added example when removed from habtm relation

[exocode]

I've added rspec example for a habtm relation.

• Given a product.create("A product", categories: [old_category])
• When product.update(categories: [new_category1, new_category2]) is updating,
• The old_category relation is deleted, but the counter for old_category stays untouched.

(The new_categories are counted properly!)

The other spec tests are mostly sanity checks

[exocode]

Regarding to this issue #358

[magnusvk]

Ok, so I think the issue here must be that the callbacks on the has-and-belongs-to-many model aren't firing when you're updating the relation directly. Doing a little bit of googling on this, it seems like the way to do this would be to add after_remove and after_add callbacks on the relation. But then I'd imagine it getting tricky in that we'd have to make sure that only one or the other callback fires, not both, to prevent double-decrementing or -incrementing.

This unfortunately looks like it might end up being a pretty big change, so I won't have time to work on that. If you're willing to invest the time, I'd be happy to look at a PR that adds this. Alternatively, I think if you stopped using update and manipulated the has-and-belongs-to-many model directly instead, that it would end up counting correctly for you.

Thanks for opening this, it does make the problem easy to understand.