[Rails] please explain me why nil?

I have 3 models
Skill, User, UserSkill

User has_many skills :through => :user_skills
User has_many :user_skills

UserSkill.new # => { :id => nil, :user_id => nil, :skill_id => nil }
us = UserSkill.create(:user_id => 1, :skill_id => 1) # => { :id => nil,
:user_id => 1, :skill_id => 1 }

Why id is nil?
In the database record was created and id is not null...

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