Didn't get much love on the google group about this, so thought I'd go straight for it and open an issue (I do think it's something that should be supported - stuff like this is one of the useful features of mongodb).

Mongodb allows primary keys (_id field) of any type, including Hash and Array. The problem:

class Foo
  include Mongoid::Document
  field :_id, :type => Hash
end

f = Foo.create! {|f| f.id = {a: 1, b: 2} } # don't forget _id attr_protected-ness!
Foo.first.id
 => {"a"=>1, "b"=>2} # all good so far, but...
Foo.find({"a"=>1, "b"=>2})
 => [#<Foo _id: {"a"=>1, "b"=>2}, _type: nil>] # an array!

This is because Mongoid is seeing the _id hash as "resizable", and inferring that you're looking for multiple documents, as you would be in the more common case:

Bar.find([1, 2, 3])
=> [#<Bar _id: 1>, #<Bar _id: 2>, #<Bar _id: 3>]

This is a change of behaviour between v2 and v3 that isn't mentioned in the release notes, so may be accidental..?

But I think this is not too hard to fix, because Mongoid also knows the type of the id field, so it can apply logic like "if my _id field is a hash, and find has received a hash, then I'm looking for one document. If find receives an _array of hashes, only then is it multiple documents" etc. But I suppose this is an api change, so would have to wait for 4.0... even though it was unintentional (maybe)?