How to prevent ActiveRecord instance destruction

November 21st, 2005

In one of my projects, I wanted to prevent instances of particular classes to be deleted if they were in any way associated to another object. In database terms, I wanted an ON DELETE RESTRICT constraint.

Since I cannot rely on the database to enforce it for me (MySQL 4, MyISAM engine), I coded the following: 
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require File.dirname(__FILE__) + '/../test_helper'

class CityTest < Test::Unit::TestCase
  fixtures :cities, :contacts

  def setup
    @city = City.find(:first)
  end

  def test_prevent_destruction_if_associated_to_any_contact
    @city.contacts << contacts(:jill)
    @city.destroy
    assert_not_nil City.find(@city.id), 'should not have been destroyed'
    assert_match /cannot destroy.*contacts/i, @city.errors.on_base,
        'reports error condition to user'
  end

  def test_allow_destruction_if_not_associated_to_any_contact
    @city.destroy
    assert_raises ActiveRecord::RecordNotFound do
      City.find(@city.id)
    end
  end
end
 
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class City < ActiveRecord::Base
  has_and_belongs_to_many :contacts, :join_table => 'contacts_cities'

  def destroy
    unless self.contacts.empty?
      self.errors.add_to_base \
          'We cannot destroy this instance since one or more contacts refer to it')
      return
    end

    super
  end
end

I had to override destroy because in has_and_belongs_to_many relationships, Rails deletes join table records before deleting the record. This means that in before_destroy filters, self.contacts.empty? will always report true. Ticket #1183: dependents are destroyed before client before_destroy hooks are called is already opened on this issue.

UPDATE (2006-03-08) Fixed since 2006-02-13

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I am François Beausoleil, a Ruby on Rails coder. During the day, I work on XLsuite. At night, I am interested many things. Read my biography

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