Restricting the Deletion of Children

[Stevenwulf]

Does anyone know if ActiveRecord has a way for you to specify that a Parent object can not be deleted if it has children?

I know that you can specify that a parent should--by default--delete all its children. But I can't seem to find anything on restricting this behaviour

has_many :dependent => :delete_all

Thanks

[Fenrir2]

Maybe:

class Post < ActiveRecord::Base
  before_destroy :check_comments

  def check_comments
     self.comments.empty?
  end
end

If the before_destroy callback returns false, that is, if the associated-comments list is not empty, the object will not be destroyed (as far as I know ;-)).

[Luke Redpath]

Yes, it will.

If a before_* callback returns false, all the later callbacks and the associated action are cancelled. If an after_* callback returns false, all the later callbacks are cancelled. Callbacks are generally run in the order they are defined, with the exception of callbacks defined as methods on the model, which are called last.

From Rails api docs.

[Sgarissta]

Yes, it will.

If a before_* callback returns false, all the later callbacks and the associated action are cancelled. If an after_* callback returns false, all the later callbacks are cancelled. Callbacks are generally run in the order they are defined, with the exception of callbacks defined as methods on the model, which are called last.

From Rails api docs.

Which still reads to me that the check_comments callback will be checked before destroy is called, it will just be the LAST callback checked.

[Fenrir2]

Yes, but maybe the actual DELETE FROM ... will be cancelled too.

[Stevenwulf]

Thanks guys. This works great. Just to clarify, if the callback returns false, the object will not be destroyed.

   class Vendor < ActiveRecord::Base
   
     # relationships
     has_many :software
   
     before_destroy :dont_destroy_if_children
   
     def dont_destroy_if_children()
   	software.empty? ? true : false
     end
   	
   end

The following test passes

    def setup()
  	@vendor = Vendor.find(1)
    end
  
     def test_delete()
   	# If vendor has childen, do not delete
   	assert_equal(false, @vendor.destroy)
   	assert_equal(1, @vendor.software.size)
   	assert_nothing_raised(ActiveRecord::RecordNotFound) { Vendor.find(@vendor.id) }
   	# You can delete the vendor, after you have deleted its children
   	@vendor.software.clear
   	assert(@vendor.destroy)
   	assert_raise(ActiveRecord::RecordNotFound) { Vendor.find(@vendor.id) }
     end

   rooster > ruby test/unit/vendor_test.rb
   Loaded suite test/unit/vendor_test
   Started
   ....
   Finished in 0.147694 seconds.
   
   4 tests, 17 assertions, 0 failures, 0 errors

[Luke Redpath]

Which still reads to me that the check_comments callback will be checked before destroy is called, it will just be the LAST callback checked.

It says all the later callbacks and the associated action (create/delete whatever).

By the way, this:

software.empty? ? true : false 

Can simply be:

software.empty? 

Which will return a boolean value anyway!