Not so far ago I have found a weird bug in the Open Source Ruby gem called composite_primary_keys, occurred when you specify :primary_key option for has_one or has_many association. There are two ways to get it fixed: submit an issue and wait till someone will work out this problem or fix it by yourself and then pull request to get the patch merged into the core. This is a great library and I use it in almost all my project, so I decided to help the author and fix this bug by myself. Here I will show you how to do that.