1 files changed, 43 insertions, 26 deletions
diff --git a/activerecord/lib/active_record/associations/has_many_through_association.rb b/activerecord/lib/active_record/associations/has_many_through_association.rb
index 59929b8c4e..0d384950fe 100644
--- a/activerecord/lib/active_record/associations/has_many_through_association.rb
+++ b/activerecord/lib/active_record/associations/has_many_through_association.rb
@@ -21,20 +21,6 @@ module ActiveRecord
         super
       end
 
-      def concat_records(records)
-        ensure_not_nested
-
-        records = super(records, true)
-
-        if owner.new_record? && records
-          records.flatten.each do |record|
-            build_through_record(record)
-          end
-        end
-
-        records
-      end
-
       def insert_record(record, validate = true, raise = false)
         ensure_not_nested
 
@@ -48,6 +34,20 @@ module ActiveRecord
       end
 
       private
+        def concat_records(records)
+          ensure_not_nested
+
+          records = super(records, true)
+
+          if owner.new_record? && records
+            records.flatten.each do |record|
+              build_through_record(record)
+            end
+          end
+
+          records
+        end
+
         # The through record (built with build_record) is temporarily cached
         # so that it may be reused if insert_record is subsequently called.
         #
@@ -57,21 +57,14 @@ module ActiveRecord
           @through_records[record.object_id] ||= begin
             ensure_mutable
 
-            through_record = through_association.build(*options_for_through_record)
-            through_record.send("#{source_reflection.name}=", record)
+            attributes = through_scope_attributes
+            attributes[source_reflection.name] = record
+            attributes[source_reflection.foreign_type] = options[:source_type] if options[:source_type]
 
-            if options[:source_type]
-              through_record.send("#{source_reflection.foreign_type}=", options[:source_type])
-            end
-
-            through_record
+            through_association.build(attributes)
           end
         end
 
-        def options_for_through_record
-          [through_scope_attributes]
-        end
-
         def through_scope_attributes
           scope.where_values_hash(through_association.reflection.name.to_s).
             except!(through_association.reflection.foreign_key,
@@ -90,7 +83,7 @@ module ActiveRecord
         def build_record(attributes)
           ensure_not_nested
 
-          record = super(attributes)
+          record = super
 
           inverse = source_reflection.inverse_of
           if inverse
@@ -161,6 +154,30 @@ module ActiveRecord
           else
             update_counter(-count)
           end
+
+          count
+        end
+
+        def difference(a, b)
+          distribution = distribution(b)
+
+          a.reject { |record| mark_occurrence(distribution, record) }
+        end
+
+        def intersection(a, b)
+          distribution = distribution(b)
+
+          a.select { |record| mark_occurrence(distribution, record) }
+        end
+
+        def mark_occurrence(distribution, record)
+          distribution[record] > 0 && distribution[record] -= 1
+        end
+
+        def distribution(array)
+          array.each_with_object(Hash.new(0)) do |record, distribution|
+            distribution[record] += 1
+          end
         end
 
         def through_records_for(record)