Improve views support with Relationship handler

I'm not sure you need a custom handler here.

Defining a relationship from {commerce_product}.product_id to {recommender}.source_id with the basic relationship handler should do it.

Now, you *could* add a custom handler in order to limit the relationship join on {recommender}.app_id. That would require a custom class.

The best place to look for a model of a relationship handler that limits on the join would be the user->profile2 relationship handler which limits on profile type. That's handled by entity_views_handler_relationship_by_bundle in entity module.

That looks very nifty; more complex than what I had in mind (which was a hack job :D), so much much more reusable! I really like the way this boils down to just being a relationship from one entity to another via the similarity.

A few points:

> 'source eid' => 'product_id',

I think it would be better to use the 'relationship field' definition item that the Views basic relationship handler provides, since that does the same thing. It means you have that waiting for you in $this->real_field, and it's clearer DX for people who implement this.

I'm completely baffled why you're joining to {recommender_app}:

    $join->definition = array(
      'table' => 'recommender_app',
      'field' => 'id',
      'left_table' => $this->first_join,
      'left_field' => 'app_id',
      'type' => !empty($this->options['required']) ? 'INNER' : 'LEFT',
    );

From what I can gather (and I may be wrong!) the {recommender_app} just stores a list of the different things that make use of the Recommender API. So it could well have only one row in it, ever, if we're the only module using the API.

Here's an updated patch that requires commerce_rec_views_data_alter() to switch to using 'relationship field' too. I've taken the liberty of adding some class docs, though I've not moved them to their own .inc files which will need doing too.

> $this->query->add_where($this->relationship, $this->second_join . '.name', $def['recommender app']);

I'm having trouble decoding that, but wouldn't it be simpler to just do a standalone db_query() on the {recommender app} table and pull out the data you need for the join?

Or if the app_id is fixed once you've installed the module, sniff it when saving the handler options and save it in $handler->options, essentially as a hidden setting.