Giving same association in :include and :join can result in table being joined twice

This is a bit of a corner case. Try and wrap your head around it.

If you give the same association in both :include and :join for a find then, as per ticket #528, this forces preloading where eager loading might have otherwise been used... only this is not entirely true! One of AR's limitations is that for any given bunch of associations, it'll try to preload them all but if it can't, it'll eager load them all instead. It does not consider each association for preloading independently.

Consequently, if you give an association in both :include and :join but some other association requires eager loading then both of the associations will be eager loaded. The problem with this is that eager loading doesn't check for any overlap between the given includes and joins and thus the table gets joined twice. ERROR!

This begs the question, why would you specify an association in both when you know it's just going to get eager loaded anyway? In my case, I have a scope where I always do a comparison on some association (join) but only sometimes I want to also include that association (include). Strictly speaking, that scope should not have to determine whether to use :join or :include by itself.

I do like to do the right thing where possible but it's easy to work around in my application and the proper fix could be a little messy. add_joins! needs to filter the given joins based on which associations are being eagerly loaded. But if :join or :include is a hash... hmm. Feel free to tackle this one if you're bored!