query_methods.rb

require "active_record/relation/from_clause"
require "active_record/relation/query_attribute"
require "active_record/relation/where_clause"
require "active_record/relation/where_clause_factory"
require "active_model/forbidden_attributes_protection"
require "active_support/core_ext/string/filters"

module ActiveRecord
  module QueryMethods
    extend ActiveSupport::Concern

    include ActiveModel::ForbiddenAttributesProtection

    # WhereChain objects act as placeholder for queries in which #where does not have any parameter.
    # In this case, #where must be chained with #not to return a new relation.
    class WhereChain
      include ActiveModel::ForbiddenAttributesProtection

      def initialize(scope)
        @scope = scope
      end

      # Returns a new relation expressing WHERE + NOT condition according to
      # the conditions in the arguments.
      #
      # #not accepts conditions as a string, array, or hash. See QueryMethods#where for
      # more details on each format.
      #
      #    User.where.not("name = 'Jon'")
      #    # SELECT * FROM users WHERE NOT (name = 'Jon')
      #
      #    User.where.not(["name = ?", "Jon"])
      #    # SELECT * FROM users WHERE NOT (name = 'Jon')
      #
      #    User.where.not(name: "Jon")
      #    # SELECT * FROM users WHERE name != 'Jon'
      #
      #    User.where.not(name: nil)
      #    # SELECT * FROM users WHERE name IS NOT NULL
      #
      #    User.where.not(name: %w(Ko1 Nobu))
      #    # SELECT * FROM users WHERE name NOT IN ('Ko1', 'Nobu')
      #
      #    User.where.not(name: "Jon", role: "admin")
      #    # SELECT * FROM users WHERE name != 'Jon' AND role != 'admin'
      def not(opts, *rest)
        opts = sanitize_forbidden_attributes(opts)

        where_clause = @scope.send(:where_clause_factory).build(opts, rest)

        @scope.references!(PredicateBuilder.references(opts)) if Hash === opts
        @scope.where_clause += where_clause.invert
        @scope
      end
    end

    FROZEN_EMPTY_ARRAY = [].freeze
    FROZEN_EMPTY_HASH = {}.freeze

    Relation::VALUE_METHODS.each do |name|
      method_name = \
        case name
        when *Relation::MULTI_VALUE_METHODS then "#{name}_values"
        when *Relation::SINGLE_VALUE_METHODS then "#{name}_value"
        when *Relation::CLAUSE_METHODS then "#{name}_clause"
        end
      class_eval <<-CODE, __FILE__, __LINE__ + 1
        def #{method_name}                   # def includes_values
          get_value(#{name.inspect})         #   get_value(:includes)
        end                                  # end

        def #{method_name}=(value)           # def includes_values=(value)
          set_value(#{name.inspect}, value)  #   set_value(:includes, value)
        end                                  # end
      CODE
    end

    def bound_attributes
      if limit_value
        limit_bind = Attribute.with_cast_value(
          "LIMIT".freeze,
          connection.sanitize_limit(limit_value),
          Type.default_value,
        )
      end
      if offset_value
        offset_bind = Attribute.with_cast_value(
          "OFFSET".freeze,
          offset_value.to_i,
          Type.default_value,
        )
      end
      connection.combine_bind_parameters(
        from_clause: from_clause.binds,
        join_clause: arel.bind_values,
        where_clause: where_clause.binds,
        having_clause: having_clause.binds,
        limit: limit_bind,
        offset: offset_bind,
      )
    end

    alias extensions extending_values

    # 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.
    def includes(*args)
      check_if_method_has_arguments!(:includes, args)
      spawn.includes!(*args)
    end

    def includes!(*args) # :nodoc:
      args.reject!(&:blank?)
      args.flatten!

      self.includes_values |= args
      self
    end

    # 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"
    def eager_load(*args)
      check_if_method_has_arguments!(:eager_load, args)
      spawn.eager_load!(*args)
    end

    def eager_load!(*args) # :nodoc:
      self.eager_load_values += args
      self
    end

    # 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)
    def preload(*args)
      check_if_method_has_arguments!(:preload, args)
      spawn.preload!(*args)
    end

    def preload!(*args) # :nodoc:
      self.preload_values += args
      self
    end

    # 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
    def references(*table_names)
      check_if_method_has_arguments!(:references, table_names)
      spawn.references!(*table_names)
    end

    def references!(*table_names) # :nodoc:
      table_names.flatten!
      table_names.map!(&:to_s)

      self.references_values |= table_names
      self
    end

    # 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
    def select(*fields)
      if block_given?
        if fields.any?
          ActiveSupport::Deprecation.warn(<<-WARNING.squish)
            When select is called with a block, it ignores other arguments. This behavior is now deprecated and will result in an ArgumentError in Rails 5.1. You can safely remove the arguments to resolve the deprecation warning because they do not have any effect on the output of the call to the select method with a block.
          WARNING
        end

        return super()
      end

      raise ArgumentError, "Call this with at least one field" if fields.empty?
      spawn._select!(*fields)
    end

    def _select!(*fields) # :nodoc:
      fields.flatten!
      fields.map! do |field|
        klass.attribute_alias?(field) ? klass.attribute_alias(field).to_sym : field
      end
      self.select_values += fields
      self
    end

    # 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">]
    def group(*args)
      check_if_method_has_arguments!(:group, args)
      spawn.group!(*args)
    end

    def group!(*args) # :nodoc:
      args.flatten!

      self.group_values += args
      self
    end

    # 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
    def order(*args)
      check_if_method_has_arguments!(:order, args)
      spawn.order!(*args)
    end

    def order!(*args) # :nodoc:
      preprocess_order_args(args)

      self.order_values += args
      self
    end

    # 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'.
    def reorder(*args)
      check_if_method_has_arguments!(:reorder, args)
      spawn.reorder!(*args)
    end

    def reorder!(*args) # :nodoc:
      preprocess_order_args(args)

      self.reordering_value = true
      self.order_values = args
      self
    end

    VALID_UNSCOPING_VALUES = Set.new([:where, :select, :group, :order, :lock,
                                     :limit, :offset, :joins, :includes, :from,
                                     :readonly, :having])

    # 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) }
    #
    def unscope(*args)
      check_if_method_has_arguments!(:unscope, args)
      spawn.unscope!(*args)
    end

    def unscope!(*args) # :nodoc:
      args.flatten!
      self.unscope_values += args

      args.each do |scope|
        case scope
        when Symbol
          if !VALID_UNSCOPING_VALUES.include?(scope)
            raise ArgumentError, "Called unscope() with invalid unscoping argument ':#{scope}'. Valid arguments are :#{VALID_UNSCOPING_VALUES.to_a.join(", :")}."
          end
          set_value(scope, nil)
        when Hash
          scope.each do |key, target_value|
            if key != :where
              raise ArgumentError, "Hash arguments in .unscope(*args) must have :where as the key."
            end

            target_values = Array(target_value).map(&:to_s)
            self.where_clause = where_clause.except(*target_values)
          end
        else
          raise ArgumentError, "Unrecognized scoping: #{args.inspect}. Use .unscope(where: :attribute_name) or .unscope(:order), for example."
        end
      end

      self
    end

    # 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" "comments_posts"
    #   #   ON "comments_posts"."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
    def joins(*args)
      check_if_method_has_arguments!(:joins, args)
      spawn.joins!(*args)
    end

    def joins!(*args) # :nodoc:
      args.compact!
      args.flatten!
      self.joins_values += args
      self
    end

    # 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"
    #
    def left_outer_joins(*args)
      check_if_method_has_arguments!(:left_outer_joins, args)

      args.compact!
      args.flatten!

      spawn.left_outer_joins!(*args)
    end
    alias :left_joins :left_outer_joins

    def left_outer_joins!(*args) # :nodoc:
      self.left_outer_joins_values += args
      self
    end

    # 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.
    def where(opts = :chain, *rest)
      if :chain == opts
        WhereChain.new(spawn)
      elsif opts.blank?
        self
      else
        spawn.where!(opts, *rest)
      end
    end

    def where!(opts, *rest) # :nodoc:
      opts = sanitize_forbidden_attributes(opts)
      references!(PredicateBuilder.references(opts)) if Hash === opts
      self.where_clause += where_clause_factory.build(opts, rest)
      self
    end

    # 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 <tt>unscope(where: conditions.keys).where(conditions)</tt>.
    # Note that unlike reorder, we're only unscoping the named conditions -- not the entire where statement.
    def rewhere(conditions)
      unscope(where: conditions.keys).where(conditions)
    end

    # 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). Neither relation may have a #limit, #offset, or #distinct set.
    #
    #    Post.where("id = 1").or(Post.where("author_id = 3"))
    #    # SELECT `posts`.* FROM `posts` WHERE ((id = 1) OR (author_id = 3))
    #
    def or(other)
      unless other.is_a? Relation
        raise ArgumentError, "You have passed #{other.class.name} object to #or. Pass an ActiveRecord::Relation object instead."
      end

      spawn.or!(other)
    end

    def or!(other) # :nodoc:
      incompatible_values = structurally_incompatible_values_for_or(other)

      unless incompatible_values.empty?
        raise ArgumentError, "Relation passed to #or must be structurally compatible. Incompatible values: #{incompatible_values}"
      end

      self.where_clause = self.where_clause.or(other.where_clause)
      self.having_clause = self.having_clause.or(other.having_clause)

      self
    end

    # 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')
    def having(opts, *rest)
      opts.blank? ? self : spawn.having!(opts, *rest)
    end

    def having!(opts, *rest) # :nodoc:
      opts = sanitize_forbidden_attributes(opts)
      references!(PredicateBuilder.references(opts)) if Hash === opts

      self.having_clause += having_clause_factory.build(opts, rest)
      self
    end

    # 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'
    def limit(value)
      spawn.limit!(value)
    end

    def limit!(value) # :nodoc:
      self.limit_value = value
      self
    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")
    def offset(value)
      spawn.offset!(value)
    end

    def offset!(value) # :nodoc:
      self.offset_value = value
      self
    end

    # Specifies locking settings (default to +true+). For more information
    # on locking, please see ActiveRecord::Locking.
    def lock(locks = true)
      spawn.lock!(locks)
    end

    def lock!(locks = true) # :nodoc:
      case locks
      when String, TrueClass, NilClass
        self.lock_value = locks || true
      else
        self.lock_value = false
      end

      self
    end

    # 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
    #
    def none
      spawn.none!
    end

    def none! # :nodoc:
      where!("1=0").extending!(NullRelation)
    end

    # 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
    def readonly(value = true)
      spawn.readonly!(value)
    end

    def readonly!(value = true) # :nodoc:
      self.readonly_value = value
      self
    end

    # 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'
    def create_with(value)
      spawn.create_with!(value)
    end

    def create_with!(value) # :nodoc:
      if value
        value = sanitize_forbidden_attributes(value)
        self.create_with_value = create_with_value.merge(value)
      else
        self.create_with_value = {}
      end

      self
    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
    #
    def from(value, subquery_name = nil)
      spawn.from!(value, subquery_name)
    end

    def from!(value, subquery_name = nil) # :nodoc:
      self.from_clause = Relation::FromClause.new(value, subquery_name)
      self
    end

    # 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
    def distinct(value = true)
      spawn.distinct!(value)
    end
    alias uniq distinct
    deprecate uniq: :distinct

    # Like #distinct, but modifies relation in place.
    def distinct!(value = true) # :nodoc:
      self.distinct_value = value
      self
    end
    alias uniq! distinct!
    deprecate uniq!: :distinct!

    # 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
    def extending(*modules, &block)
      if modules.any? || block
        spawn.extending!(*modules, &block)
      else
        self
      end
    end

    def extending!(*modules, &block) # :nodoc:
      modules << Module.new(&block) if block
      modules.flatten!

      self.extending_values += modules
      extend(*extending_values) if extending_values.any?

      self
    end

    # Reverse the existing order clause on the relation.
    #
    #   User.order('name ASC').reverse_order # generated SQL has 'ORDER BY name DESC'
    def reverse_order
      spawn.reverse_order!
    end

    def reverse_order! # :nodoc:
      orders = order_values.uniq
      orders.reject!(&:blank?)
      self.order_values = reverse_sql_order(orders)
      self
    end

    # Returns the Arel object associated with the relation.
    def arel # :nodoc:
      @arel ||= build_arel
    end

    # Returns a relation value with a given name
    def get_value(name) # :nodoc:
      @values[name] || default_value_for(name)
    end

    # Sets the relation value with the given name
    def set_value(name, value) # :nodoc:
      assert_mutability!
      @values[name] = value
    end

    private

      def assert_mutability!
        raise ImmutableRelation if @loaded
        raise ImmutableRelation if defined?(@arel) && @arel
      end

      def build_arel
        arel = Arel::SelectManager.new(table)

        build_joins(arel, joins_values.flatten) unless joins_values.empty?
        build_left_outer_joins(arel, left_outer_joins_values.flatten) unless left_outer_joins_values.empty?

        arel.where(where_clause.ast) unless where_clause.empty?
        arel.having(having_clause.ast) unless having_clause.empty?
        arel.take(Arel::Nodes::BindParam.new) if limit_value
        arel.skip(Arel::Nodes::BindParam.new) if offset_value
        arel.group(*arel_columns(group_values.uniq.reject(&:blank?))) unless group_values.empty?

        build_order(arel)

        build_select(arel)

        arel.distinct(distinct_value)
        arel.from(build_from) unless from_clause.empty?
        arel.lock(lock_value) if lock_value

        arel
      end

      def build_from
        opts = from_clause.value
        name = from_clause.name
        case opts
        when Relation
          name ||= "subquery"
          opts.arel.as(name.to_s)
        else
          opts
        end
      end

      def build_left_outer_joins(manager, outer_joins)
        buckets = outer_joins.group_by do |join|
          case join
          when Hash, Symbol, Array
            :association_join
          else
            raise ArgumentError, "only Hash, Symbol and Array are allowed"
          end
        end

        build_join_query(manager, buckets, Arel::Nodes::OuterJoin)
      end

      def build_joins(manager, joins)
        buckets = joins.group_by do |join|
          case join
          when String
            :string_join
          when Hash, Symbol, Array
            :association_join
          when ActiveRecord::Associations::JoinDependency
            :stashed_join
          when Arel::Nodes::Join
            :join_node
          else
            raise "unknown class: %s" % join.class.name
          end
        end

        build_join_query(manager, buckets, Arel::Nodes::InnerJoin)
      end

      def build_join_query(manager, buckets, join_type)
        buckets.default = []

        association_joins         = buckets[:association_join]
        stashed_association_joins = buckets[:stashed_join]
        join_nodes                = buckets[:join_node].uniq
        string_joins              = buckets[:string_join].map(&:strip).uniq

        join_list = join_nodes + convert_join_strings_to_ast(manager, string_joins)

        join_dependency = ActiveRecord::Associations::JoinDependency.new(
          @klass,
          association_joins,
          join_list
        )

        join_infos = join_dependency.join_constraints stashed_association_joins, join_type

        join_infos.each do |info|
          info.joins.each { |join| manager.from(join) }
          manager.bind_values.concat info.binds
        end

        manager.join_sources.concat(join_list)

        manager
      end

      def convert_join_strings_to_ast(table, joins)
        joins
          .flatten
          .reject(&:blank?)
          .map { |join| table.create_string_join(Arel.sql(join)) }
      end

      def build_select(arel)
        if select_values.any?
          arel.project(*arel_columns(select_values.uniq))
        else
          arel.project(@klass.arel_table[Arel.star])
        end
      end

      def arel_columns(columns)
        columns.map do |field|
          if (Symbol === field || String === field) && (klass.has_attribute?(field) || klass.attribute_alias?(field)) && !from_clause.value
            arel_attribute(field)
          elsif Symbol === field
            connection.quote_table_name(field.to_s)
          else
            field
          end
        end
      end

      def reverse_sql_order(order_query)
        if order_query.empty?
          return [arel_attribute(primary_key).desc] if primary_key
          raise IrreversibleOrderError,
            "Relation has no current order and table has no primary key to be used as default order"
        end

        order_query.flat_map do |o|
          case o
          when Arel::Attribute
            o.desc
          when Arel::Nodes::Ordering
            o.reverse
          when String
            if does_not_support_reverse?(o)
              raise IrreversibleOrderError, "Order #{o.inspect} can not be reversed automatically"
            end
            o.split(",").map! do |s|
              s.strip!
              s.gsub!(/\sasc\Z/i, " DESC") || s.gsub!(/\sdesc\Z/i, " ASC") || s.concat(" DESC")
            end
          else
            o
          end
        end
      end

      def does_not_support_reverse?(order)
        # Uses SQL function with multiple arguments.
        (order.include?(",") && order.split(",").find { |section| section.count("(") != section.count(")") }) ||
          # Uses "nulls first" like construction.
          /nulls (first|last)\Z/i.match?(order)
      end

      def build_order(arel)
        orders = order_values.uniq
        orders.reject!(&:blank?)

        arel.order(*orders) unless orders.empty?
      end

      VALID_DIRECTIONS = [:asc, :desc, :ASC, :DESC,
                          "asc", "desc", "ASC", "DESC"] # :nodoc:

      def validate_order_args(args)
        args.each do |arg|
          next unless arg.is_a?(Hash)
          arg.each do |_key, value|
            raise ArgumentError, "Direction \"#{value}\" is invalid. Valid " \
                                 "directions are: #{VALID_DIRECTIONS.inspect}" unless VALID_DIRECTIONS.include?(value)
          end
        end
      end

      def preprocess_order_args(order_args)
        order_args.map! do |arg|
          klass.send(:sanitize_sql_for_order, arg)
        end
        order_args.flatten!
        validate_order_args(order_args)

        references = order_args.grep(String)
        references.map! { |arg| arg =~ /^([a-zA-Z]\w*)\.(\w+)/ && $1 }.compact!
        references!(references) if references.any?

        # if a symbol is given we prepend the quoted table name
        order_args.map! do |arg|
          case arg
          when Symbol
            arel_attribute(arg).asc
          when Hash
            arg.map { |field, dir|
              arel_attribute(field).send(dir.downcase)
            }
          else
            arg
          end
        end.flatten!
      end

      # Checks to make sure that the arguments are not blank. Note that if some
      # blank-like object were initially passed into the query method, then this
      # method will not raise an error.
      #
      # Example:
      #
      #    Post.references()   # raises an error
      #    Post.references([]) # does not raise an error
      #
      # This particular method should be called with a method_name and the args
      # passed into that method as an input. For example:
      #
      # def references(*args)
      #   check_if_method_has_arguments!("references", args)
      #   ...
      # end
      def check_if_method_has_arguments!(method_name, args)
        if args.blank?
          raise ArgumentError, "The method .#{method_name}() must contain arguments."
        end
      end

      STRUCTURAL_OR_METHODS = Relation::VALUE_METHODS - [:extending, :where, :having]
      def structurally_incompatible_values_for_or(other)
        STRUCTURAL_OR_METHODS.reject do |method|
          get_value(method) == other.get_value(method)
        end
      end

      def where_clause_factory
        @where_clause_factory ||= Relation::WhereClauseFactory.new(klass, predicate_builder)
      end
      alias having_clause_factory where_clause_factory

      def default_value_for(name)
        case name
        when :create_with
          FROZEN_EMPTY_HASH
        when :readonly
          false
        when :where, :having
          Relation::WhereClause.empty
        when :from
          Relation::FromClause.empty
        when *Relation::MULTI_VALUE_METHODS
          FROZEN_EMPTY_ARRAY
        when *Relation::SINGLE_VALUE_METHODS
          nil
        else
          raise ArgumentError, "unknown relation value #{name.inspect}"
        end
      end
  end
end