Module: ActiveRecord::QueryMethods

Specifies whether the records should be unique or not. For example:

User.select(:name)
# => Might return two records with the same name

User.select(:name).distinct
# => Returns 1 record per distinct name

User.select(:name).distinct.distinct(false)
# => You can also remove the uniqueness

Forces eager loading by performing a LEFT OUTER JOIN on args:

User.eager_load(:posts)
#=> SELECT "users"."id" AS t0_r0, "users"."name" AS t0_r1, ...
FROM "users" LEFT OUTER JOIN "posts" ON "posts"."user_id" =
"users"."id"

Used to extend a scope with additional methods, either through a module or through a block provided.

The object returned is a relation, which can be further extended.

Using a module

module Pagination
  def page(number)
    # pagination code goes here
  end
end

scope = Model.all.extending(Pagination)
scope.page(params[:page])

You can also pass a list of modules:

scope = Model.all.extending(Pagination, SomethingElse)

Using a block

scope = Model.all.extending do
  def page(number)
    # pagination code goes here
  end
end
scope.page(params[:page])

You can also use a block and a module list:

scope = Model.all.extending(Pagination) do
  def per_page(number)
    # pagination code goes here
  end
end

Specifies table from which the records will be fetched. For example:

Topic.select('title').from('posts')
# => SELECT title FROM posts

Can accept other relation objects. For example:

Topic.select('title').from(Topic.approved)
# => SELECT title FROM (SELECT * FROM topics WHERE approved = 't') subquery

Topic.select('a.title').from(Topic.approved, :a)
# => SELECT a.title FROM (SELECT * FROM topics WHERE approved = 't') a

Allows to specify a group attribute:

User.group(:name)
#=> SELECT "users".* FROM "users" GROUP BY name

Returns an array with distinct records based on the group attribute:

User.select([:id, :name])
#=> [#<User id: 1, name: "Oscar">, #<User id: 2, name: "Oscar">, #<User id: 3, name: "Foo">

User.group(:name)
#=> [#<User id: 3, name: "Foo", ...>, #<User id: 2, name: "Oscar", ...>]

User.group('name AS grouped_name, age')
#=> [#<User id: 3, name: "Foo", age: 21, ...>, #<User id: 2, name: "Oscar", age: 21, ...>, #<User id: 5, name: "Foo", age: 23, ...>]

Passing in an array of attributes to group by is also supported.

User.select([:id, :first_name]).group(:id, :first_name).first(3)
#=> [#<User id: 1, first_name: "Bill">, #<User id: 2, first_name: "Earl">, #<User id: 3, first_name: "Beto">]

Allows to specify a HAVING clause. Note that you can't use HAVING without also specifying a GROUP clause.

Order.having('SUM(price) > 30').group('user_id')

Specify relationships to be included in the result set. For example:

users = User.includes(:address)
users.each do |user|
  user.address.city
end

allows you to access the address attribute of the User model without firing an additional query. This will often result in a performance improvement over a simple join.

You can also specify multiple relationships, like this:

users = User.includes(:address, :friends)

Loading nested relationships is possible using a ::Hash:

users = User.includes(:address, friends: [:address, :followers])

conditions

If you want to add conditions to your included models you'll have to explicitly reference them. For example:

User.includes(:posts).where('posts.name = ?', 'example')

Will throw an error, but this will work:

User.includes(:posts).where('posts.name = ?', 'example').references(:posts)

Note that includes works with association names while #references needs the actual table name.

Performs a joins on args:

User.joins(:posts)
#=> SELECT "users".* FROM "users" INNER JOIN "posts" ON "posts"."user_id" = "users"."id"

You can use strings in order to customize your joins:

User.joins("LEFT JOIN bookmarks ON bookmarks.bookmarkable_type = 'Post' AND bookmarks.user_id = users.id")
#=> SELECT "users".* FROM "users" LEFT JOIN bookmarks ON bookmarks.bookmarkable_type = 'Post' AND bookmarks.user_id = users.id

Specifies a limit for the number of records to retrieve.

User.limit(10) # generated SQL has 'LIMIT 10'

User.limit(10).limit(20) # generated SQL has 'LIMIT 20'

Specifies locking settings (default to true). For more information on locking, please see Locking.

Returns a chainable relation with zero records.

The returned relation implements the Null Object pattern. It is an object with defined null behavior and always returns an empty array of records without querying the database.

Any subsequent condition chained to the returned relation will continue generating an empty relation and will not fire any query to the database.

Used in cases where a method or scope could return zero records but the result needs to be chainable.

For example:

@posts = current_user.visible_posts.where(name: params[:name])
# => the visible_posts method is expected to return a chainable Relation

def visible_posts
  case role
  when 'Country Manager'
    Post.where(country: country)
  when 'Reviewer'
    Post.published
  when 'Bad User'
    Post.none # It can't be chained if [] is returned.
  end
end

Specifies the number of rows to skip before returning rows.

User.offset(10) # generated SQL has "OFFSET 10"

Should be used with order.

User.offset(10).order("name ASC")

Allows to specify an order attribute:

User.order(:name)
#=> SELECT "users".* FROM "users" ORDER BY "users"."name" ASC

User.order(email: :desc)
#=> SELECT "users".* FROM "users" ORDER BY "users"."email" DESC

User.order(:name, email: :desc)
#=> SELECT "users".* FROM "users" ORDER BY "users"."name" ASC, "users"."email" DESC

User.order('name')
#=> SELECT "users".* FROM "users" ORDER BY name

User.order('name DESC')
#=> SELECT "users".* FROM "users" ORDER BY name DESC

User.order('name DESC, email')
#=> SELECT "users".* FROM "users" ORDER BY name DESC, email

Allows preloading of args, in the same way that #includes does:

User.preload(:posts)
#=> SELECT "posts".* FROM "posts" WHERE "posts"."user_id" IN (1, 2, 3)

Sets readonly attributes for the returned relation. If value is true (default), attempting to update a record will result in an error.

users = User.readonly
users.first.save
#=> ActiveRecord::ReadOnlyRecord: ActiveRecord::ReadOnlyRecord

Use to indicate that the given table_names are referenced by an SQL string, and should therefore be JOINed in any query rather than loaded separately. This method only works in conjunction with #includes. See #includes for more details.

User.includes(:posts).where("posts.name = 'foo'")
# => Doesn't JOIN the posts table, resulting in an error.

User.includes(:posts).where("posts.name = 'foo'").references(:posts)
# => Query now knows the string references posts, so adds a JOIN

Replaces any existing order defined on the relation with the specified order.

User.order('email DESC').reorder('id ASC') # generated SQL has 'ORDER BY id ASC'

Subsequent calls to order on the same relation will be appended. For example:

User.order('email DESC').reorder('id ASC').order('name ASC')

generates a query with 'ORDER BY id ASC, name ASC'.

Reverse the existing order clause on the relation.

User.order('name ASC').reverse_order # generated SQL has 'ORDER BY name DESC'

Allows you to change a previously set where condition for a given attribute, instead of appending to that condition.

Post.where(trashed: true).where(trashed: false)                       # => WHERE `trashed` = 1 AND `trashed` = 0
Post.where(trashed: true).rewhere(trashed: false)                     # => WHERE `trashed` = 0
Post.where(active: true).where(trashed: true).rewhere(trashed: false) # => WHERE `active` = 1 AND `trashed` = 0

This is short-hand for unscope(where: conditions.keys).where(conditions). Note that unlike reorder, we're only unscoping the named conditions – not the entire where statement.

Works in two unique ways.

First: takes a block so it can be used just like Array#select.

Model.all.select { |m| m.field == value }

This will build an array of objects from the database for the scope, converting them into an array and iterating through them using Array#select.

Second: Modifies the SELECT statement for the query so that only certain fields are retrieved:

Model.select(:field)
# => [#<Model id: nil, field: "value">]

Although in the above example it looks as though this method returns an array, it actually returns a relation object and can have other query methods appended to it, such as the other methods in QueryMethods.

The argument to the method can also be an array of fields.

Model.select(:field, :other_field, :and_one_more)
# => [#<Model id: nil, field: "value", other_field: "value", and_one_more: "value">]

You can also use one or more strings, which will be used unchanged as SELECT fields.

Model.select('field AS field_one', 'other_field AS field_two')
# => [#<Model id: nil, field: "value", other_field: "value">]

If an alias was specified, it will be accessible from the resulting objects:

Model.select('field AS field_one').first.field_one
# => "value"

Accessing attributes of an object that do not have fields retrieved by a select except id will throw ::ActiveModel::MissingAttributeError:

Model.select(:field).first.other_field
# => ActiveModel::MissingAttributeError: missing attribute: other_field

Alias for #distinct.

Alias for #distinct!.

Removes an unwanted relation that is already defined on a chain of relations. This is useful when passing around chains of relations and would like to modify the relations without reconstructing the entire chain.

User.order('email DESC').unscope(:order) == User.all

The method arguments are symbols which correspond to the names of the methods which should be unscoped. The valid arguments are given in VALID_UNSCOPING_VALUES. The method can also be called with multiple arguments. For example:

User.order('email DESC').select('id').where(name: "John")
    .unscope(:order, :select, :where) == User.all

One can additionally pass a hash as an argument to unscope specific :where values. This is done by passing a hash with a single key-value pair. The key should be :where and the value should be the where value to unscope. For example:

User.where(name: "John", active: true).unscope(where: :name)
    == User.where(active: true)

This method is similar to except, but unlike except, it persists across merges:

User.order('email').merge(User.except(:order))
    == User.order('email')

User.order('email').merge(User.unscope(:order))
    == User.all

This means it can be used in association definitions:

has_many :comments, -> { unscope where: :trashed }

Returns a new relation, which is the result of filtering the current relation according to the conditions in the arguments.

#where accepts conditions in one of several formats. In the examples below, the resulting SQL is given as an illustration; the actual query generated may be different depending on the database adapter.

string

A single string, without additional arguments, is passed to the query constructor as an SQL fragment, and used in the where clause of the query.

Client.where("orders_count = '2'")
# SELECT * from clients where orders_count = '2';

Note that building your own string from user input may expose your application to injection attacks if not done properly. As an alternative, it is recommended to use one of the following methods.

array

If an array is passed, then the first element of the array is treated as a template, and the remaining elements are inserted into the template to generate the condition. Active Record takes care of building the query to avoid injection attacks, and will convert from the ruby type to the database type where needed. Elements are inserted into the string in the order in which they appear.

User.where(["name = ? and email = ?", "Joe", "joe@example.com"])
# SELECT * FROM users WHERE name = 'Joe' AND email = 'joe@example.com';

Alternatively, you can use named placeholders in the template, and pass a hash as the second element of the array. The names in the template are replaced with the corresponding values from the hash.

User.where(["name = :name and email = :email", { name: "Joe", email: "joe@example.com" }])
# SELECT * FROM users WHERE name = 'Joe' AND email = 'joe@example.com';

This can make for more readable code in complex queries.

Lastly, you can use sprintf-style % escapes in the template. This works slightly differently than the previous methods; you are responsible for ensuring that the values in the template are properly quoted. The values are passed to the connector for quoting, but the caller is responsible for ensuring they are enclosed in quotes in the resulting SQL. After quoting, the values are inserted using the same escapes as the Ruby core method Kernel.sprintf.

User.where(["name = '%s' and email = '%s'", "Joe", "joe@example.com"])
# SELECT * FROM users WHERE name = 'Joe' AND email = 'joe@example.com';

If #where is called with multiple arguments, these are treated as if they were passed as the elements of a single array.

User.where("name = :name and email = :email", { name: "Joe", email: "joe@example.com" })
# SELECT * FROM users WHERE name = 'Joe' AND email = 'joe@example.com';

When using strings to specify conditions, you can use any operator available from the database. While this provides the most flexibility, you can also unintentionally introduce dependencies on the underlying database. If your code is intended for general consumption, test with multiple database backends.

hash

#where will also accept a hash condition, in which the keys are fields and the values are values to be searched for.

Fields can be symbols or strings. Values can be single values, arrays, or ranges.

User.where({ name: "Joe", email: "joe@example.com" })
# SELECT * FROM users WHERE name = 'Joe' AND email = 'joe@example.com'

User.where({ name: ["Alice", "Bob"]})
# SELECT * FROM users WHERE name IN ('Alice', 'Bob')

User.where({ created_at: (Time.now.midnight - 1.day)..Time.now.midnight })
# SELECT * FROM users WHERE (created_at BETWEEN '2012-06-09 07:00:00.000000' AND '2012-06-10 07:00:00.000000')

In the case of a belongs_to relationship, an association key can be used to specify the model if an ::ActiveRecord object is used as the value.

author = Author.find(1)

# The following queries will be equivalent:
Post.where(author: author)
Post.where(author_id: author)

This also works with polymorphic belongs_to relationships:

treasure = Treasure.create(name: 'gold coins')
treasure.price_estimates << PriceEstimate.create(price: 125)

# The following queries will be equivalent:
PriceEstimate.where(estimate_of: treasure)
PriceEstimate.where(estimate_of_type: 'Treasure', estimate_of_id: treasure)

Joins

If the relation is the result of a join, you may create a condition which uses any of the tables in the join. For string and array conditions, use the table name in the condition.

User.joins(:posts).where("posts.created_at < ?", Time.now)

For hash conditions, you can either use the table name in the key, or use a sub-hash.

User.joins(:posts).where({ "posts.published" => true })
User.joins(:posts).where({ posts: { published: true } })

no argument

If no argument is passed, #where returns a new instance of QueryMethods::WhereChain, that can be chained with #not to return a new relation that negates the where clause.

User.where.not(name: "Jon")
# SELECT * FROM users WHERE name != 'Jon'

See WhereChain for more details on #not.

blank condition

If the condition is any blank-ish object, then #where is a no-op and returns the current relation.