Allow reordering of many to many foreign keys. Fixes #1705.

CHANGELOG.md

@@ -245,6 +245,8 @@ For instructions on upgrading to newer versions, visit
 
 ### Resolved Issues
 
+* \#1705 Allow changing the order of many to many foreign keys.
+
 * \#1703 Updated at is now versioned again. (Lucas Souza)
 
 * \#1686 Set the base metadata on unbind as well as bind for belongs to

lib/mongoid/fields/internal/foreign_keys/array.rb

@@ -13,6 +13,8 @@ class Array
           # @example Add the atomic changes.
           #   field.add_atomic_changes(doc, "key", {}, [], [])
           #
+          # @todo: Durran: Refactor, big time.
+          #
           # @param [ Document ] document The document to add to.
           # @param [ String ] name The name of the field.
           # @param [ String ] key The atomic location of the field.
@@ -32,6 +34,8 @@ def add_atomic_changes(document, name, key, mods, new, old)
               document.atomic_array_add_to_sets[key] = pushes
             elsif !pulls.empty?
               document.atomic_array_pulls[key] = pulls
+            elsif new != old
+              mods[key] = document.attributes[name]
             end
           end
 

spec/mongoid/relations/referenced/many_to_many_spec.rb

@@ -2562,4 +2562,40 @@
       end
     end
   end
+
+  context "when changing the order of existing ids" do
+
+    let(:person) do
+      Person.new
+    end
+
+    let(:preference_one) do
+      Preference.create(:name => "one")
+    end
+
+    let(:preference_two) do
+      Preference.create(:name => "two")
+    end
+
+    before do
+      person.preference_ids = [ preference_one.id, preference_two.id ]
+      person.save
+    end
+
+    context "and the order is changed" do
+
+      before do
+        person.preference_ids = [ preference_two.id, preference_one.id ]
+        person.save
+      end
+
+      let(:reloaded) do
+        Person.find(person.id)
+      end
+
+      it "persists the change in id order" do
+        reloaded.preference_ids.should eq([ preference_two.id, preference_one.id ])
+      end
+    end
+  end
 end