Seems to be a bug. I can't dig into the core of it (since too many models), but here is what I know.
I have a model user, which has primary key :id. I also have a model of action. Two users interact with each other through the model of action. Say, one user sends a comment to the other. So, action model has primary key :id, and also :sender_id and :receiver_id, which are ids of sender and receiver, correspondingly. There is also a concept of rating. Receiver can rate the sender for his comment. That is, rating doesn't have a primary key, yet it has :user_id (which is the id of receiver) and :action_id.
The model for ACTION has: belongs_to sender, :class_name => 'User', :foreign_key => :sender_id belongs_to receiver, :class_name => 'User', :foreign_key => :receiver_id
The model for USER has: has_many :sent_actions, :class_name => 'Action', :foreign_key => :sender_id has_many :received_actions, :class_name => 'Action', :foreign_key => :receiver_id
The model for RATING has: belongs_to :user belongs_to :action
Now the funniest part. When I try to save new rating, it's not saved, while rating has belongs_to :action. If this line is commented, then everything is ok - rating is saved and rating.action_id is in place in the database.
I dug into into save! of the ActiveRecord class and put a "raise 'testing...'" there instead of its usual code. Now, if belongs_to :action is inside the model, then save! doesn't fire off with an error, which means that rails doesn't even go inside save!. If belongs_to :action is commented, then 'testing...' appear on the screen, while trying to save a rating.
Did anyone bump into anything similar? How to override? Or it's a bug
and I need to forget about it and find working workaround? 
Thanks.
