Merge pull request #5453 from JonRowe/patch_uniq_has_and_belongs_to_many_when_already_loaded

When Active Record has already loaded a unique association `.size` returns the wrong number.

activerecord/lib/active_record/associations/collection_association.rb

@@ -242,8 +242,12 @@ module ActiveRecord
       # This method is abstract in the sense that it relies on
       # +count_records+, which is a method descendants have to provide.
       def size
-        if !find_target? || (loaded? && !options[:uniq])
-          target.size
+        if !find_target? || loaded?
+          if options[:uniq]
+            target.uniq.size
+          else
+            target.size
+          end
         elsif !loaded? && options[:group]
           load_target.size
         elsif !loaded? && !options[:uniq] && target.is_a?(Array)

activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb

@@ -333,6 +333,12 @@ class HasAndBelongsToManyAssociationsTest < ActiveRecord::TestCase
     assert_equal 3, project.developers.size
   end
 
+  def test_uniq_when_association_already_loaded
+    project = projects(:active_record)
+    project.developers << [ developers(:jamis), developers(:david), developers(:jamis), developers(:david) ]
+    assert_equal 3, Project.includes(:developers).find(project.id).developers.size
+  end
+
   def test_deleting
     david = Developer.find(1)
     active_record = Project.find(1)