Prevent duplicate HABTM associations

Rails
1

Hi,

I am trying to restrict duplicate has_and_belongs_to_many associations.

I tried to find support for a association validation for duplicates but
couldn’t find anything.

I understand I could use :uniq=>true on the model association but this
would only prevent it from displaying duplicates, I want to stop
duplicate associations being inserted in the first place.

I have a vague idea of something using a callback to check before
adding.

has_and_belongs_to_many :catalogue_categories, :before_add =>
:evaluate_duplicate_category

def evaluate_duplicate_category(catalogue_category)
#check duplicate
#if so raise error and abort add
end

However I am unsure how to abort the add at this stage and return an
error. Could anyone help point me in the right direction?

Thanks,

Andrew.

2

Is this what you’re looking for?

validates_uniqueness_of

http://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#M000816

3

lisa-u wrote:

Is this what you’re looking for?

validates_uniqueness_of

ActiveRecord::Validations::ClassMethods

Thanks.

I had looked at this but thought it would only be useful where the
association is represented by a physical model.

In my system products and categories are linked, but just by a plain
habtm association with a join table that has no corresponding model. If
I gave the association a physical model I probably could use the
validates_uniqueness_of within that model but at this stage such a model
is otherwise not neccesary.

If I have misunderstood its capability let me know.

4

Andrew E. wrote:

I had looked at this but thought it would only be useful where the
association is represented by a physical model.

In my system products and categories are linked, but just by a plain
habtm association with a join table that has no corresponding model.

You do need a table categories_products that contains category_id and
product_id to store the link in the case of HABTM. (Btw, tablea_tableb
should be in alphabetical order.) Although HABTM by itself does not
require you to generate a model, to validates_uniqueness_of, you need
the Category_Product model (I’m not 100% sure about the naming
convention in this case)

If you really want to avoid making this table, use has_many/belongs_to
relationships. And validates_uniqueness_of category with scope
product_id