I wrote a helper method to get attributes hash/hashes out of some known data types like this

case obj
when Hash then obj
when Mongoid::Relations::Targets::Enumerable, 
     Array,
     Mongoid::Criteria then obj.map(&:attrs)
when Error, Mongoid::Document then obj.attrs
end

while obj.class == Mongoid::Relations::Targets::Enumerable the second branch get skipped. I dug into the code, found === method had been overridden:

def ===(other)
  other.class == Class ? Array == other : self == other
end

such that its instances cannot match it in a case statement.