Support resources as aggregate targets in expressions

[vonagam]

Right now aggregates (exists, max, count and so on) in expressions (expr) work only with relationships expr(exists(friends, name == "John")). It would be handy in some cases to have ability to specify aggregates using resources as targets - expr(exists(User, name == "John")).

Recent case where I've encountered it is about create policies as they do not have relationships setup yet. I can use simple check but expression one would have been shorter and cleaner.

I've seen hacks with declaring a has_many relationship with no_attributes?: true and then using that relationship as target for aggregates but would prefer for it to be supported officially so to speak.

[zachdaniel]

I'm a fan of this proposal, but it will be a pretty significant change, as all aggregates are currently expecting to be at some relationship path to the resource. Its definitely doable, and should be doable without being a breaking change for users though.