app/models/user_show_association.rb
class UserShowAssociation < ActiveRecord::Base
belongs_to :user, inverse_of: :user_show_associations
belongs_to :show, inverse_of: :user_show_associations
validates :user, presence: true, uniqueness: { scope: :show }
validates :show, presence: true
validates :is_owner, uniqueness: { scope: :show, if: :is_owner? }
end