Association type cast or: Building instances of a given class on polymorphic/STI collections « mediapeers development blog

In quite some cases you want to build items on a rails collection instead of appending them, especially when the parent object is already stored in the database and you do not want the new item being saved automatically. However, this approach does not work on polymorphic collections as you cannot specify the class of the item being built in the build call. You can find some solutions on the net interpreting the type attribute, however I consider this unsafe because attributes tend to be taken directly from the params hash (actually, type attributes are filtered by default on mass assignments which does certainly make sense). The following code snippet adds a build_a method to has_many collections which allows you to say