Preventing destory when grandchildren records exist?

OK, I’m hoping there’s a better way to do this, but searching returns
many more old hits than “current” ones.

I am working on an application that has Products belong to a Category
and categories belong to a Group. I want to restrict the removal of
a Category when it still has Products. I also want to restrict the
removal of a Group if any of it’s categories can’t be removed.

class Group < ActiveRecord::Base
before_destroy :ensure_no_categories_with_products
has_many :categories, :dependent => :destroy

def ensure_no_categories_with_products
unless self.categories.all? {|c| c.products.empty?}
self.errors.add_to_base “Can’t destroy a group referenced by a
category having products”
return false
end
end
end

class Category < ActiveRecord::Base
belongs_to :group
before_destroy :ensure_no_products
has_many :products

def ensure_no_products
unless self.products.empty?
self.errors.add_to_base “Can’t destroy a category having
products”
return false
end
# raise “Can’t destroy a category having products” unless
self.products.empty?
end
end

class Product < ActiveRecord::Base
belongs_to :category
end

As a side note, I couldn’t find definitive guidance on whether
returning false or raising an exception was “better”, but returning
false ended up working out better for my controllers.

So here’s the big question: It feels wrong to duplicate the
condition “unless self.categories.all? {|c| c.products.empty?}” in
the Group model, but I’m not sure changing it to “unless
self.categories.all? {|c| c.ensure_no_products }” feels any better.

I realize it’s a bit asymmetric letting a Group be removed if there
are existing Categories that happen to be Productless, but I’m OK
with the :dependent => :destroy taking care of that.

What I’d love to know is if there’s a better way. I can see this
same kind of relationship other places:
Group=>Category=>Product
Author=>Post=>Comment
House=>Room=>Furniture

I found a thread on the Rails-core list (http://wrath.rubyonrails.org/
pipermail/rails-core/2006-July/001986.html) that offered a similar
solution via a new :dependent=>:restrict option on has_many (to mimic
the RESTRICT option in some SQL dialects such as MySql http://
dev.mysql.com/doc/refman/5.1/en/innodb-foreign-key-constraints.html),
but I don’t know if that led to this ticket (http://
dev.rubyonrails.org/ticket/3837) which has a patch which is currently
available as a plugin. (These are concerned with just the one-level
dependency however.)

-Rob

Rob B. http://agileconsultingllc.com
[email protected]

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