Ruby on Rails Guides: 액티브 레코드 쿼리 인터페이스

이 가이드는 레일즈 3.0에 기초합니다. 기존 버전의 레일즈에서는 이 문서의 코드가 동작하지 않을 수 있습니다.

이 가이드의 원본은 영문 버전인 Active Record Query Interface 입니다. 번역 과정에서 원본과 차이가 발생할 수 있습니다. 번역본은 원본의 참고로 생각해 주세요.

만약 여러분이 데이터베이스의 레코드를 찾기 위해 표준 SQL을 이용한다면, 일반적으로 여러분은 레일즈로 동일 작업에 대하여 더 좋은 방법을 알게 될 것입니다. 액티브 레코드는 SQL 사용이 필요한 많은 경우에서 여러분을 해방 시킵니다.

이 가이드의 코드 예제는 다음의 한가지 혹은 그 이상의 모델을 참고합니다.:

특별한 경우가 아니면, 아래의 모든 모델은 id를 기본키로 사용합니다.

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

액티브 레코드는 데이터베이스 상에서 쿼리를 실행하며 대부분의 데이터베이스와 호환됩니다. (MySQL, PostgresSQL, SQLite). 여러분이 사용하는 데이터베이스가 상이해도, 액티브 레코드 메소드 사용 방식은 항상 동일합니다.

1 데이터베이스에서 객체 조회하기

데이터베이스에서 객체를 조회하기 위해, 액티브 레코드는 몇가지 찾기(finder) 메소드를 제공합니다. 각 찾기 메소드는 기본 SQL을 사용하지 않고, 데이터베이스에 정확한 쿼리를 수행하기 위해서 인자를 받습니다.

메소드는 다음과 같습니다.:

where
select
group
order
limit
offset
joins
includes
lock
readonly
from
having

위의 모든 메소드는 ActiveRecord::Relation의 인스턴스를 반환합니다.

Model.find(options)의 주요 동작은 다음으로 요약됩니다.:

옵션을 동일 SQL 쿼리로 변환
SQL 쿼리를 실행하고, 데이터베이스에서 관련 결과를 검색
모든 결과 열을 수용하는 적절한 루비 객체의 인스턴스를 생성
after_find 콜백 실행

단일 객체 검색

액티브 레코드는 단일 객체 조회를 위해 3가지 다른 방법을 제공합니다.

기본키 이용하기

Model.find(primary_key)를 사용해서, 주어진 기본키(primary key)와 조건에 일치하는 객체를 찾을 수 있습니다. 예를들어:

# 기본키(id) 10 을 가지고 client 찾기
client = Client.find(10)
=> #<Client id: 10, first_name: => "Ryan">

위와 동일한 SQL은 다음과 같습니다.:

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

만약 일치하는 레코드가 존재하지 않으면, Model.find(primary_key) 호출은 ActiveRecord::RecordNotFound을 발생시킵니다.

`first`

Model.first는 주어진 조건과 일치하는 첫번째 레코드를 찾습니다. 예를들어:

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

위와 동일한 SQL은 다음과 같습니다.:

SELECT * FROM clients LIMIT 1

만약 일치하는 레코드가 없다면, Model.first는 nil을 반환합니다. 예외가 발생하지는 않습니다.

`last`

Model.last는 주어진 조건과 일치하는 마지막 레코드를 찾습니다. 예를들어:

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

위와 동일한 SQL은 다음과 같습니다.:

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

만약 일치하는 레코드가 없다면, Model.last는 nil을 반환합니다. 예외가 발생하지는 않습니다.

1.1 복수 객체 검색

1.1.1 복수 기본키 사용하기

Model.find(array_of_primary_key)는 기본키(primary key)의 배열도 허용합니다. 주어진 여러 기본키와 일치하는 모든 레코드 배열이 반환됩니다. 예를들어:

# 기본키 1과 0에 해당하는 client 객체 찾기
client = Client.find(1, 10) # Or even Client.find([1, 10])
=> [#<Client id: 1, first_name: => "Lifo">, #<Client id: 10, first_name: => "Ryan">]

위와 동일한 SQL은 다음과 같습니다.:

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

Model.find(array_of_primary_key)는 입력된 여러 기본키 값들과 일치하는 레코드를 모두 찾을수 없다면 ActiveRecord::RecordNotFound 예외를 발생합니다. (한개라도 못찾으면 발생)

1.2 일괄 작업(Batch)에서 여러 객체 검색하기

때로 많은 양의 레코드를 순회할 필요가 있습니다. 예를들어, 모든 사용자에게 뉴스레터를 보낼때나, 필요한 데이터를 빼낼때(export) 등 말이죠.

다음 코드는 아마도 맨 처음으로 곧바로 생각할 수 있는 방법일 겁니니다.:

# 열의 수가 수천개가 되는 사용자 테이블이라면 매우 비효율적입니다.
User.all.each do |user|
  NewsLetter.weekly_deliver(user)
end

그러나 테이블의 전체 열의 수가 매우 크다면, 위의 접근 방식은 아마도 But if the total number of rows in the table is very large, the above approach may vary from being under performant to just plain impossible.

User.all.each는 액티브 레코드로 하여금 전체 테이블 정보를 가지고 오게 만들고, 각 행마다 모델 객체를 만들며 메모리에 전체 배열을 유지하게 하기 때문입니다. 때로는 너무 많은 객체를 만들고 굉장히 많은 매모리를 차지합니다.

1.2.1 `find_each`

큰 테이블을 효율적으로 순회하기 위해서, 액티브 레코드는 find_each로 불리는 일괄 작업용 파인더 메소드를 제공합니다.:

User.find_each do |user|
  NewsLetter.weekly_deliver(user)
end

일괄 작업(batch) 크기 설정

find_each는 기본적으로 1000개 단위로 열을 가지고 와서 하나씩 블록을 실행합니다. 이 단위는 :batch_size 옵션을 통해서 설정할 수 있습니다.

User 레코드를 5000개 단위로 가지고 와서 처리하려면:

User.find_each(:batch_size => 5000) do |user|
  NewsLetter.weekly_deliver(user)
end

정해진 기본키 부터 일괄 작업(batch) 조회 시작하기

find_each는 레일즈 2.3.8까지만 존재하며, 이후 버전에서는 제거되었습니다. find_in_batches를 사용하세요.

레코드는 정수형 기본키에 의해 오름차순으로 불립니다. :start 옵션으로 최소 기본키를 설정할 수 있으며, 이 옵션은 이미 처리된 마지막 ID를 체크포인트 용도로 저장했다가, 다시 그 지점부터 일괄 작업을 재시작할때 유용하게 사용할 수 있습니다.

기본키 2000 이상의 사용자에게만 뉴스레터를 보내려면:

User.find_each(:batch_size => 5000, :start => 2000) do |user|
  NewsLetter.weekly_deliver(user)
end

부가 옵션

find_each는 표준 find 메소드와 동일한 옵션을 허용합니다. 그렇지만 :order와 :limit는 내부적으로 필요해서 이 옵션들은 명시적으로 넘길수 없습니다.

1.2.2 `find_in_batches`

You can also work by chunks instead of row by row using find_in_batches. This method is analogous to find_each, but it yields arrays of models instead:

# Works in chunks of 1000 invoices at a time.
Invoice.find_in_batches(:include => :invoice_lines) do |invoices|
  export.add_invoices(invoices)
end

The above will yield the supplied block with 1000 invoices every time.

2 Conditions

The find method allows you to specify conditions to limit the records returned, representing the WHERE-part of the SQL statement. Conditions can either be specified as a string, array, or hash.

2.1 Pure String Conditions

If you’d like to add conditions to your find, you could just specify them in there, just like Client.where("orders_count = '2'"). This will find all clients where the orders_count field’s value is 2.

Building your own conditions as pure strings can leave you vulnerable to SQL injection exploits. For example, Client.where("first_name LIKE '%#{params[:first_name]}%'") is not safe. See the next section for the preferred way to handle conditions using an array.

2.2 Array Conditions

Now what if that number could vary, say as an argument from somewhere? The find then becomes something like:

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

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

Or if you want to specify two conditions, you can do it like:

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

In this example, the first question mark will be replaced with the value in params[:orders] and the second will be replaced with the SQL representation of false, which depends on the adapter.

The reason for doing code like:

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

instead of:

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

is 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 he or she can exploit your database they can do just about anything to it. Never ever put your arguments directly inside the conditions string.

For more information on the dangers of SQL injection, see the Ruby on Rails Security Guide.

2.2.1 Placeholder Conditions

Similar to the (?) replacement style of params, you can also specify keys/values hash in your array conditions:

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

This makes for clearer readability if you have a large number of variable conditions.

2.2.2 Range Conditions

If you’re looking for a range inside of a table (for example, users created in a certain timeframe) you can use the conditions option coupled with the IN SQL statement for this. If you had two dates coming in from a controller you could do something like this to look for a range:

Client.where(:created_at => (params[:start_date].to_date)..(params[:end_date].to_date))

This query will generate something similar to the following SQL:

SELECT "clients".* FROM "clients" WHERE ("clients"."created_at" BETWEEN '2010-09-29' AND '2010-11-30')

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)

The field name can also be a string:

Client.where('locked' => true)

2.3.2 Range Conditions

The good thing about this is that we can pass in a range for our fields without it generating a large query as shown in the preamble of this section.

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

This will find all clients created yesterday by using a BETWEEN SQL statement:

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

If you want to find records using the IN expression you can pass an array to the conditions hash:

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

This code will generate SQL like this:

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

3 Ordering

To retrieve records from the database in a specific order, you can use the order method.

For example, if you’re getting a set of records and want to order them in ascending order by the created_at field in your table:

Client.order("created_at")

You could specify ASC or DESC as well:

Client.order("created_at DESC")
# OR
Client.order("created_at ASC")

Or ordering by multiple fields:

Client.order("orders_count ASC, created_at DESC")

4 Selecting Specific Fields

By default, Model.find selects all the fields from the result set using select *.

To select only a subset of fields from the result set, you can specify the subset via the select method.

If the select method is used, all the returning objects will be read only.

For example, to select only viewable_by and locked columns:

Client.select("viewable_by, locked")

The SQL query used by this find call will be somewhat like:

SELECT viewable_by, locked FROM clients

Be careful because this also means you’re initializing a model object with only the fields that you’ve selected. If you attempt to access a field that is not in the initialized record you’ll receive:

ActiveRecord::MissingAttributeError: missing attribute: <attribute>

Where <attribute> is the attribute you asked for. The id method will not raise the ActiveRecord::MissingAttributeError, so just be careful when working with associations because they need the id method to function properly.

You can also call SQL functions within the select option. For example, if you would like to only grab a single record per unique value in a certain field by using the DISTINCT function you can do it like this:

Client.select("DISTINCT(name)")

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, 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.group("date(created_at)").order("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 * FROM orders GROUP BY date(created_at) ORDER BY created_at

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 option to the find.

For example:

Order.group("date(created_at)").having("created_at > ?", 1.month.ago)

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

SELECT * FROM orders GROUP BY date(created_at) HAVING created_at > '2009-01-15'

This will return single order objects for each day, but only for the last month.

8 Overriding Conditions

You can specify certain conditions to be excepted by using the except method.

For example:

Post.where('id > 10').limit(20).order('id asc').except(:order)

The SQL that would be executed:

SELECT * FROM posts WHERE id > 10 LIMIT 20

You can also override conditions using the only method.

For example:

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

The SQL that would be executed:

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

9 Readonly Objects

Active Record provides readonly method on a relation to explicitly disallow modification or deletion of any of the returned object. Any attempt to alter or destroy 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.

10 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

10.1 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. 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 a 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.

You must ensure that your database schema defaults the lock_version column to 0.

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 method called set_locking_column:

class Client < ActiveRecord::Base
  set_locking_column :lock_client_column
end

10.2 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

11 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.

11.1 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

11.2 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 clause for those associations when using the joins method.

For example, consider the following Category, Post, Comments and Guest models:

class Category < ActiveRecord::Base
  has_many :posts
end

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

class Comments < ActiveRecord::Base
  belongs_to :post
  has_one :guest
end

class Guest < ActiveRecord::Base
  belongs_to :comment
end

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

11.2.1 Joining a Single Association

Category.joins(:posts)

This produces:

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

11.2.2 Joining Multiple Associations

Post.joins(:category, :comments)

This produces:

SELECT posts.* FROM posts
  INNER JOIN categories ON posts.category_id = categories.id
  INNER JOIN comments ON comments.post_id = posts.id

11.2.3 Joining Nested Associations (Single Level)

Post.joins(:comments => :guest)

11.2.4 Joining Nested Associations (Multiple Level)

Category.joins(:posts => [{:comments => :guest}, :tags])

11.3 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.

12 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.all 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))

12.1 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.

12.1.1 Array of Multiple Associations

Post.includes(:category, :comments)

This loads all the posts and the associated category and comments for each post.

12.1.2 Nested Associations Hash

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

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

12.2 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.

13 Scopes

Scoping allows you to specify commonly-used ARel 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 ARel query that we’d like run when this scope is called:

class Post < ActiveRecord::Base
  scope :published, where(:published => true)
end

Just like before, these methods are also chainable:

class Post < ActiveRecord::Base
  scope :published, where(:published => true).joins(:category)
end

Scopes are also chainable within scopes:

class Post < ActiveRecord::Base
  scope :published, where(:published => true)
  scope :published_and_commented, published.and(self.arel_table[:comments_count].gt(0))
end

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

Post.published => [published posts]

Or on an association consisting of Post objects:

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

13.1 Working with times

If you’re working with dates or times within scopes, due to how they are evaluated, you will need to use a lambda so that the scope is evaluated every time.

class Post < ActiveRecord::Base
  scope :last_week, lambda { where("created_at < ?", Time.zone.now ) }
end

Without the lambda, this Time.zone.now will only be called once.

13.2 Passing in arguments

When a lambda is used for a scope, it can take arguments:

class Post < ActiveRecord::Base
  scope :1_week_before, lambda { |time| where("created_at < ?", time)
end

This may then be called using this:

Post.1_week_before(Time.zone.now)

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

class Post < ActiveRecord::Base
  def self.1_week_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.posts.1_week_before(time)

14 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 and find_all_by_first_name for free from Active Record. If you have a locked field on the Client model, you also get find_by_locked and find_all_by_locked methods.

You can also use find_last_by_* methods which will find the last record matching your argument.

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).

There’s another set of dynamic finders that let you find or create/initialize objects if they aren’t found. These work in a similar fashion to the other finders and can be used like find_or_create_by_first_name(params[:first_name]). Using this will first perform a find and then create if the find returns nil. The SQL looks like this for Client.find_or_create_by_first_name("Ryan"):

SELECT * FROM clients WHERE (clients.first_name = 'Ryan') LIMIT 1
BEGIN
INSERT INTO clients (first_name, updated_at, created_at, orders_count, locked)
  VALUES('Ryan', '2008-09-28 15:39:12', '2008-09-28 15:39:12', 0, '0')
COMMIT

find_or_create’s sibling, find_or_initialize, will find an object and if it does not exist will act similarly to calling new with the arguments you passed in. For example:

client = Client.find_or_initialize_by_first_name('Ryan')

will either assign an existing client object with the name “Ryan” to the client local variable, or initialize a new object similar to calling Client.new(:first_name => 'Ryan'). From here, you can modify other fields in client by calling the attribute setters on it: client.locked = true and when you want to write it to the database just call save on it.

15 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 clients.created_at desc")

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

16 `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 * FROM clients WHERE id = '1'")

17 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 ids, but the catch is that it will return true if any one of those records exists.

Client.exists?(1,2,3)
# or
Client.exists?([1,2,3])

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.

18 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')

18.1 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).