has_many :through checkboxes + select

Hello all.

I have 3 models:

Profile.rb

has_many :knowledges
has_many :skills, :through => :knowledges

table (id,name)

Knowledge.rb

belongs_to :profile
belongs_to :skill

table (profile_id,skill_id,level)

Skill.rb

has_many :knowledges
has_many :profiles, :through => :knowledges

table(id,name)

Thats all ok if I want to populate, in the profile "_form", the
checkboxes using "profile[skill_ids][]".

So, what about the LEVEL field? How can I fill that? Anyone can help
with this??