models.rb

class Outage < ApplicationRecord
  has_many :relationships
  has_many :children, through: :relationships
end


class Child < ApplicationRecord
  has_many :relationships
  has_many :outages, through: :relationships
end

class Relationship < ApplicationRecord
  belongs_to :child
  belongs_to :outage
  validate :ensure_one_relationship

  def ensure_one_relationship
    if Relationship.where(child_id: self.child_id).present?
        errors.add(:base, "A trouble ticket can only be associated with one outage ticket.")
    end
  end
end