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'

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

    Relation::MULTI_VALUE_METHODS.each do |name|
      class_eval <<-CODE, __FILE__, __LINE__ + 1
        def #{name}_values                    # def select_values
          @values[:#{name}] || []             #   @values[:select] || []
        end                                   # end
                                              #
        def #{name}_values=(values)           # def select_values=(values)
          assert_mutability!                  #   assert_mutability!
          @values[:#{name}] = values          #   @values[:select] = values
        end                                   # end
      CODE
    end

    (Relation::SINGLE_VALUE_METHODS - [:create_with]).each do |name|
      class_eval <<-CODE, __FILE__, __LINE__ + 1
        def #{name}_value                    # def readonly_value
          @values[:#{name}]                  #   @values[:readonly]
        end                                  # end
      CODE
    end

    Relation::SINGLE_VALUE_METHODS.each do |name|
      class_eval <<-CODE, __FILE__, __LINE__ + 1
        def #{name}_value=(value)            # def readonly_value=(value)
          assert_mutability!                 #   assert_mutability!
          @values[:#{name}] = value          #   @values[:readonly] = value
        end                                  # end
      CODE
    end

    Relation::CLAUSE_METHODS.each do |name|
      class_eval <<-CODE, __FILE__, __LINE__ + 1
        def #{name}_clause                           # def where_clause
          @values[:#{name}] || new_#{name}_clause    #   @values[:where] || new_where_clause
        end                                          # end
                                                     #
        def #{name}_clause=(value)                   # def where_clause=(value)
          assert_mutability!                         #   assert_mutability!
          @values[:#{name}] = value                  #   @values[:where] = value
        end                                          # end
      CODE
    end

    def bound_attributes
      result = from_clause.binds + arel.bind_values + where_clause.binds + having_clause.binds
      if limit_value && !string_containing_comma?(limit_value)
        result << Attribute.with_cast_value(
          "LIMIT".freeze,
          connection.sanitize_limit(limit_value),
          Type::Value.new,
        )
      end
      if offset_value
        result << Attribute.with_cast_value(
          "OFFSET".freeze,
          offset_value.to_i,
          Type::Value.new,
        )
      end
      result
    end

    def create_with_value # :nodoc:
      @values[:create_with] || {}
    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)
      return super if block_given?
      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
          symbol_unscoping(scope)
        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
    alias :left_joins! :left_outer_joins!

    # 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("id = 2"))
    #    # SELECT `posts`.* FROM `posts`  WHERE (('id = 1' OR 'id = 2'))
    #
    def or(other)
      spawn.or!(other)
    end

    def or!(other) # :nodoc:
      unless structurally_compatible_for_or?(other)
        raise ArgumentError, 'Relation passed to #or must be structurally compatible'
      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:
      if string_containing_comma?(value)
        # Remove `string_containing_comma?` when removing this deprecation
        ActiveSupport::Deprecation.warn(<<-WARNING)
          Passing a string to limit in the form "1,2" is deprecated and will be
          removed in Rails 5.1. Please call `offset` explicitly instead.
        WARNING
      end
      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
      where("1=0").extending!(NullRelation)
    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

    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?
      if limit_value
        if string_containing_comma?(limit_value)
          arel.take(connection.sanitize_limit(limit_value))
        else
          arel.take(Arel::Nodes::BindParam.new)
        end
      end
      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 symbol_unscoping(scope)
      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

      clause_method = Relation::CLAUSE_METHODS.include?(scope)
      multi_val_method = Relation::MULTI_VALUE_METHODS.include?(scope)
      if clause_method
        unscope_code = "#{scope}_clause="
      else
        unscope_code = "#{scope}_value#{'s' if multi_val_method}="
      end

      case scope
      when :order
        result = []
      else
        result = [] if multi_val_method
      end

      self.send(unscope_code, result)
    end

    def association_for_table(table_name)
      table_name = table_name.to_s
      @klass._reflect_on_association(table_name) ||
        @klass._reflect_on_association(table_name.singularize)
    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) && columns_hash.key?(field.to_s) && !from_clause.value
          arel_table[field]
        elsif Symbol === field
          connection.quote_table_name(field.to_s)
        else
          field
        end
      end
    end

    def reverse_sql_order(order_query)
      order_query = ["#{quoted_table_name}.#{quoted_primary_key} ASC"] if order_query.empty?

      order_query.flat_map do |o|
        case o
        when Arel::Nodes::Ordering
          o.reverse
        when String
          o.to_s.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 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
          arg = klass.attribute_alias(arg) if klass.attribute_alias?(arg)
          table[arg].asc
        when Hash
          arg.map { |field, dir|
            field = klass.attribute_alias(field) if klass.attribute_alias?(field)
            table[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

    def structurally_compatible_for_or?(other)
      Relation::SINGLE_VALUE_METHODS.all? { |m| send("#{m}_value") == other.send("#{m}_value") } &&
        (Relation::MULTI_VALUE_METHODS - [:extending]).all? { |m| send("#{m}_values") == other.send("#{m}_values") } &&
        (Relation::CLAUSE_METHODS - [:having, :where]).all? { |m| send("#{m}_clause") != other.send("#{m}_clause") }
    end

    def new_where_clause
      Relation::WhereClause.empty
    end
    alias new_having_clause new_where_clause

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

    def new_from_clause
      Relation::FromClause.empty
    end

    def string_containing_comma?(value)
      ::String === value && value.include?(",")
    end
  end
end