Rails: Relate two records after creation -

i have model in rails:

class commission < activerecord::base   has_and_belongs_to_many :books end  class book < activerecord::base   has_and_belongs_to_many :commissions end 

this model generates 3 tables: commission, book , books_commissions. in first process of app generate , save commissions , books without relating entries.

question: how create record in books_commissions if know book_id , commission_id?

lets have object book model book , object commission model commission.

then book.commissions << commission should trick.