User exists? instead of find_first

[Dijkztra]

Issue: #5168

Benchmark in progress

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[gabrielbaldao]

In the another change you was using { key: value } and now { key => value }, what do you think about use one as default, I suggest { key: value }

[carlosantoniodasilva]

@Dijkztra thanks for your contribution.

Devise uses https://github.com/ianwhite/orm_adapter in order to work with Active Record and Mongo without having to worry about specifics of each, which is why it uses to_adapter.find_first. I'm sure there's a slight gain here, as I understand exists? is meant to be faster, so if ORM Adapter provided us with a way to run it under the hood, we'd be all in in those spots. For now, however, I think we'll stick with find_first.

I am also unsure if mongo is working 100% in master right now, but we'll take a look at that eventually.

[carlosantoniodasilva]

I want to remove orm_adapter: #5823, so we might be able to implement something like this on our own side now.