mongodb/mongoid

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lib/mongoid/criteria/queryable/selectable.rb

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# frozen_string_literal: true
# encoding: utf-8

module Mongoid
  class Criteria
    module Queryable

      # An queryable selectable is selectable, in that it has the ability to select
      # document from the database. The selectable module brings all functionality
      # to the selectable that has to do with building MongoDB selectors.
      module Selectable
        extend Macroable

        # Constant for a LineString $geometry.
        #
        # @since 2.0.0
        LINE_STRING = "LineString"

        # Constant for a Point $geometry.
        #
        # @since 2.0.0
        POINT = "Point"

        # Constant for a Polygon $geometry.
        #
        # @since 2.0.0
        POLYGON = "Polygon"

        # @attribute [rw] negating If the next expression is negated.
        # @attribute [rw] selector The query selector.
        attr_accessor :negating, :selector

        # Add the $all criterion.
        #
        # @example Add the criterion.
        #   selectable.all(field: [ 1, 2 ])
        #
        # @example Execute an $all in a where query.
        #   selectable.where(:field.all => [ 1, 2 ])
        #
        # @param [ Hash ] criterion The key value pairs for $all matching.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def all(*criteria)
          if criteria.empty?
            return clone.tap do |query|
              query.reset_strategies!
            end
          end

          criteria.inject(clone) do |query, condition|
            if condition.nil?
              raise Errors::CriteriaArgumentRequired, :all
            end

            condition = expand_condition_to_array_values(condition)

            if strategy
              send(strategy, condition, "$all")
            else
              condition.inject(query) do |_query, (field, value)|
                v = {'$all' => value}
                if negating?
                  v = {'$not' => v}
                end
                _query.add_field_expression(field.to_s, v)
              end
            end
          end.reset_strategies!
        end
        alias :all_in :all
        key :all, :union, "$all"

        # Add the $and criterion.
        #
        # @example Add the criterion.
        #   selectable.and({ field: value }, { other: value })
        #
        # @param [ Array<Hash | Criteria> ] criteria Multiple key/value pair
        #   matches or Criteria objects that all must match to return results.
        #
        # @return [ Selectable ] The new selectable.
        #
        # @since 1.0.0
        def and(*criteria)
          _mongoid_flatten_arrays(criteria).inject(self.clone) do |c, new_s|
            if new_s.is_a?(Selectable)
              new_s = new_s.selector
            end
            normalized = _mongoid_expand_keys(new_s)
            normalized.each do |k, v|
              k = k.to_s
              if c.selector[k]
                # There is already a condition on k.
                # If v is an operator, and all existing conditions are
                # also operators, and v isn't present in existing conditions,
                # we can add to existing conditions.
                # Otherwise use $and.
                if v.is_a?(Hash) &&
                  v.length == 1 &&
                  (new_k = v.keys.first).start_with?('$') &&
                  (existing_kv = c.selector[k]).is_a?(Hash) &&
                  !existing_kv.key?(new_k) &&
                  existing_kv.keys.all? { |sub_k| sub_k.start_with?('$') }
                then
                  merged_v = c.selector[k].merge(v)
                  c.selector.store(k, merged_v)
                else
                  c = c.send(:__multi__, [k => v], '$and')
                end
              else
                c.selector.store(k, v)
              end
            end
            c
          end
        end
        alias :all_of :and

        # Add the range selection.
        #
        # @example Match on results within a single range.
        #   selectable.between(field: 1..2)
        #
        # @example Match on results between multiple ranges.
        #   selectable.between(field: 1..2, other: 5..7)
        #
        # @param [ Hash ] criterion Multiple key/range pairs.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def between(criterion)
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :between
          end

          selection(criterion) do |selector, field, value|
            selector.store(
              field,
              { "$gte" => value.min, "$lte" => value.max }
            )
          end
        end

        # Select with an $elemMatch.
        #
        # @example Add criterion for a single match.
        #   selectable.elem_match(field: { name: "value" })
        #
        # @example Add criterion for multiple matches.
        #   selectable.elem_match(
        #     field: { name: "value" },
        #     other: { name: "value"}
        #   )
        #
        # @example Execute an $elemMatch in a where query.
        #   selectable.where(:field.elem_match => { name: "value" })
        #
        # @param [ Hash ] criterion The field/match pairs.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def elem_match(criterion)
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :elem_match
          end

          __override__(criterion, "$elemMatch")
        end
        key :elem_match, :override, "$elemMatch"

        # Add the $exists selection.
        #
        # @example Add a single selection.
        #   selectable.exists(field: true)
        #
        # @example Add multiple selections.
        #   selectable.exists(field: true, other: false)
        #
        # @example Execute an $exists in a where query.
        #   selectable.where(:field.exists => true)
        #
        # @param [ Hash ] criterion The field/boolean existence checks.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def exists(criterion)
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :exists
          end

          typed_override(criterion, "$exists") do |value|
            Mongoid::Boolean.evolve(value)
          end
        end
        key :exists, :override, "$exists" do |value|
          Mongoid::Boolean.evolve(value)
        end

        # Add a $geoIntersects or $geoWithin selection. Symbol operators must
        # be used as shown in the examples to expand the criteria.
        #
        # @note The only valid geometry shapes for a $geoIntersects are:
        #   :intersects_line, :intersects_point, and :intersects_polygon.
        #
        # @note The only valid options for a $geoWithin query are the geometry
        #   shape :within_polygon and the operator :within_box.
        #
        # @note The :within_box operator for the $geoWithin query expects the
        #   lower left (south west) coordinate pair as the first argument and
        #   the upper right (north east) as the second argument.
        #   Important: When latitude and longitude are passed, longitude is
        #   expected as the first element of the coordinate pair.
        #   Source: https://docs.mongodb.com/manual/reference/operator/query/box/
        #
        # @example Add a geo intersect criterion for a line.
        #   query.geo_spatial(:location.intersects_line => [[ 1, 10 ], [ 2, 10 ]])
        #
        # @example Add a geo intersect criterion for a point.
        #   query.geo_spatial(:location.intersects_point => [[ 1, 10 ]])
        #
        # @example Add a geo intersect criterion for a polygon.
        #   query.geo_spatial(:location.intersects_polygon => [[ 1, 10 ], [ 2, 10 ], [ 1, 10 ]])
        #
        # @example Add a geo within criterion for a polygon.
        #   query.geo_spatial(:location.within_polygon => [[ 1, 10 ], [ 2, 10 ], [ 1, 10 ]])
        #
        # @example Add a geo within criterion for a box.
        #   query.geo_spatial(:location.within_box => [[ 1, 10 ], [ 2, 10 ])
        #
        # @param [ Hash ] criterion The criterion.
        #
        # @return [ Selectable ] The cloned selectable.
        def geo_spatial(criterion)
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :geo_spatial
          end

          __merge__(criterion)
        end

        # Alias for +geo_spatial+.
        #
        # @deprecated
        def geo_spacial(criterion)
          # Duplicate method body so that we can raise this exception with
          # geo_spacial as the indicated operator rather than geo_spatial.
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :geo_spacial
          end

          __merge__(criterion)
        end

        key :intersects_line, :override, "$geoIntersects", "$geometry" do |value|
          { "type" => LINE_STRING, "coordinates" => value }
        end
        key :intersects_point, :override, "$geoIntersects", "$geometry" do |value|
          { "type" => POINT, "coordinates" => value }
        end
        key :intersects_polygon, :override, "$geoIntersects", "$geometry" do |value|
          { "type" => POLYGON, "coordinates" => value }
        end
        key :within_polygon, :override, "$geoWithin", "$geometry" do |value|
          { "type" => POLYGON, "coordinates" => value }
        end
        key :within_box, :override, "$geoWithin", "$box"

        # Add the $gt criterion to the selector.
        #
        # @example Add the $gt criterion.
        #   selectable.gt(age: 60)
        #
        # @example Execute an $gt in a where query.
        #   selectable.where(:field.gt => 10)
        #
        # @param [ Hash ] criterion The field/value pairs to check.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def gt(criterion)
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :gt
          end

          __override__(criterion, "$gt")
        end
        key :gt, :override, "$gt"

        # Add the $gte criterion to the selector.
        #
        # @example Add the $gte criterion.
        #   selectable.gte(age: 60)
        #
        # @example Execute an $gte in a where query.
        #   selectable.where(:field.gte => 10)
        #
        # @param [ Hash ] criterion The field/value pairs to check.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def gte(criterion)
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :gte
          end

          __override__(criterion, "$gte")
        end
        key :gte, :override, "$gte"

        # Adds the $in selection to the selectable.
        #
        # @example Add $in selection on an array.
        #   selectable.in(age: [ 1, 2, 3 ])
        #
        # @example Add $in selection on a range.
        #   selectable.in(age: 18..24)
        #
        # @example Execute an $in in a where query.
        #   selectable.where(:field.in => [ 1, 2, 3 ])
        #
        # @param [ Hash ] condition The field/value criterion pairs.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def in(condition)
          if condition.nil?
            raise Errors::CriteriaArgumentRequired, :in
          end

          condition = expand_condition_to_array_values(condition)

          if strategy
            send(strategy, condition, "$in")
          else
            condition.inject(clone) do |query, (field, value)|
              v = {'$in' => value}
              if negating?
                v = {'$not' => v}
              end
              query.add_field_expression(field.to_s, v)
            end.reset_strategies!
          end
        end
        alias :any_in :in
        key :in, :intersect, "$in"

        # Add the $lt criterion to the selector.
        #
        # @example Add the $lt criterion.
        #   selectable.lt(age: 60)
        #
        # @example Execute an $lt in a where query.
        #   selectable.where(:field.lt => 10)
        #
        # @param [ Hash ] criterion The field/value pairs to check.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def lt(criterion)
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :lt
          end

          __override__(criterion, "$lt")
        end
        key :lt, :override, "$lt"

        # Add the $lte criterion to the selector.
        #
        # @example Add the $lte criterion.
        #   selectable.lte(age: 60)
        #
        # @example Execute an $lte in a where query.
        #   selectable.where(:field.lte => 10)
        #
        # @param [ Hash ] criterion The field/value pairs to check.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def lte(criterion)
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :lte
          end

          __override__(criterion, "$lte")
        end
        key :lte, :override, "$lte"

        # Add a $maxDistance selection to the selectable.
        #
        # @example Add the $maxDistance selection.
        #   selectable.max_distance(location: 10)
        #
        # @param [ Hash ] criterion The field/distance pairs.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def max_distance(criterion)
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :max_distance
          end

          # $maxDistance must be given together with $near
          __add__(criterion, "$maxDistance")
        end

        # Adds $mod selection to the selectable.
        #
        # @example Add the $mod selection.
        #   selectable.mod(field: [ 10, 1 ])
        #
        # @example Execute an $mod in a where query.
        #   selectable.where(:field.mod => [ 10, 1 ])
        #
        # @param [ Hash ] criterion The field/mod selections.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def mod(criterion)
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :mod
          end

          __override__(criterion, "$mod")
        end
        key :mod, :override, "$mod"

        # Adds $ne selection to the selectable.
        #
        # @example Query for a value $ne to something.
        #   selectable.ne(field: 10)
        #
        # @example Execute an $ne in a where query.
        #   selectable.where(:field.ne => "value")
        #
        # @param [ Hash ] criterion The field/ne selections.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def ne(criterion)
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :ne
          end

          __override__(criterion, "$ne")
        end
        alias :excludes :ne
        key :ne, :override, "$ne"

        # Adds a $near criterion to a geo selection.
        #
        # @example Add the $near selection.
        #   selectable.near(location: [ 23.1, 12.1 ])
        #
        # @example Execute an $near in a where query.
        #   selectable.where(:field.near => [ 23.2, 12.1 ])
        #
        # @param [ Hash ] criterion The field/location pair.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def near(criterion)
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :near
          end

          __override__(criterion, "$near")
        end
        key :near, :override, "$near"

        # Adds a $nearSphere criterion to a geo selection.
        #
        # @example Add the $nearSphere selection.
        #   selectable.near_sphere(location: [ 23.1, 12.1 ])
        #
        # @example Execute an $nearSphere in a where query.
        #   selectable.where(:field.near_sphere => [ 10.11, 3.22 ])
        #
        # @param [ Hash ] criterion The field/location pair.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def near_sphere(criterion)
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :near_sphere
          end

          __override__(criterion, "$nearSphere")
        end
        key :near_sphere, :override, "$nearSphere"

        # Adds the $nin selection to the selectable.
        #
        # @example Add $nin selection on an array.
        #   selectable.nin(age: [ 1, 2, 3 ])
        #
        # @example Add $nin selection on a range.
        #   selectable.nin(age: 18..24)
        #
        # @example Execute an $nin in a where query.
        #   selectable.where(:field.nin => [ 1, 2, 3 ])
        #
        # @param [ Hash ] condition The field/value criterion pairs.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def nin(condition)
          if condition.nil?
            raise Errors::CriteriaArgumentRequired, :nin
          end

          condition = expand_condition_to_array_values(condition)

          if strategy
            send(strategy, condition, "$nin")
          else
            condition.inject(clone) do |query, (field, value)|
              v = {'$nin' => value}
              if negating?
                v = {'$not' => v}
              end
              query.add_field_expression(field.to_s, v)
            end.reset_strategies!
          end
        end
        alias :not_in :nin
        key :nin, :intersect, "$nin"

        # Adds $nor selection to the selectable.
        #
        # @example Add the $nor selection.
        #   selectable.nor(field: 1, field: 2)
        #
        # @param [ Array<Hash | Criteria> ] criteria Multiple key/value pair
        #   matches or Criteria objects.
        #
        # @return [ Selectable ] The new selectable.
        #
        # @since 1.0.0
        def nor(*criteria)
          _mongoid_add_top_level_operation('$nor', criteria)
        end

        # Is the current selectable negating the next selection?
        #
        # @example Is the selectable negating?
        #   selectable.negating?
        #
        # @return [ true, false ] If the selectable is negating.
        #
        # @since 1.0.0
        def negating?
          !!negating
        end

        # Negate the arguments, or the next selection if no arguments are given.
        #
        # @example Negate the next selection.
        #   selectable.not.in(field: [ 1, 2 ])
        #
        # @example Add the $not criterion.
        #   selectable.not(name: /Bob/)
        #
        # @example Execute a $not in a where query.
        #   selectable.where(:field.not => /Bob/)
        #
        # @param [ Array<Hash | Criteria> ] criteria Multiple key/value pair
        #   matches or Criteria objects to negate.
        #
        # @return [ Selectable ] The new selectable.
        #
        # @since 1.0.0
        def not(*criteria)
          if criteria.empty?
            dup.tap { |query| query.negating = true }
          else
            criteria.compact.inject(self.clone) do |c, new_s|
              if new_s.is_a?(Selectable)
                new_s = new_s.selector
              end
              _mongoid_expand_keys(new_s).each do |k, v|
                k = k.to_s
                if c.selector[k] || k[0] == ?$
                  c = c.send(:__multi__, [{'$nor' => [{k => v}]}], '$and')
                else
                  if v.is_a?(Hash)
                    c = c.send(:__multi__, [{'$nor' => [{k => v}]}], '$and')
                  else
                    if v.is_a?(Regexp)
                      negated_operator = '$not'
                    else
                      negated_operator = '$ne'
                    end
                    c = c.send(:__override__, {k => v}, negated_operator)
                  end
                end
              end
              c
            end
          end
        end
        key :not, :override, "$not"

        # Creates a disjunction using $or from the existing criteria in the
        # receiver and the provided arguments.
        #
        # This behavior (receiver becoming one of the disjunction operands)
        # matches ActiveRecord's +or+ behavior.
        #
        # Use +any_of+ to add a disjunction of the arguments as an additional
        # constraint to the criteria already existing in the receiver.
        #
        # Each argument can be a Hash, a Criteria object, an array of
        # Hash or Criteria objects, or a nested array. Nested arrays will be
        # flattened and can be of any depth. Passing arrays is deprecated.
        #
        # @example Add the $or selection where both fields must have the specified values.
        #   selectable.or(field: 1, field: 2)
        #
        # @example Add the $or selection where either value match is sufficient.
        #   selectable.or({field: 1}, {field: 2})
        #
        # @example Same as previous example but using the deprecated array wrap.
        #   selectable.or([{field: 1}, {field: 2}])
        #
        # @example Same as previous example, also deprecated.
        #   selectable.or([{field: 1}], [{field: 2}])
        #
        # @param [ Hash | Criteria | Array<Hash | Criteria>, ... ] criteria
        #   Multiple key/value pair matches or Criteria objects, or arrays
        #   thereof. Passing arrays is deprecated.
        #
        # @return [ Selectable ] The new selectable.
        #
        # @since 1.0.0
        def or(*criteria)
          _mongoid_add_top_level_operation('$or', criteria)
        end

        # Adds a disjunction of the arguments as an additional constraint
        # to the criteria already existing in the receiver.
        #
        # Use +or+ to make the receiver one of the disjunction operands.
        #
        # Each argument can be a Hash, a Criteria object, an array of
        # Hash or Criteria objects, or a nested array. Nested arrays will be
        # flattened and can be of any depth. Passing arrays is deprecated.
        #
        # @example Add the $or selection where both fields must have the specified values.
        #   selectable.any_of(field: 1, field: 2)
        #
        # @example Add the $or selection where either value match is sufficient.
        #   selectable.any_of({field: 1}, {field: 2})
        #
        # @example Same as previous example but using the deprecated array wrap.
        #   selectable.any_of([{field: 1}, {field: 2}])
        #
        # @example Same as previous example, also deprecated.
        #   selectable.any_of([{field: 1}], [{field: 2}])
        #
        # @param [ Hash | Criteria | Array<Hash | Criteria>, ... ] criteria
        #   Multiple key/value pair matches or Criteria objects, or arrays
        #   thereof. Passing arrays is deprecated.
        #
        # @return [ Selectable ] The new selectable.
        #
        # @since 1.0.0
        def any_of(*criteria)
          criteria = _mongoid_flatten_arrays(criteria)
          case criteria.length
          when 0
            clone
          when 1
            # When we have a single criteria, any_of behaves like and.
            # Note: criteria can be a Query object, which #where method does
            # not support.
            self.and(*criteria)
          else
            # When we have multiple criteria, combine them all with $or
            # and add the result to self.
            exprs = criteria.map do |criterion|
              if criterion.is_a?(Selectable)
                _mongoid_expand_keys(criterion.selector)
              else
                Hash[criterion.map do |k, v|
                  if k.is_a?(Symbol)
                    [k.to_s, v]
                  else
                    [k, v]
                  end
                end]
              end
            end
            # Should be able to do:
            #where('$or' => exprs)
            # But since that is broken do instead:
            clone.tap do |query|
              if query.selector['$or']
                query.selector.store('$or', query.selector['$or'] + exprs)
              else
                query.selector.store('$or', exprs)
              end
            end
          end
        end

        # Add a $size selection for array fields.
        #
        # @example Add the $size selection.
        #   selectable.with_size(field: 5)
        #
        # @note This method is named #with_size not to conflict with any existing
        #   #size method on enumerables or symbols.
        #
        # @example Execute an $size in a where query.
        #   selectable.where(:field.with_size => 10)
        #
        # @param [ Hash ] criterion The field/size pairs criterion.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def with_size(criterion)
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :with_size
          end

          typed_override(criterion, "$size") do |value|
            ::Integer.evolve(value)
          end
        end
        key :with_size, :override, "$size" do |value|
          ::Integer.evolve(value)
        end

        # Adds a $type selection to the selectable.
        #
        # @example Add the $type selection.
        #   selectable.with_type(field: 15)
        #
        # @example Execute an $type in a where query.
        #   selectable.where(:field.with_type => 15)
        #
        # @note http://vurl.me/PGOU contains a list of all types.
        #
        # @param [ Hash ] criterion The field/type pairs.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def with_type(criterion)
          if criterion.nil?
            raise Errors::CriteriaArgumentRequired, :with_type
          end

          typed_override(criterion, "$type") do |value|
            ::Integer.evolve(value)
          end
        end
        key :with_type, :override, "$type" do |value|
          ::Integer.evolve(value)
        end

        # Construct a text search selector.
        #
        # @example Construct a text search selector.
        #   selectable.text_search("testing")
        #
        # @example Construct a text search selector with options.
        #   selectable.text_search("testing", :$language => "fr")
        #
        # @note Per https://docs.mongodb.com/manual/reference/operator/query/text/
        #   it is not currently possible to supply multiple text search
        #   conditions in a query. Mongoid will build such a query but the
        #   server will return an error when trying to execute it.
        #
        # @param [ String, Symbol ] terms A string of terms that MongoDB parses
        #   and uses to query the text index.
        # @param [ Hash ] opts Text search options. See MongoDB documentation
        #   for options.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 2.2.0
        def text_search(terms, opts = nil)
          if terms.nil?
            raise Errors::CriteriaArgumentRequired, :terms
          end

          clone.tap do |query|
            criterion = {'$text' => { '$search' => terms }}
            criterion['$text'].merge!(opts) if opts
            if query.selector['$text']
              # Per https://docs.mongodb.com/manual/reference/operator/query/text/
              # multiple $text expressions are not currently supported by
              # MongoDB server, but build the query correctly instead of
              # overwriting previous text search condition with the currently
              # given one.
              Mongoid.logger.warn('Multiple $text expressions per query are not currently supported by the server')
              query.selector = {'$and' => [query.selector]}.merge(criterion)
            else
              query.selector = query.selector.merge(criterion)
            end
          end
        end

        # This is the general entry point for most MongoDB queries. This either
        # creates a standard field: value selection, and expanded selection with
        # the use of hash methods, or a $where selection if a string is provided.
        #
        # @example Add a standard selection.
        #   selectable.where(name: "syd")
        #
        # @example Add a javascript selection.
        #   selectable.where("this.name == 'syd'")
        #
        # @param [ String, Hash ] criterion The javascript or standard selection.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        def where(*criteria)
          criteria.inject(clone) do |query, criterion|
            if criterion.nil?
              raise Errors::CriteriaArgumentRequired, :where
            end

            # We need to save the criterion in an instance variable so
            # Modifiable methods know how to create a polymorphic object.
            # Note that this method in principle accepts multiple criteria,
            # but only the first one will be stored in @criterion. This
            # works out to be fine because first_or_create etc. methods
            # only ever specify one criterion to #where.
            @criterion = criterion
            if criterion.is_a?(String)
              js_query(criterion)
            else
              expr_query(criterion)
            end
          end.reset_strategies!
        end

        private

        # Adds the specified expression to the query.
        #
        # Criterion must be a hash in one of the following forms:
        # - {field_name: value}
        # - {'field_name' => value}
        # - {key_instance: value}
        # - {'$operator' => operator_value_expression}
        #
        # Field name and operator may be given as either strings or symbols.
        #
        # @example Create the selection.
        #   selectable.expr_query(age: 50)
        #
        # @param [ Hash ] criterion The field/value pairs.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        # @api private
        def expr_query(criterion)
          if criterion.nil?
            raise ArgumentError, 'Criterion cannot be nil here'
          end
          unless Hash === criterion
            raise Errors::InvalidQuery, "Expression must be a Hash: #{Errors::InvalidQuery.truncate_expr(criterion)}"
          end

          normalized = _mongoid_expand_keys(criterion)
          clone.tap do |query|
            normalized.each do |field, value|
              field_s = field.to_s
              if field_s[0] == ?$
                # Query expression-level operator, like $and or $where
                query.add_operator_expression(field_s, value)
              else
                query.add_field_expression(field, value)
              end
            end
            query.reset_strategies!
          end
        end

        # Force the values of the criterion to be evolved.
        #
        # @api private
        #
        # @example Force values to booleans.
        #   selectable.force_typing(criterion) do |val|
        #     Boolean.evolve(val)
        #   end
        #
        # @param [ Hash ] criterion The criterion.
        #
        # @since 1.0.0
        def typed_override(criterion, operator)
          if criterion
            criterion.update_values do |value|
              yield(value)
            end
          end
          __override__(criterion, operator)
        end

        # Create a javascript selection.
        #
        # @api private
        #
        # @example Create the javascript selection.
        #   selectable.js_query("this.age == 50")
        #
        # @param [ String ] criterion The javascript as a string.
        #
        # @return [ Selectable ] The cloned selectable
        #
        # @since 1.0.0
        def js_query(criterion)
          clone.tap do |query|
            if negating?
              query.add_operator_expression('$and',
                [{'$nor' => [{'$where' => criterion}]}])
            else
              query.add_operator_expression('$where', criterion)
            end
            query.reset_strategies!
          end
        end

        # Take the provided criterion and store it as a selection in the query
        # selector.
        #
        # @example Store the selection.
        #   selectable.selection({ field: "value" })
        #
        # @param [ Hash ] criterion The selection to store.
        #
        # @return [ Selectable ] The cloned selectable.
        #
        # @since 1.0.0
        # @api private
        def selection(criterion = nil)
          clone.tap do |query|
            if criterion
              criterion.each_pair do |field, value|
                yield(query.selector, field.is_a?(Key) ? field : field.to_s, value)
              end
            end
            query.reset_strategies!
          end
        end

        class << self

          # Get the methods on the selectable that can be forwarded to from a model.
          #
          # @example Get the forwardable methods.
          #   Selectable.forwardables
          #
          # @return [ Array<Symbol> ] The names of the forwardable methods.
          #
          # @since 1.0.0
          def forwardables
            public_instance_methods(false) -
              [ :negating, :negating=, :negating?, :selector, :selector= ]
          end
        end
      end
    end
  end
end