You can do the same with strong by
user.strong_associated_products << product
You may need to set to your relations this way:
class User < ActiveRecord::Base
has_many :medium_associations, class_name: 'Association', condition: ["associations.strength = ?", "medium"]
has_many :strong_associations, class_name: 'Association', condition: ["associations.strength = ?", "strong"]
has_many :medium_associated_products, class_name: 'Product', through: :medium_associations, source: :product
has_many :strong_associated_products, class_name: 'Product', through: :strong_associations, source: :product
end