> use test
switched to db test
> db.main.find()
{"_id" : "b45c046c-c502-11dd-b0b6-001d7dc91168" , "_ref" : {"$ref" : "test" , "$id" : "b44ab2b6-c502-11dd-b0b6-001d7dc91168"}}
> db.main.find({"_ref":

{'$id' : "b44ab2b6-c502-11dd-b0b6-001d7dc91168", '$ref': 'test'}

})
error:

{"$err" : "invalid $operator: $id"}

> db.main.find({"_ref":

{"$ref" : "test" , "$id" : "b44ab2b6-c502-11dd-b0b6-001d7dc91168"}

})
{"_id" : "b45c046c-c502-11dd-b0b6-001d7dc91168" , "_ref" : {"$ref" : "test" , "$id" : "b44ab2b6-c502-11dd-b0b6-001d7dc91168"}}

Since objects/dictionaries/associative arrays are unordered, one would expect the first example to work.