How to query document using key

[meetjaydeep]

In below code I want to fetch only the values of User1 in descending order of Item values.
but I'm unable to fetch it using below query.

op = Output()
op.data =  {"User1": {"Item2": 2, "Item3": -1, "Item4": 3}, "User2": {"Item4": 2, "Item5": 1, "Item6": 2}}
r = op.query.filter({Output.data:{"User1":{'$exists':True}}})
result = r.all()

Expected result:
["Item4","Item2","Item3"]