has_and_belongs_to_many or has_many :through?

As you may (or may not) know, there are two ways to build a many-to-many (or N-N) relationship with Rails.

1. The first way uses has_and_belongs_to_many in both models. You’ll have to create a join table that has no corresponding model or primary key. ( ActiveRecord will look by default for a join table called groups_users )

class Group < ActiveRecord::Base
has_and_belongs_to_many :users # foreign keys in the join table
end
class User < ActiveRecord::Base
has_and_belongs_to_many :groups # foreign keys in the join table
end


The join table can have more attributes which can be filled with the push_with_attributes method:

class User < ActiveRecord::Base
has_and_belongs_to_many :groups # foreign keys in the join table

def join_group(group)
groups.push_with_attributes(group, :joined_at => Time.now)
end
end

2. The second way uses a has_many association with the :through option and a join model:
   class Subscriptions < ActiveRecord::Base
belongs_to :group # foreign key - programmer_id
belongs_to :user # foreign key - project_id
end
class Group :subscriptions
end
class User :subscriptions
end


If you have to work with the relationship model as its own entity, thent you'll need to use has_many :through. Otherwise, you can just stick to has_and_belongs_to_many.