Sometimes you might want to limit the number of associated records in a has_many association, but cannot add a foreign key to the other model (using belongs_to).

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There are many Show archive.org snapshot takes Show archive.org snapshot on limiting the number of associated records in has_many associations, but none feels smooth.

However, when your limit is 1, there is an elegant solution: Simply add validates_uniqueness_of :association_id to your join model. In the following example, a project may have only one feedback contact.

class FeedbackContact < ActiveRecord::Base
  has_many :projects
  has_many :feedback_contacts, :class_name => 'User'
  validates_uniqueness_of :project_id
end