Merge pull request #15701 from zzak/issue_15496

Open extension point for defining options in build_through_record

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -84,12 +84,16 @@ def build_through_record(record)
           @through_records[record.object_id] ||= begin
             ensure_mutable
 
-            through_record = through_association.build through_scope_attributes
+            through_record = through_association.build(*options_for_through_record)
             through_record.send("#{source_reflection.name}=", record)
             through_record
           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)
         end