Two content types: subscriptions and sites. Each site belongs to a subscription. I use the UUID module, since subscriptions and sites are synced from websites elsewhere. Each site has a field_subscription_uuid field to identify the subscription it belongs to. There can be multiple sites on each subscription.
I want to make a view that lists sites alongside some of the data from their subscription. I thought I would use the Equi Join module to relate a site's field_subscription_uuid to a subscription's uuid. However, Equi Join only lists custom fields (and body), not UUID.
Although I could create a new custom field for subscriptions and sync its own UUID to that field to make it visible to Equi Join, I would rather not because it would be redundant (since the UUID is already a column in the node table).
So I think I have to write a views relationship handler in order to bring a node's uuid into the scope of Views relationships, but I don't know where to start and the API documentation has only confused me more.
How can use a specific node type's UUID in a views relationship without storing the UUID in a custom field?