The following bulk operation results in a NoMethodError:

> bulk.find({:_id => 3}).upsert.update({"$set" => {:b => 3}}) 
> result = bulk.execute
NoMethodError: undefined method `collect' for 3:Fixnum

The very likely culprit is this line of code , which assumes that if the "upserted" field in the result from a bulk operation is not an ObjectId, then it must be an array.