Mongoid unique children
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I'm creating a multi-tenant application and am trying to figure out how to model this relationship with Mongoid. I have a `Site` model which `has_many :domains`, or `embeds_many`, or even uses an array of strings if that will work. I want to validate that only unique domains can be associated (so two `Sites` can't have the same domain). I also want to be able to find a site by a given domain.

How would I represent this using Mongoid? Also, how would I query for the `Site` by a given domain?