This fixes CVE-2011-0448

ActiveRecord::Relation#primary_key should return a string, just like ActiveRecord::Base.primary_key does.

Make Relation#create_with always merge rather than overwrite, not just when merging two relations. If you wish to overwrite, you can do relation.create_with(nil), or for a specific attribute, relation.create_with(:attr => nil).

Refactor JoinDependency and friends so that a JoinAssociation can produce an arbitrary number of joins, which will be needed in order to support nested through associations.