On Oct 29, 2011, at 5:40 AM, Michael Q wrote:
> What is the most DRY way to prevent deletion of a parent if children exist?
>
> I assume it would be a
> before_destroy :check_that_no_child_exists
>
> But an object might have several different types of children and we would
> ideally want to iterate through all the has_many's with just one command.
>
> Is there an elegant way of doing this, and returning a helpful error message
> indicating which type of child exists?
>
> I'm amazed that Rails still doesn't seem to have a default facility for this
> as its a very basic requirement.
Rails 3 added the :restrict option for dependent associations - to use it:
has_many :foos, :dependent => :restrict
Attempting to delete a model that still has associated foos will raise an
ActiveRecord::DeleteRestrictionError with the message:
"Cannot delete record because of dependent #{reflection.name}"
You'll need to catch this exception (probably in your controller) and figure
out how to present it to the user.
DB-level constraints are certainly another option - note that you may need to
change the default schema format to :sql to be able to test them. You'll still
have to catch the exception, but it will be a database-specific one.
Finally, you can prevent users *seeing* the delete option by adding code like
this to the model:
def destroy_permitted?
foos.empty?
end
This will remove Rapid-generated delete buttons from your interface, and
attempting to delete a record will raise a PermissionDeniedError.
--Matt Jones
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