Add migration.

db/migrate/20150324155957_set_incorrect_assignee_id_to_null.rb

+class SetIncorrectAssigneeIdToNull < ActiveRecord::Migration
+  def up
+    execute "UPDATE issues SET assignee_id = NULL WHERE assignee_id = -1"
+    execute "UPDATE merge_requests SET assignee_id = NULL WHERE assignee_id = -1"
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
-ActiveRecord::Schema.define(version: 20150320234437) do
+ActiveRecord::Schema.define(version: 20150324155957) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"