

Rails cung cấp 2 phương thức giải quyết vấn đề này bằng việc chia các bản ghi vào các memory-friendly batches để xử lý. Phương thức đầu tiên,find_each, lấy ra một loạt các bản ghi và sau đó yields each record to the block individually as a model. Phương thức thứ hai, find_in_batches, lưu trữ một loạt các bản ghi và sau đó yields the entire batch to the block as an array of models.

Phương thức find_each và find_in_batches được dụng ý để sử dụng cho batch processing một lượng lớn các bản ghi vượt quá memory. Nếu bạn chỉ cần lặp vài nghìn bản ghi thì phương thức finds thông thường là lựa chọn tốt hơn.

1.2.1 `find_each`

The find_each method retrieves a batch of records and then yields each record to the block individually as a model. In the following example, find_each will retrieve 1000 records (the current default for both find_each and find_in_batches) and then yield each record individually to the block as a model. This process is repeated until all of the records have been processed:

User.find_each do |user|
  NewsMailer.weekly(user).deliver_now
end

To add conditions to a find_each operation you can chain other Active Record methods such as where:

User.where(weekly_subscriber: true).find_each do |user|
  NewsMailer.weekly(user).deliver_now
end

1.2.1.1 Options for `find_each`

Phương thức find_each chấp nhận hầu hết các options được chấp nhận bởi phương thức find , trừ :order và :limit, which are reserved for internal use by find_each.

Ba options :batch_size, :begin_at and :end_at, are available as well.

:batch_size

:batch_size option cho phép bạn chỉ định số lượng bản ghi được lấy ra trong môi batch, trước khi được passed individually to the block. Ví dụ để lấy 5000 bản ghi vào batch:

User.find_each(batch_size: 5000) do |user|
  NewsMailer.weekly(user).deliver_now
end

:begin_at

Mặc định, các bản ghi được lấy ra theo thứ tự tăng dần của khoá chính, có kiểu integer. :begin_at option cho phép bạn chỉ định ID đầu tiên của dãy được lấy ra trong trường hợp bạn không cần ID nhỏ nhất. Ví dụ, nếu bạn muốn resume an interrupted batch process, provided you saved the last processed ID as a checkpoint.

Ví dụ, để gửi newsletters cho người dùng với khoá chính bắt đầu từ 2000 và lấy ra trong batches độ lớn 5000 bản ghi.

User.find_each(begin_at: 2000, batch_size: 5000) do |user|
  NewsMailer.weekly(user).deliver_now
end

Một ví dụ khác, nếu bạn muốn nhiều workers handling the same processing queue. Bạn có thể có mỗi worker xử lý 10000 bản ghi bằng cách chỉ định :begin_at option tương ứng cho mỗi worker.

:end_at

Tương tự như :begin_at option, :end_at cho phép bạn chỉ định ID cuối cùng của chuỗi các bản ghi lấy ra khi bạn không cần lấy tới các bản ghi có ID lớn nhất.

Ví dụ, nếu bạn muốn chạy một batch process, sử dụng tập con các bản ghi dựa trên :begin_at và :end_at

Ví dụ, bạn muốn gửi newsletters cho người dùng có khoá chính bắt đầu từ 2000 tới 10000 và oaays chúng theo các batch có độ lớn 5000 bản ghi:

User.find_each(begin_at: 2000, end_at: 10000, batch_size: 5000) do |user|
  NewsMailer.weekly(user).deliver_now
end

1.2.2 `find_in_batches`

Phương thức find_in_batches gần giống như find_each, cả hai đều retrieve batches of records. Điểm khấc biệt ở đây là find_in_batches yields batches to the block as an array of models, instead of individually. The following example will yield to the supplied block an array of up to 1000 invoices at a time, with the final block containing any remaining invoices:

# Gán cho add_invoices một mảng 1000 invoices trong một lần
Invoice.find_in_batches do |invoices|
  export.add_invoices(invoices)
end

1.2.2.1 Options for `find_in_batches`

Phương thức find_in_batches chấp nhận các option :batch_size, :begin_at and :end_at giống như find_each.

2 Conditions ( Điều kiện)

Phương thức where cho phép bạn định rõ điều kiện trả về của các bản ghi, representing the WHERE-part of the SQL statement. Các điều kiện có thể được chỉ định bằng chuỗi ( string), mảng (array) hoặc bảng băm ( hash).

2.1 Pure String Conditions ( Điều kiện giới hạn bằng chuỗi)

Nếu bạn muốn thêm điều kiện, bạn chỉ cần xác định nó , giống như Client.where("orders_count = '2'") sẽ tìm toàn bộ các client có giá trị của trường orders_count bằng 2.

Xây dựng các điều kiện bằng string có thể gây ra lỗ hổng SQL injection. Ví dụ câu tìm kiếm Client.where("first_name LIKE '%#{params[:first_name]}%'") không an toàn. Xem phần tiếp kế tiếp để biết về cách xử lý tốt hơn các điều kiện sử dụng Array.

2.2 Array Conditions

Trong trường hợp tham số được truyền vào từ nơi khác ( giá trị của tham số thay đổi), câu lệnh tìm kiếm sẽ có dạng như dưới:

Client.where("orders_count = ?", params[:orders])

Active Record sẽ will go through the first element in the conditions value and any additional elements will replace the question marks (?) in the first element.

Nếu bạn muốn chỉ định nhiều điều kiện:

Client.where("orders_count = ? AND locked = ?", params[:orders], false)

Trong ví dụ này, dấu hỏi chấm (?) đầu tiên sẽ được thay thế bằng giá trị của params[:orders] và dấu hỏi chấm thứ hai sẽ được thay thế bằng and the second will be replaced with the SQL representation of false, phụ thuộc vào hệ quản trị cơ sở dữ liệu.

This code is highly preferable:

Client.where("orders_count = ?", params[:orders])

to this code:

Client.where("orders_count = #{params[:orders]}")

because of argument safety. Putting the variable directly into the conditions string will pass the variable to the database as-is. This means that it will be an unescaped variable directly from a user who may have malicious intent. If you do this, you put your entire database at risk because once a user finds out they can exploit your database they can do just about anything to it. Never ever put your arguments directly inside the conditions string.

Thông tin thêm về sự nguy hiểm của SQL injection có thể xem tại Ruby on Rails Security Guide.

2.2.1 Placeholder Conditions

Tương tự như kiểu thay thế params (?), bạn cũng có thể chỉ định theo kiểu bảng băm keys/value trong mảng các điều kiện :

Client.where("created_at >= :start_date AND created_at <= :end_date",
  {start_date: params[:start_date], end_date: params[:end_date]})

kiểu truyền tham số này giúp source code của bạn dễ đọc hơn nếu bạn có một số lượng lớn các điều kiện khác nhau.

2.3 Hash Conditions

Active Record also allows you to pass in hash conditions which can increase the readability of your conditions syntax. With hash conditions, you pass in a hash with keys of the fields you want conditionalised and the values of how you want to conditionalise them:

Only equality, range and subset checking are possible with Hash conditions.

2.3.1 Equality Conditions

Client.where(locked: true)

Tên trường cũng có thể viết dưới dạng string:

Client.where('locked' => true)

Trong trường hợp quan hệ belongs_to, an association key can be used to specify the model if an Active Record object is used as the value. This method works with polymorphic relationships as well.

Article.where(author: author)
Author.joins(:articles).where(articles: { author: author })

Giá trị không thể là symbols. Ví dụ bạn không thể viết Client.where(status: :active).

2.3.2 Range Conditions

Client.where(created_at: (Time.now.midnight - 1.day)..Time.now.midnight)

Câu lệnh trên sẽ tìm toàn bộ các clients được tạo hôm qua bằng cách sử dụng lệnh SQL BETWEEN:

SELECT * FROM clients WHERE (clients.created_at BETWEEN '2008-12-21 00:00:00' AND '2008-12-22 00:00:00')

This demonstrates a shorter syntax for the examples in Array Conditions

2.3.3 Subset Conditions

Nếu bạn muốn tìm các bản ghi sử dụng câu lệnh IN bạn có thể truyền mảng vào bảng băm các điều kiện :

Client.where(orders_count: [1,3,5])

Câu lệnh trên sẽ sinh ra câu lệnh SQL như bên dưới:

SELECT * FROM clients WHERE (clients.orders_count IN (1,3,5))

2.4 NOT Conditions

Câu lệnh NOT trong truy vấn SQL có thể được xây dựng bằng lệnh where.not.

Article.where.not(author: author)

Nói cách khác, truy vấn này có thể được sinh ra bằng cách gọi lệnh where không có tham số đầu vào, then immediately chain with not passing where conditions.

3 Ordering ( Sắp xếp)

Để lấy các bản ghi từ cơ sở dữ liệu theo một thứ tự xác định, bạn có thể dùng phương thức order

Ví dụ, nếu bạn lấy một tập các bản ghi và muốn sắp xếp chúng theo thứ tự tăng dần của trường created_at trong bảng:

Client.order(:created_at)
# OR
Client.order("created_at")

Bạn cũng có thể chỉ định ASC hoặc DESC:

Client.order(created_at: :desc)
# OR
Client.order(created_at: :asc)
# OR
Client.order("created_at DESC")
# OR
Client.order("created_at ASC")

Hoặc sắp xếp theo nhiều trường :

Client.order(orders_count: :asc, created_at: :desc)
# OR
Client.order(:orders_count, created_at: :desc)
# OR
Client.order("orders_count ASC, created_at DESC")
# OR
Client.order("orders_count ASC", "created_at DESC")

Nếu bạn muốn gọi phương thức order nhiều lần. Ví dụ trong những ngữ cảnh khác nhau thứ tự sắp xếp mới sẽ được set thêm vào thứ tự trước đó:

Client.order("orders_count ASC").order("created_at DESC")
# SELECT * FROM clients ORDER BY orders_count ASC, created_at DESC

4 Selecting Specific Fields ( Lựa chọn các trường xác định)

Mặc định, Model.find lựa chọn toàn bộ các trường trong tập kết quả bằng cách dùng select *.

Để lựa chọn một tập các trường từ tập kết quả, bạn có thể xác định tập các trường đó thông qua phương thức select .

Ví dụ, chỉ lựa chọn cột viewable_by và locked:

Client.select("viewable_by, locked")

Câu lệnh truy vấn SQL được sinh từ câu lệnh trên có thể như câu lệnh bên dưới:

SELECT viewable_by, locked FROM clients

Hãy cẩn thận, bởi vì điều đó cũng có nghĩa là bạn chỉ khởi tạo đối tượng model với các trường bạn đã lựa chọn. Nếu như bạn muốn truy cập vào một trường không có trong các trường được khởi tạo bạn có thể nhận được exception:

ActiveModel::MissingAttributeError: missing attribute: <attribute>

trong đó <attribute> là thuộc tính ( attribute) bạn yêu cầu. Phương thức id sẽ không raise ActiveRecord::MissingAttributeError, nên hãy cẩn thận khi dùng với associations vì các phương thức này cần id để hoạt động đúng.

If you would like to only grab a single record per unique value in a certain field, you can use distinct:

Client.select(:name).distinct

Câu lệnh trên sẽ sinh ra câu lệnh SQL như phía dưới

SELECT DISTINCT name FROM clients

Bạn cũng có thể xoá bỏ uniqueness constraint:

query = Client.select(:name).distinct
# => Returns unique names

query.distinct(false)
# => Returns all names, even if there are duplicates

5 Limit and Offset

To apply LIMIT to the SQL fired by the Model.find, you can specify the LIMIT using limit and offset methods on the relation.

You can use limit to specify the number of records to be retrieved, and use offset to specify the number of records to skip before starting to return the records. For example

Client.limit(5)

will return a maximum of 5 clients and because it specifies no offset it will return the first 5 in the table. The SQL it executes looks like this:

SELECT * FROM clients LIMIT 5

Adding offset to that

Client.limit(5).offset(30)

will return instead a maximum of 5 clients beginning with the 31st. The SQL looks like:

SELECT * FROM clients LIMIT 5 OFFSET 30

6 Group

To apply a GROUP BY clause to the SQL fired by the finder, you can specify the group method on the find.

For example, if you want to find a collection of the dates orders were created on:

Order.select("date(created_at) as ordered_date, sum(price) as total_price").group("date(created_at)")

And this will give you a single Order object for each date where there are orders in the database.

The SQL that would be executed would be something like this:

SELECT date(created_at) as ordered_date, sum(price) as total_price
FROM orders
GROUP BY date(created_at)

6.1 Total of grouped items

To get the total of grouped items on a single query call count after the group.

Order.group(:status).count
# => { 'awaiting_approval' => 7, 'paid' => 12 }

The SQL that would be executed would be something like this:

SELECT COUNT (*) AS count_all, status AS status
FROM "orders"
GROUP BY status

7 Having

SQL uses the HAVING clause to specify conditions on the GROUP BY fields. You can add the HAVING clause to the SQL fired by the Model.find by adding the having method to the find.

For example:

Order.select("date(created_at) as ordered_date, sum(price) as total_price").
  group("date(created_at)").having("sum(price) > ?", 100)

The SQL that would be executed would be something like this:

SELECT date(created_at) as ordered_date, sum(price) as total_price
FROM orders
GROUP BY date(created_at)
HAVING sum(price) > 100

This will return single order objects for each day, but only those that are ordered more than $100 in a day.

8 Overriding Conditions

8.1 `unscope`

You can specify certain conditions to be removed using the unscope method. For example:

Article.where('id > 10').limit(20).order('id asc').unscope(:order)

The SQL that would be executed:

SELECT * FROM articles WHERE id > 10 LIMIT 20

# Original query without `unscope`
SELECT * FROM articles WHERE id > 10 ORDER BY id asc LIMIT 20

You can also unscope specific where clauses. For example:

Article.where(id: 10, trashed: false).unscope(where: :id)
# SELECT "articles".* FROM "articles" WHERE trashed = 0

A relation which has used unscope will affect any relation it is merged in to:

Article.order('id asc').merge(Article.unscope(:order))
# SELECT "articles".* FROM "articles"

8.2 `only`

You can also override conditions using the only method. For example:

Article.where('id > 10').limit(20).order('id desc').only(:order, :where)

The SQL that would be executed:

SELECT * FROM articles WHERE id > 10 ORDER BY id DESC

# Original query without `only`
SELECT "articles".* FROM "articles" WHERE (id > 10) ORDER BY id desc LIMIT 20

8.3 `reorder`

The reorder method overrides the default scope order. For example:

class Article < ActiveRecord::Base
  has_many :comments, -> { order('posted_at DESC') }
end

Article.find(10).comments.reorder('name')

The SQL that would be executed:

SELECT * FROM articles WHERE id = 10
SELECT * FROM comments WHERE article_id = 10 ORDER BY name

In case the reorder clause is not used, the SQL executed would be:

SELECT * FROM articles WHERE id = 10
SELECT * FROM comments WHERE article_id = 10 ORDER BY posted_at DESC

8.4 `reverse_order`

The reverse_order method reverses the ordering clause if specified.

Client.where("orders_count > 10").order(:name).reverse_order

The SQL that would be executed:

SELECT * FROM clients WHERE orders_count > 10 ORDER BY name DESC

If no ordering clause is specified in the query, the reverse_order orders by the primary key in reverse order.

Client.where("orders_count > 10").reverse_order

The SQL that would be executed:

SELECT * FROM clients WHERE orders_count > 10 ORDER BY clients.id DESC

This method accepts no arguments.

8.5 `rewhere`

The rewhere method overrides an existing, named where condition. For example:

Article.where(trashed: true).rewhere(trashed: false)

The SQL that would be executed:

SELECT * FROM articles WHERE `trashed` = 0

In case the rewhere clause is not used,

Article.where(trashed: true).where(trashed: false)

the SQL executed would be:

SELECT * FROM articles WHERE `trashed` = 1 AND `trashed` = 0

9 Null Relation

The none method returns a chainable relation with no records. Any subsequent conditions chained to the returned relation will continue generating empty relations. This is useful in scenarios where you need a chainable response to a method or a scope that could return zero results.

Article.none # returns an empty Relation and fires no queries.

# The visible_articles method below is expected to return a Relation.
@articles = current_user.visible_articles.where(name: params[:name])

def visible_articles
  case role
  when 'Country Manager'
    Article.where(country: country)
  when 'Reviewer'
    Article.published
  when 'Bad User'
    Article.none # => returning [] or nil breaks the caller code in this case
  end
end

10 Readonly Objects

Active Record provides readonly method on a relation to explicitly disallow modification of any of the returned objects. Any attempt to alter a readonly record will not succeed, raising an ActiveRecord::ReadOnlyRecord exception.

client = Client.readonly.first
client.visits += 1
client.save

As client is explicitly set to be a readonly object, the above code will raise an ActiveRecord::ReadOnlyRecord exception when calling client.save with an updated value of visits.

11 Locking Records for Update

Locking is helpful for preventing race conditions when updating records in the database and ensuring atomic updates.

Active Record provides two locking mechanisms:

Optimistic Locking
Pessimistic Locking

11.1 Optimistic Locking

Optimistic locking allows multiple users to access the same record for edits, and assumes a minimum of conflicts with the data. It does this by checking whether another process has made changes to a record since it was opened. An ActiveRecord::StaleObjectError exception is thrown if that has occurred and the update is ignored.

Optimistic locking column

In order to use optimistic locking, the table needs to have a column called lock_version of type integer. Each time the record is updated, Active Record increments the lock_version column. If an update request is made with a lower value in the lock_version field than is currently in the lock_version column in the database, the update request will fail with an ActiveRecord::StaleObjectError. Example:

c1 = Client.find(1)
c2 = Client.find(1)

c1.first_name = "Michael"
c1.save

c2.name = "should fail"
c2.save # Raises an ActiveRecord::StaleObjectError

You're then responsible for dealing with the conflict by rescuing the exception and either rolling back, merging, or otherwise apply the business logic needed to resolve the conflict.

This behavior can be turned off by setting ActiveRecord::Base.lock_optimistically = false.

To override the name of the lock_version column, ActiveRecord::Base provides a class attribute called locking_column:

class Client < ActiveRecord::Base
  self.locking_column = :lock_client_column
end

11.2 Pessimistic Locking

Pessimistic locking uses a locking mechanism provided by the underlying database. Using lock when building a relation obtains an exclusive lock on the selected rows. Relations using lock are usually wrapped inside a transaction for preventing deadlock conditions.

For example:

Item.transaction do
  i = Item.lock.first
  i.name = 'Jones'
  i.save
end

The above session produces the following SQL for a MySQL backend:

SQL (0.2ms)   BEGIN
Item Load (0.3ms)   SELECT * FROM `items` LIMIT 1 FOR UPDATE
Item Update (0.4ms)   UPDATE `items` SET `updated_at` = '2009-02-07 18:05:56', `name` = 'Jones' WHERE `id` = 1
SQL (0.8ms)   COMMIT

You can also pass raw SQL to the lock method for allowing different types of locks. For example, MySQL has an expression called LOCK IN SHARE MODE where you can lock a record but still allow other queries to read it. To specify this expression just pass it in as the lock option:

Item.transaction do
  i = Item.lock("LOCK IN SHARE MODE").find(1)
  i.increment!(:views)
end

If you already have an instance of your model, you can start a transaction and acquire the lock in one go using the following code:

item = Item.first
item.with_lock do
  # This block is called within a transaction,
  # item is already locked.
  item.increment!(:views)
end

12 Joining Tables

Active Record provides a finder method called joins for specifying JOIN clauses on the resulting SQL. There are multiple ways to use the joins method.

12.1 Using a String SQL Fragment

You can just supply the raw SQL specifying the JOIN clause to joins:

Client.joins('LEFT OUTER JOIN addresses ON addresses.client_id = clients.id')

This will result in the following SQL:

SELECT clients.* FROM clients LEFT OUTER JOIN addresses ON addresses.client_id = clients.id

12.2 Using Array/Hash of Named Associations

This method only works with INNER JOIN.

Active Record lets you use the names of the associations defined on the model as a shortcut for specifying JOIN clauses for those associations when using the joins method.

For example, consider the following Category, Article, Comment, Guest and Tag models:

class Category < ActiveRecord::Base
  has_many :articles
end

class Article < ActiveRecord::Base
  belongs_to :category
  has_many :comments
  has_many :tags
end

class Comment < ActiveRecord::Base
  belongs_to :article
  has_one :guest
end

class Guest < ActiveRecord::Base
  belongs_to :comment
end

class Tag < ActiveRecord::Base
  belongs_to :article
end

Now all of the following will produce the expected join queries using INNER JOIN:

12.2.1 Joining a Single Association

Category.joins(:articles)

This produces:

SELECT categories.* FROM categories
  INNER JOIN articles ON articles.category_id = categories.id

Or, in English: "return a Category object for all categories with articles". Note that you will see duplicate categories if more than one article has the same category. If you want unique categories, you can use Category.joins(:articles).uniq.

12.2.2 Joining Multiple Associations

Article.joins(:category, :comments)

This produces:

SELECT articles.* FROM articles
  INNER JOIN categories ON articles.category_id = categories.id
  INNER JOIN comments ON comments.article_id = articles.id

Or, in English: "return all articles that have a category and at least one comment". Note again that articles with multiple comments will show up multiple times.

12.2.3 Joining Nested Associations (Single Level)

Article.joins(comments: :guest)

This produces:

SELECT articles.* FROM articles
  INNER JOIN comments ON comments.article_id = articles.id
  INNER JOIN guests ON guests.comment_id = comments.id

Or, in English: "return all articles that have a comment made by a guest."

12.2.4 Joining Nested Associations (Multiple Level)

Category.joins(articles: [{ comments: :guest }, :tags])

This produces:

SELECT categories.* FROM categories
  INNER JOIN articles ON articles.category_id = categories.id
  INNER JOIN comments ON comments.article_id = articles.id
  INNER JOIN guests ON guests.comment_id = comments.id
  INNER JOIN tags ON tags.article_id = articles.id

12.3 Specifying Conditions on the Joined Tables

You can specify conditions on the joined tables using the regular Array and String conditions. Hash conditions provides a special syntax for specifying conditions for the joined tables:

time_range = (Time.now.midnight - 1.day)..Time.now.midnight
Client.joins(:orders).where('orders.created_at' => time_range)

An alternative and cleaner syntax is to nest the hash conditions:

time_range = (Time.now.midnight - 1.day)..Time.now.midnight
Client.joins(:orders).where(orders: { created_at: time_range })

This will find all clients who have orders that were created yesterday, again using a BETWEEN SQL expression.

13 Eager Loading Associations

Eager loading is the mechanism for loading the associated records of the objects returned by Model.find using as few queries as possible.

N + 1 queries problem

Consider the following code, which finds 10 clients and prints their postcodes:

clients = Client.limit(10)

clients.each do |client|
  puts client.address.postcode
end

This code looks fine at the first sight. But the problem lies within the total number of queries executed. The above code executes 1 (to find 10 clients) + 10 (one per each client to load the address) = 11 queries in total.

Solution to N + 1 queries problem

Active Record lets you specify in advance all the associations that are going to be loaded. This is possible by specifying the includes method of the Model.find call. With includes, Active Record ensures that all of the specified associations are loaded using the minimum possible number of queries.

Revisiting the above case, we could rewrite Client.limit(10) to use eager load addresses:

clients = Client.includes(:address).limit(10)

clients.each do |client|
  puts client.address.postcode
end

The above code will execute just 2 queries, as opposed to 11 queries in the previous case:

SELECT * FROM clients LIMIT 10
SELECT addresses.* FROM addresses
  WHERE (addresses.client_id IN (1,2,3,4,5,6,7,8,9,10))

13.1 Eager Loading Multiple Associations

Active Record lets you eager load any number of associations with a single Model.find call by using an array, hash, or a nested hash of array/hash with the includes method.

13.1.1 Array of Multiple Associations

Article.includes(:category, :comments)

This loads all the articles and the associated category and comments for each article.

13.1.2 Nested Associations Hash

Category.includes(articles: [{ comments: :guest }, :tags]).find(1)

This will find the category with id 1 and eager load all of the associated articles, the associated articles' tags and comments, and every comment's guest association.

13.2 Specifying Conditions on Eager Loaded Associations

Even though Active Record lets you specify conditions on the eager loaded associations just like joins, the recommended way is to use joins instead.

However if you must do this, you may use where as you would normally.

Article.includes(:comments).where(comments: { visible: true })

This would generate a query which contains a LEFT OUTER JOIN whereas the joins method would generate one using the INNER JOIN function instead.

  SELECT "articles"."id" AS t0_r0, ... "comments"."updated_at" AS t1_r5 FROM "articles" LEFT OUTER JOIN "comments" ON "comments"."article_id" = "articles"."id" WHERE (comments.visible = 1)

If there was no where condition, this would generate the normal set of two queries.

Using where like this will only work when you pass it a Hash. For SQL-fragments you need to use references to force joined tables:

Article.includes(:comments).where("comments.visible = true").references(:comments)

If, in the case of this includes query, there were no comments for any articles, all the articles would still be loaded. By using joins (an INNER JOIN), the join conditions must match, otherwise no records will be returned.

14 Scopes

Scoping allows you to specify commonly-used queries which can be referenced as method calls on the association objects or models. With these scopes, you can use every method previously covered such as where, joins and includes. All scope methods will return an ActiveRecord::Relation object which will allow for further methods (such as other scopes) to be called on it.

To define a simple scope, we use the scope method inside the class, passing the query that we'd like to run when this scope is called:

class Article < ActiveRecord::Base
  scope :published, -> { where(published: true) }
end

This is exactly the same as defining a class method, and which you use is a matter of personal preference:

class Article < ActiveRecord::Base
  def self.published
    where(published: true)
  end
end

Scopes are also chainable within scopes:

class Article < ActiveRecord::Base
  scope :published,               -> { where(published: true) }
  scope :published_and_commented, -> { published.where("comments_count > 0") }
end

To call this published scope we can call it on either the class:

Article.published # => [published articles]

Or on an association consisting of Article objects:

category = Category.first
category.articles.published # => [published articles belonging to this category]

14.1 Passing in arguments

Your scope can take arguments:

class Article < ActiveRecord::Base
  scope :created_before, ->(time) { where("created_at < ?", time) }
end

Call the scope as if it were a class method:

Article.created_before(Time.zone.now)

However, this is just duplicating the functionality that would be provided to you by a class method.

class Article < ActiveRecord::Base
  def self.created_before(time)
    where("created_at < ?", time)
  end
end

Using a class method is the preferred way to accept arguments for scopes. These methods will still be accessible on the association objects:

category.articles.created_before(time)

14.2 Applying a default scope

If we wish for a scope to be applied across all queries to the model we can use the default_scope method within the model itself.

class Client < ActiveRecord::Base
  default_scope { where("removed_at IS NULL") }
end

When queries are executed on this model, the SQL query will now look something like this:

SELECT * FROM clients WHERE removed_at IS NULL

If you need to do more complex things with a default scope, you can alternatively define it as a class method:

class Client < ActiveRecord::Base
  def self.default_scope
    # Should return an ActiveRecord::Relation.
  end
end

14.3 Merging of scopes

Just like where clauses scopes are merged using AND conditions.

class User < ActiveRecord::Base
  scope :active, -> { where state: 'active' }
  scope :inactive, -> { where state: 'inactive' }
end

User.active.inactive
# SELECT "users".* FROM "users" WHERE "users"."state" = 'active' AND "users"."state" = 'inactive'

We can mix and match scope and where conditions and the final sql will have all conditions joined with AND.

User.active.where(state: 'finished')
# SELECT "users".* FROM "users" WHERE "users"."state" = 'active' AND "users"."state" = 'finished'

If we do want the last where clause to win then Relation#merge can be used.

User.active.merge(User.inactive)
# SELECT "users".* FROM "users" WHERE "users"."state" = 'inactive'

One important caveat is that default_scope will be prepended in scope and where conditions.

class User < ActiveRecord::Base
  default_scope { where state: 'pending' }
  scope :active, -> { where state: 'active' }
  scope :inactive, -> { where state: 'inactive' }
end

User.all
# SELECT "users".* FROM "users" WHERE "users"."state" = 'pending'

User.active
# SELECT "users".* FROM "users" WHERE "users"."state" = 'pending' AND "users"."state" = 'active'

User.where(state: 'inactive')
# SELECT "users".* FROM "users" WHERE "users"."state" = 'pending' AND "users"."state" = 'inactive'

As you can see above the default_scope is being merged in both scope and where conditions.

14.4 Removing All Scoping

If we wish to remove scoping for any reason we can use the unscoped method. This is especially useful if a default_scope is specified in the model and should not be applied for this particular query.

Client.unscoped.load

This method removes all scoping and will do a normal query on the table.

Note that chaining unscoped with a scope does not work. In these cases, it is recommended that you use the block form of unscoped:

Client.unscoped {
  Client.created_before(Time.zone.now)
}

15 Dynamic Finders

For every field (also known as an attribute) you define in your table, Active Record provides a finder method. If you have a field called first_name on your Client model for example, you get find_by_first_name for free from Active Record. If you have a locked field on the Client model, you also get find_by_locked method.

You can specify an exclamation point (!) on the end of the dynamic finders to get them to raise an ActiveRecord::RecordNotFound error if they do not return any records, like Client.find_by_name!("Ryan")

If you want to find both by name and locked, you can chain these finders together by simply typing "and" between the fields. For example, Client.find_by_first_name_and_locked("Ryan", true).

16 Understanding The Method Chaining

The Active Record pattern implements Method Chaining, which allow us to use multiple Active Record methods together in a simple and straightforward way.

You can chain methods in a statement when the previous method called returns an ActiveRecord::Relation, like all, where, and joins. Methods that return a single object (see Retrieving a Single Object Section) have to be at the end of the statement.

There are some examples below. This guide won't cover all the possibilities, just a few as examples. When an Active Record method is called, the query is not immediately generated and sent to the database, this just happens when the data is actually needed. So each example below generates a single query.

16.1 Retrieving filtered data from multiple tables

Person
  .select('people.id, people.name, comments.text')
  .joins(:comments)
  .where('comments.created_at > ?', 1.week.ago)

The result should be something like this:

SELECT people.id, people.name, comments.text
FROM people
INNER JOIN comments
  ON comments.person_id = people.id
WHERE comments.created_at = '2015-01-01'

16.2 Retrieving specific data from multiple tables

Person
  .select('people.id, people.name, companies.name')
  .joins(:company)
  .find_by('people.name' => 'John') # this should be the last

The above should generate:

SELECT people.id, people.name, companies.name
FROM people
INNER JOIN companies
  ON companies.person_id = people.id
WHERE people.name = 'John'
LIMIT 1

Note that if a query matches multiple records, find_by will fetch only the first one and ignore the others (see the LIMIT 1 statement above).

17 Find or Build a New Object

It's common that you need to find a record or create it if it doesn't exist. You can do that with the find_or_create_by and find_or_create_by! methods.

17.1 `find_or_create_by`

The find_or_create_by method checks whether a record with the attributes exists. If it doesn't, then create is called. Let's see an example.

Suppose you want to find a client named 'Andy', and if there's none, create one. You can do so by running:

Client.find_or_create_by(first_name: 'Andy')
# => #<Client id: 1, first_name: "Andy", orders_count: 0, locked: true, created_at: "2011-08-30 06:09:27", updated_at: "2011-08-30 06:09:27">

The SQL generated by this method looks like this:

SELECT * FROM clients WHERE (clients.first_name = 'Andy') LIMIT 1
BEGIN
INSERT INTO clients (created_at, first_name, locked, orders_count, updated_at) VALUES ('2011-08-30 05:22:57', 'Andy', 1, NULL, '2011-08-30 05:22:57')
COMMIT

find_or_create_by returns either the record that already exists or the new record. In our case, we didn't already have a client named Andy so the record is created and returned.

The new record might not be saved to the database; that depends on whether validations passed or not (just like create).

Suppose we want to set the 'locked' attribute to false if we're creating a new record, but we don't want to include it in the query. So we want to find the client named "Andy", or if that client doesn't exist, create a client named "Andy" which is not locked.

We can achieve this in two ways. The first is to use create_with:

Client.create_with(locked: false).find_or_create_by(first_name: 'Andy')

The second way is using a block:

Client.find_or_create_by(first_name: 'Andy') do |c|
  c.locked = false
end

The block will only be executed if the client is being created. The second time we run this code, the block will be ignored.

17.2 `find_or_create_by!`

You can also use find_or_create_by! to raise an exception if the new record is invalid. Validations are not covered on this guide, but let's assume for a moment that you temporarily add

validates :orders_count, presence: true

to your Client model. If you try to create a new Client without passing an orders_count, the record will be invalid and an exception will be raised:

Client.find_or_create_by!(first_name: 'Andy')
# => ActiveRecord::RecordInvalid: Validation failed: Orders count can't be blank

17.3 `find_or_initialize_by`

The find_or_initialize_by method will work just like find_or_create_by but it will call new instead of create. This means that a new model instance will be created in memory but won't be saved to the database. Continuing with the find_or_create_by example, we now want the client named 'Nick':

nick = Client.find_or_initialize_by(first_name: 'Nick')
# => <Client id: nil, first_name: "Nick", orders_count: 0, locked: true, created_at: "2011-08-30 06:09:27", updated_at: "2011-08-30 06:09:27">

nick.persisted?
# => false

nick.new_record?
# => true

Because the object is not yet stored in the database, the SQL generated looks like this:

SELECT * FROM clients WHERE (clients.first_name = 'Nick') LIMIT 1

When you want to save it to the database, just call save:

nick.save
# => true

18 Finding by SQL

If you'd like to use your own SQL to find records in a table you can use find_by_sql. The find_by_sql method will return an array of objects even if the underlying query returns just a single record. For example you could run this query:

Client.find_by_sql("SELECT * FROM clients
  INNER JOIN orders ON clients.id = orders.client_id
  ORDER BY clients.created_at desc")
# =>  [
  #<Client id: 1, first_name: "Lucas" >,
  #<Client id: 2, first_name: "Jan" >,
  # ...
]

find_by_sql provides you with a simple way of making custom calls to the database and retrieving instantiated objects.

18.1 `select_all`

find_by_sql has a close relative called connection#select_all. select_all will retrieve objects from the database using custom SQL just like find_by_sql but will not instantiate them. Instead, you will get an array of hashes where each hash indicates a record.

Client.connection.select_all("SELECT first_name, created_at FROM clients WHERE id = '1'")
# => [
  {"first_name"=>"Rafael", "created_at"=>"2012-11-10 23:23:45.281189"},
  {"first_name"=>"Eileen", "created_at"=>"2013-12-09 11:22:35.221282"}
]

18.2 `pluck`

pluck can be used to query single or multiple columns from the underlying table of a model. It accepts a list of column names as argument and returns an array of values of the specified columns with the corresponding data type.

Client.where(active: true).pluck(:id)
# SELECT id FROM clients WHERE active = 1
# => [1, 2, 3]

Client.distinct.pluck(:role)
# SELECT DISTINCT role FROM clients
# => ['admin', 'member', 'guest']

Client.pluck(:id, :name)
# SELECT clients.id, clients.name FROM clients
# => [[1, 'David'], [2, 'Jeremy'], [3, 'Jose']]

pluck makes it possible to replace code like:

Client.select(:id).map { |c| c.id }
# or
Client.select(:id).map(&:id)
# or
Client.select(:id, :name).map { |c| [c.id, c.name] }

with:

Client.pluck(:id)
# or
Client.pluck(:id, :name)

Unlike select, pluck directly converts a database result into a Ruby Array, without constructing ActiveRecord objects. This can mean better performance for a large or often-running query. However, any model method overrides will not be available. For example:

class Client < ActiveRecord::Base
  def name
    "I am #{super}"
  end
end

Client.select(:name).map &:name
# => ["I am David", "I am Jeremy", "I am Jose"]

Client.pluck(:name)
# => ["David", "Jeremy", "Jose"]

Furthermore, unlike select and other Relation scopes, pluck triggers an immediate query, and thus cannot be chained with any further scopes, although it can work with scopes already constructed earlier:

Client.pluck(:name).limit(1)
# => NoMethodError: undefined method `limit' for #<Array:0x007ff34d3ad6d8>

Client.limit(1).pluck(:name)
# => ["David"]

18.3 `ids`

ids can be used to pluck all the IDs for the relation using the table's primary key.

Person.ids
# SELECT id FROM people

class Person < ActiveRecord::Base
  self.primary_key = "person_id"
end

Person.ids
# SELECT person_id FROM people

19 Existence of Objects

If you simply want to check for the existence of the object there's a method called exists?. This method will query the database using the same query as find, but instead of returning an object or collection of objects it will return either true or false.

Client.exists?(1)

The exists? method also takes multiple values, but the catch is that it will return true if any one of those records exists.

Client.exists?(id: [1,2,3])
# or
Client.exists?(name: ['John', 'Sergei'])

It's even possible to use exists? without any arguments on a model or a relation.

Client.where(first_name: 'Ryan').exists?

The above returns true if there is at least one client with the first_name 'Ryan' and false otherwise.

Client.exists?

The above returns false if the clients table is empty and true otherwise.

You can also use any? and many? to check for existence on a model or relation.

# via a model
Article.any?
Article.many?

# via a named scope
Article.recent.any?
Article.recent.many?

# via a relation
Article.where(published: true).any?
Article.where(published: true).many?

# via an association
Article.first.categories.any?
Article.first.categories.many?

20 Calculations

This section uses count as an example method in this preamble, but the options described apply to all sub-sections.

All calculation methods work directly on a model:

Client.count
# SELECT count(*) AS count_all FROM clients

Or on a relation:

Client.where(first_name: 'Ryan').count
# SELECT count(*) AS count_all FROM clients WHERE (first_name = 'Ryan')

You can also use various finder methods on a relation for performing complex calculations:

Client.includes("orders").where(first_name: 'Ryan', orders: { status: 'received' }).count

Which will execute:

SELECT count(DISTINCT clients.id) AS count_all FROM clients
  LEFT OUTER JOIN orders ON orders.client_id = client.id WHERE
  (clients.first_name = 'Ryan' AND orders.status = 'received')

20.1 Count

If you want to see how many records are in your model's table you could call Client.count and that will return the number. If you want to be more specific and find all the clients with their age present in the database you can use Client.count(:age).

For options, please see the parent section, Calculations.

20.2 Average

If you want to see the average of a certain number in one of your tables you can call the average method on the class that relates to the table. This method call will look something like this:

Client.average("orders_count")

This will return a number (possibly a floating point number such as 3.14159265) representing the average value in the field.

For options, please see the parent section, Calculations.

20.3 Minimum

If you want to find the minimum value of a field in your table you can call the minimum method on the class that relates to the table. This method call will look something like this:

Client.minimum("age")

For options, please see the parent section, Calculations.

20.4 Maximum

If you want to find the maximum value of a field in your table you can call the maximum method on the class that relates to the table. This method call will look something like this:

Client.maximum("age")

For options, please see the parent section, Calculations.

20.5 Sum

If you want to find the sum of a field for all records in your table you can call the sum method on the class that relates to the table. This method call will look something like this:

Client.sum("orders_count")

For options, please see the parent section, Calculations.

21 Running EXPLAIN

You can run EXPLAIN on the queries triggered by relations. For example,

User.where(id: 1).joins(:articles).explain

may yield

EXPLAIN for: SELECT `users`.* FROM `users` INNER JOIN `articles` ON `articles`.`user_id` = `users`.`id` WHERE `users`.`id` = 1
+----+-------------+----------+-------+---------------+
| id | select_type | table    | type  | possible_keys |
+----+-------------+----------+-------+---------------+
|  1 | SIMPLE      | users    | const | PRIMARY       |
|  1 | SIMPLE      | articles | ALL   | NULL          |
+----+-------------+----------+-------+---------------+
+---------+---------+-------+------+-------------+
| key     | key_len | ref   | rows | Extra       |
+---------+---------+-------+------+-------------+
| PRIMARY | 4       | const |    1 |             |
| NULL    | NULL    | NULL  |    1 | Using where |
+---------+---------+-------+------+-------------+

2 rows in set (0.00 sec)

under MySQL.

Active Record performs a pretty printing that emulates the one of the database shells. So, the same query running with the PostgreSQL adapter would yield instead

EXPLAIN for: SELECT "users".* FROM "users" INNER JOIN "articles" ON "articles"."user_id" = "users"."id" WHERE "users"."id" = 1
                                  QUERY PLAN
------------------------------------------------------------------------------
 Nested Loop Left Join  (cost=0.00..37.24 rows=8 width=0)
   Join Filter: (articles.user_id = users.id)
   ->  Index Scan using users_pkey on users  (cost=0.00..8.27 rows=1 width=4)
         Index Cond: (id = 1)
   ->  Seq Scan on articles  (cost=0.00..28.88 rows=8 width=4)
         Filter: (articles.user_id = 1)
(6 rows)

Eager loading may trigger more than one query under the hood, and some queries may need the results of previous ones. Because of that, explain actually executes the query, and then asks for the query plans. For example,

User.where(id: 1).includes(:articles).explain

yields

EXPLAIN for: SELECT `users`.* FROM `users`  WHERE `users`.`id` = 1
+----+-------------+-------+-------+---------------+
| id | select_type | table | type  | possible_keys |
+----+-------------+-------+-------+---------------+
|  1 | SIMPLE      | users | const | PRIMARY       |
+----+-------------+-------+-------+---------------+
+---------+---------+-------+------+-------+
| key     | key_len | ref   | rows | Extra |
+---------+---------+-------+------+-------+
| PRIMARY | 4       | const |    1 |       |
+---------+---------+-------+------+-------+

1 row in set (0.00 sec)

EXPLAIN for: SELECT `articles`.* FROM `articles`  WHERE `articles`.`user_id` IN (1)
+----+-------------+----------+------+---------------+
| id | select_type | table    | type | possible_keys |
+----+-------------+----------+------+---------------+
|  1 | SIMPLE      | articles | ALL  | NULL          |
+----+-------------+----------+------+---------------+
+------+---------+------+------+-------------+
| key  | key_len | ref  | rows | Extra       |
+------+---------+------+------+-------------+
| NULL | NULL    | NULL |    1 | Using where |
+------+---------+------+------+-------------+


1 row in set (0.00 sec)

under MySQL.

21.1 Interpreting EXPLAIN

Interpretation of the output of EXPLAIN is beyond the scope of this guide. The following pointers may be helpful:

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Guides.rubyonrails.org

Chapters

1 Lấy các đối tượng từ Database

1.1 Lấy một Object đơn từ Database

1.1.1 find

1.1.2 take

1.1.3 first

1.1.4 last

1.1.5 find_by

1.2 Retrieving Multiple Objects in Batches ( Lấy nhiều đối tượng)

1.2.1 find_each

1.2.1.1 Options for find_each

1.2.2 find_in_batches

1.2.2.1 Options for find_in_batches

2 Conditions ( Điều kiện)

2.1 Pure String Conditions ( Điều kiện giới hạn bằng chuỗi)

2.2 Array Conditions

2.2.1 Placeholder Conditions

2.3 Hash Conditions

2.3.1 Equality Conditions

2.3.2 Range Conditions

2.3.3 Subset Conditions

2.4 NOT Conditions

3 Ordering ( Sắp xếp)

4 Selecting Specific Fields ( Lựa chọn các trường xác định)

5 Limit and Offset

6 Group

6.1 Total of grouped items

7 Having

8 Overriding Conditions

8.1 unscope

8.2 only

8.3 reorder

8.4 reverse_order

8.5 rewhere

9 Null Relation

10 Readonly Objects

11 Locking Records for Update

11.1 Optimistic Locking

11.2 Pessimistic Locking

12 Joining Tables

12.1 Using a String SQL Fragment

12.2 Using Array/Hash of Named Associations

12.2.1 Joining a Single Association

12.2.2 Joining Multiple Associations

12.2.3 Joining Nested Associations (Single Level)

12.2.4 Joining Nested Associations (Multiple Level)

12.3 Specifying Conditions on the Joined Tables

13 Eager Loading Associations

13.1 Eager Loading Multiple Associations

13.1.1 Array of Multiple Associations

13.1.2 Nested Associations Hash

13.2 Specifying Conditions on Eager Loaded Associations

14 Scopes

14.1 Passing in arguments

14.2 Applying a default scope

14.3 Merging of scopes

14.4 Removing All Scoping

15 Dynamic Finders

16 Understanding The Method Chaining

16.1 Retrieving filtered data from multiple tables

16.2 Retrieving specific data from multiple tables

17 Find or Build a New Object

17.1 find_or_create_by

17.2 find_or_create_by!

17.3 find_or_initialize_by

18 Finding by SQL

18.1 select_all

18.2 pluck

18.3 ids

19 Existence of Objects

20 Calculations

20.1 Count

20.2 Average

20.3 Minimum

20.4 Maximum

20.5 Sum

21 Running EXPLAIN

21.1 Interpreting EXPLAIN

1.1.1 `find`

1.1.2 `take`

1.1.3 `first`

1.1.4 `last`

1.1.5 `find_by`

1.2.1 `find_each`

1.2.1.1 Options for `find_each`

1.2.2 `find_in_batches`

1.2.2.1 Options for `find_in_batches`

8.1 `unscope`

8.2 `only`

8.3 `reorder`

8.4 `reverse_order`

8.5 `rewhere`

17.1 `find_or_create_by`

17.2 `find_or_create_by!`

17.3 `find_or_initialize_by`

18.1 `select_all`

18.2 `pluck`

18.3 `ids`