Setting keys on many-to-many relations now sets the inverse:

- In some cases this may cause up to 2 extra atomic queries to keep both
  sides of the relation in sync.

- Fixes #622

lib/mongoid/contexts/mongo.rb

@@ -6,6 +6,14 @@ class Mongo
 
       delegate :klass, :options, :field_list, :selector, :to => :criteria
 
+      def add_to_set(field, value)
+        klass.collection.update(
+          selector,
+          { "$addToSet" => { field => value } },
+          :multi => true
+        )
+      end
+
       # Aggregate the context. This will take the internally built selector and options
       # and pass them on to the Ruby driver's +group()+ method on the collection. The
       # collection itself will be retrieved from the class provided, and once the
@@ -235,6 +243,14 @@ def min(field)
         grouped(:min, field.to_s, Javascript.min)
       end
 
+      def pull(field, value)
+        klass.collection.update(
+          selector,
+          { "$pull" => { field => value } },
+          :multi => true
+        )
+      end
+
       # Return the first result for the +Context+ and skip it
       # for successive calls.
       #
@@ -267,6 +283,8 @@ def sum(field)
       # attributes provided in the hash. Can be expanded to later for more
       # robust functionality.
       #
+      # @todo Fix safe mode options.
+      #
       # @example Update all matching documents.
       #   context.update_all(:title => "Sir")
       #

lib/mongoid/criteria.rb

@@ -42,6 +42,7 @@ class Criteria
       :field_list
 
     delegate \
+      :add_to_set,
       :aggregate,
       :avg,
       :blank?,
@@ -59,6 +60,7 @@ class Criteria
       :max,
       :min,
       :one,
+      :pull,
       :shift,
       :sum,
       :update,

lib/mongoid/relations.rb

@@ -22,6 +22,7 @@
 require "mongoid/relations/referenced/many_to_many"
 require "mongoid/relations/referenced/one"
 require "mongoid/relations/reflections"
+require "mongoid/relations/synchronization"
 require "mongoid/relations/metadata"
 require "mongoid/relations/macros"
 
@@ -40,6 +41,7 @@ module Relations
     include Macros
     include Polymorphic
     include Reflections
+    include Synchronization
 
     included do
       attr_accessor :metadata

lib/mongoid/relations/bindings/referenced/many_to_many.rb

@@ -21,6 +21,8 @@ def bind_one(doc)
               binding do
                 inverse_keys = doc.do_or_do_not(metadata.inverse_foreign_key)
                 inverse_keys.push(base.id) if inverse_keys
+                base.synced[metadata.foreign_key] = true
+                doc.synced[metadata.inverse_foreign_key] = true
               end
             end
           end
@@ -37,6 +39,8 @@ def unbind_one(doc)
                 base.send(metadata.foreign_key).delete_one(doc.id)
                 inverse_keys = doc.do_or_do_not(metadata.inverse_foreign_key)
                 inverse_keys.delete_one(base.id) if inverse_keys
+                base.synced[metadata.foreign_key] = true
+                doc.synced[metadata.inverse_foreign_key] = true
               end
             end
           end

lib/mongoid/relations/macros.rb

@@ -193,6 +193,7 @@ def references_and_referenced_in_many(name, options = {}, &block)
             reference(meta, Array)
             autosave(meta)
             validates_relation(meta)
+            synced(meta)
           end
         end
         alias :has_and_belongs_to_many :references_and_referenced_in_many

lib/mongoid/relations/metadata.rb

@@ -234,6 +234,10 @@ def foreign_key
         @foreign_key ||= determine_foreign_key
       end
 
+      def foreign_key_check
+        @foreign_key_check ||= "#{foreign_key}_changed?"
+      end
+
       # Returns the name of the method used to set the foreign key on a
       # document.
       #

lib/mongoid/relations/referenced/many_to_many.rb

@@ -37,7 +37,10 @@ def <<(*args)
                   base.send(metadata.foreign_key).push(doc.id)
                 end
               end
-              base.push_all(metadata.foreign_key, ids) if persistable?
+              if persistable?
+                base.push_all(metadata.foreign_key, ids)
+                base.synced[metadata.foreign_key] = false
+              end
             end
           end
         end
@@ -81,6 +84,7 @@ def create(attributes = nil, type = nil, &block)
           super.tap do |doc|
             base.send(metadata.foreign_key).delete_one(doc.id)
             base.push(metadata.foreign_key, doc.id)
+            base.synced[metadata.foreign_key] = false
           end
         end
 
@@ -103,6 +107,7 @@ def create!(attributes = nil, type = nil, &block)
           super.tap do |doc|
             base.send(metadata.foreign_key).delete_one(doc.id)
             base.push(metadata.foreign_key, doc.id)
+            base.synced[metadata.foreign_key] = false
           end
         end
 
@@ -122,6 +127,7 @@ def delete(document)
           super.tap do |doc|
             if doc && persistable?
               base.pull(metadata.foreign_key, doc.id)
+              base.synced[metadata.foreign_key] = false
             end
           end
         end
@@ -146,6 +152,7 @@ def initialize(base, target, metadata)
                 bind_one(doc)
                 if persistable?
                   base.push(metadata.foreign_key, doc.id)
+                  base.synced[metadata.foreign_key] = false
                   doc.save
                 else
                   base.send(metadata.foreign_key).push(doc.id)

lib/mongoid/relations/synchronization.rb

@@ -0,0 +1,113 @@
+# encoding: utf-8
+module Mongoid # :nodoc:
+  module Relations #:nodoc:
+
+    # This module handles the behaviour for synchronizing foreign keys between
+    # both sides of a many to many relations.
+    module Synchronization
+      extend ActiveSupport::Concern
+
+      # Is the document able to be synced on the inverse side? This is only if
+      # the key has changed and the relation bindings have not been run.
+      #
+      # @example Are the foreign keys syncable?
+      #   document.syncable?(metadata)
+      #
+      # @param [ Metadata ] metadata The relation metadata.
+      #
+      # @return [ true, false ] If we can sync.
+      #
+      # @since 2.1.0
+      def syncable?(metadata)
+        !synced?(metadata.foreign_key) && send(metadata.foreign_key_check)
+      end
+
+      # Get the synced foreign keys.
+      #
+      # @example Get the synced foreign keys.
+      #   document.synced
+      #
+      # @return [ Hash ] The synced foreign keys.
+      #
+      # @since 2.1.0
+      def synced
+        @synced ||= {}
+      end
+
+      # Has the document been synced for the foreign key?
+      #
+      # @todo Change the sync to be key based.
+      #
+      # @example Has the document been synced?
+      #   document.synced?
+      #
+      # @param [ String ] foreign_key The foreign key.
+      #
+      # @return [ true, false ] If we can sync.
+      #
+      # @since 2.1.0
+      def synced?(foreign_key)
+        !!synced[foreign_key]
+      end
+
+      # Update the inverse keys for the relation.
+      #
+      # @example Update the inverse keys
+      #   document.update_inverse_keys(metadata)
+      #
+      # @param [ Metadata ] meta The document metadata.
+      #
+      # @return [ Object ] The updated values.
+      #
+      # @since 2.1.0
+      def update_inverse_keys(meta)
+        old, new = changes[meta.foreign_key]
+        meta.criteria(new - old).add_to_set(meta.inverse_foreign_key, id)
+        meta.criteria(old - new).pull(meta.inverse_foreign_key, id)
+      end
+
+      module ClassMethods #:nodoc:
+
+        # Set up the syncing of many to many foreign keys.
+        #
+        # @example Set up the syncing.
+        #   Person.synced(metadata)
+        #
+        # @param [ Metadata ] metadata The relation metadata.
+        #
+        # @since 2.1.0
+        def synced(metadata)
+          synced_save(metadata)
+        end
+
+        private
+
+        # Set up the sync of inverse keys that needs to happen on a save.
+        #
+        # If the foreign key field has changed and the document is not
+        # synced, $addToSet the new ids, $pull the ones no longer in the
+        # array from the inverse side.
+        #
+        # @example Set up the save syncing.
+        #   Person.synced_save(metadata)
+        #
+        # @param [ Metadata ] metadata The relation metadata.
+        #
+        # @return [ Class ] The class getting set up.
+        #
+        # @since 2.1.0
+        def synced_save(metadata)
+          tap do
+            set_callback(
+              :save,
+              :after,
+              :if => lambda { |doc| doc.syncable?(metadata) }
+            ) do |doc|
+              doc.update_inverse_keys(metadata)
+            end
+          end
+        end
+      end
+    end
+  end
+end

spec/functional/mongoid/relations/referenced/many_to_many_spec.rb

@@ -788,12 +788,11 @@
       context "when the documents are part of the relation" do
 
         before do
-          Preference.create(:person_ids => person.id)
+          Preference.create(:person_ids => [ person.id ])
         end
 
-        pending "returns the count from the db" do
-          # @todo: Durran this gets fixed with m-t-m key fix.
-          person.preferences.count.should == 1
+        it "returns the count from the db" do
+          person.reload.preferences.count.should == 1
         end
       end