has_many and has_one associations

Question

There are two models:

1. T
2. TGroup

TGroup object may have many T objects. TGroup object has one favorite T object.

I want to define associations that will provide me the following functionality:

1. TGroup.first.ts # will return list of T objects belong to TGroup.first object
2. TGroup.first.favorite # will return the favorite T object of the TGroup.first object
3. TGroup.first.ts << T.first # will attach T.first object to the TGroup.first object
4. TGroup.first.favorite = T.first # will set the T.first object as the favorite of TGroup.first object

The solution I tried to implement was:

1. Define t_group_id field (integer) on T
2. Define favorite_id field (integer) on TGroup
3. Define the following associations on TGroup model

has_many :ts, :class_name => T.to_s

has_one :t, :class_name => T.to_s, :foreign_key => :id, :primary_key => :favorite_id

I didn't succeed to define the "favorite" (should be something like ":as => :favorite"?) alias so I started with "t" instead. Unfortunately, doing that I can't set TGroup.first.t = T.first . Instead of setting the TGroup.first.t.id to T.first.id it makes changes on T model.

Answer 1

I think that what you need is a belongs_to association rather than a has_one one.

According to the docs the foreign key goes on the table for the class declaring the belongs_to association, so something like this should do the job:

belongs_to :favorite, :class_name => T.to_s

The has_one/belongs_to associations can be sometimes a bit confusing depending on each particular context.

Hope this helps.