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Constants
FROZEN_EMPTY_ARRAY | = | [].freeze |
FROZEN_EMPTY_HASH | = | {}.freeze |
VALID_UNSCOPING_VALUES | = | Set.new([:where, :select, :group, :order, :lock, :limit, :offset, :joins, :left_outer_joins, :annotate, :includes, :from, :readonly, :having, :optimizer_hints]) |
Instance Public methods
and(other) Link
Returns a new relation, which is the logical intersection of this relation and the one passed as an argument.
The two relations must be structurally compatible: they must be scoping the same model, and they must differ only by where
(if no group
has been defined) or having
(if a group
is present).
Post.where(id: [1, 2]).and(Post.where(id: [2, 3]))
# SELECT `posts`.* FROM `posts` WHERE `posts`.`id` IN (1, 2) AND `posts`.`id` IN (2, 3)
annotate(*args) Link
Adds an SQL comment to queries generated from this relation. For example:
User.annotate("selecting user names").select(:name)
# SELECT "users"."name" FROM "users" /* selecting user names */
User.annotate("selecting", "user", "names").select(:name)
# SELECT "users"."name" FROM "users" /* selecting */ /* user */ /* names */
The SQL block comment delimiters, “/*” and “*/”, will be added automatically.
create_with(value) Link
Sets attributes to be used when creating new records from a relation object.
users = User.where(name: 'Oscar')
users.new.name # => 'Oscar'
users = users.create_with(name: 'DHH')
users.new.name # => 'DHH'
You can pass nil
to create_with
to reset attributes:
users = users.create_with(nil)
users.new.name # => 'Oscar'
distinct(value = true) Link
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
eager_load(*args) Link
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"
extending(*modules, &block) Link
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
extract_associated(association) Link
Extracts a named association
from the relation. The named association is first preloaded, then the individual association records are collected from the relation. Like so:
account.memberships.extract_associated(:user)
# => Returns collection of User records
This is short-hand for:
account.memberships.preload(:user).collect(&:user)
from(value, subquery_name = nil) Link
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
group(*args) Link
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">]
having(opts, *rest) Link
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')
includes(*args) Link
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 string 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.
If you pass the conditions via hash, you don't need to call references
explicitly, as where
references the tables for you. For example, this will work correctly:
User.includes(:posts).where(posts: { name: 'example' })
joins(*args) Link
Performs a joins on args
. The given symbol(s) should match the name of the association(s).
User.joins(:posts)
# SELECT "users".*
# FROM "users"
# INNER JOIN "posts" ON "posts"."user_id" = "users"."id"
Multiple joins:
User.joins(:posts, :account)
# SELECT "users".*
# FROM "users"
# INNER JOIN "posts" ON "posts"."user_id" = "users"."id"
# INNER JOIN "accounts" ON "accounts"."id" = "users"."account_id"
Nested joins:
User.joins(posts: [:comments])
# SELECT "users".*
# FROM "users"
# INNER JOIN "posts" ON "posts"."user_id" = "users"."id"
# INNER JOIN "comments" ON "comments"."post_id" = "posts"."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
left_outer_joins(*args) Link
Performs a left outer joins on args
:
User.left_outer_joins(:posts)
=> SELECT "users".* FROM "users" LEFT OUTER JOIN "posts" ON "posts"."user_id" = "users"."id"
limit(value) Link
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'
lock(locks = true) Link
Specifies locking settings (default to true
). For more information on locking, please see ActiveRecord::Locking
.
none() Link
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
offset(value) Link
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")
optimizer_hints(*args) Link
Specify optimizer hints to be used in the SELECT statement.
Example (for MySQL):
Topic.optimizer_hints("MAX_EXECUTION_TIME(50000)", "NO_INDEX_MERGE(topics)")
# SELECT /*+ MAX_EXECUTION_TIME(50000) NO_INDEX_MERGE(topics) */ `topics`.* FROM `topics`
Example (for PostgreSQL with pg_hint_plan):
Topic.optimizer_hints("SeqScan(topics)", "Parallel(topics 8)")
# SELECT /*+ SeqScan(topics) Parallel(topics 8) */ "topics".* FROM "topics"
or(other) Link
Returns a new relation, which is the logical union of this relation and the one passed as an argument.
The two relations must be structurally compatible: they must be scoping the same model, and they must differ only by where
(if no group
has been defined) or having
(if a group
is present).
Post.where("id = 1").or(Post.where("author_id = 3"))
# SELECT `posts`.* FROM `posts` WHERE ((id = 1) OR (author_id = 3))
order(*args) Link
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
preload(*args) Link
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)
readonly(value = true) Link
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: User is marked as readonly
references(*table_names) Link
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
reorder(*args) Link
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'.
reselect(*args) Link
Allows you to change a previously set select statement.
Post.select(:title, :body)
# SELECT `posts`.`title`, `posts`.`body` FROM `posts`
Post.select(:title, :body).reselect(:created_at)
# SELECT `posts`.`created_at` FROM `posts`
This is short-hand for unscope(:select).select(fields)
. Note that we're unscoping the entire select statement.
reverse_order() Link
Reverse the existing order clause on the relation.
User.order('name ASC').reverse_order # generated SQL has 'ORDER BY name DESC'
rewhere(conditions) Link
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.
select(*fields) Link
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 ActiveRecord::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
# File activerecord/lib/active_record/relation/query_methods.rb, line 263 def select(*fields) if block_given? if fields.any? raise ArgumentError, "`select' with block doesn't take arguments." end return super() end check_if_method_has_arguments!(:select, fields, "Call `select' with at least one field.") spawn._select!(*fields) end
strict_loading(value = true) Link
Sets the returned relation to strict_loading
mode. This will raise an error if the record tries to lazily load an association.
user = User.strict_loading.first
user.comments.to_a
=> ActiveRecord::StrictLoadingViolationError
uniq!(name) Link
Deduplicate multiple values.
unscope(*args) Link
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) }
where(*args) Link
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 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.