Given the custom validation in relationship.rb, if I try to update an outage with a child that is already associated to a different outage, it gives me a validation error. It's trying to create a new relationship, not update one. If I pass the on: :create to the custom validation, that won't work since it's trying to create a new relationship on…

child.rb

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

outage.rb

class Outage < ApplicationRecord
    has_many :relationships
    has_many :children, through: :relationships
    
    def self.destroy_current_relationship(outage_id, child_id)
      Relationship.where(outage_id: outage_id).where(child_id: child_id).destroy_all
    end

end

relationship.rb

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

show.html.erb

<%= form_for(@outage) do |f| %>
  <p>    
    <div class="form-group">
      <%= collection_check_boxes(:outage, :child_ids, Child.all, :id, :omnia_trouble_ticket) do |b| %>
        <div class="col-xs-12">
          <div class="row">
            <%= b.label(class: "check_box") do %>
              <%= b.check_box(class: "check_box") %><%= b.object.omnia_trouble_ticket %>
            <% end %>
          </div>
        </div>
      <% end %>        
    </div>
  </p>

  <%= f.submit("Update Outage", class: "btn btn-default") %>
<% end %>