In a Rails app I'm building, I need what I can best describe as a contextual auto-incrementing column. Essentially, the Bar model belongs to Foo via a foo_id column. I need to increment contextual_id on Bar rows in the context of its Foo, meaning that for Foo foo_1, its first Bar has a contextual_id of 1, the second has a contextual_id of 2, and for Foo foo_2, its first Bar also has a contextual_id of 1. Hope this makes sense.
Obviously, this should be handled at the database level, but I'm not sure how to do that in Postgres. Any help would be greatly appreciated.