Many to Many rails form

I would like to know how to do a form many to many in rails.

Here are my models:

class Course < ActiveRecord::Base
  has_and_belongs_to_many :notices, dependent: :destroy
  has_and_belongs_to_many :internships, dependent: :destroy
end

class Internship < ActiveRecord::Base
  belongs_to :enterprise
  has_and_belongs_to_many :courses
end

I also have another question about the same subject. Assuming I have 3 internship-related courses.

When I use action update and update related courses to 2, will rails automatically delete that relationship that is no longer present in the update?