如何使用 MongoDB/pyMongo 使字段路径在同一文档中包含一个值？

Question

The documents in the MongoDB I am querying can be stripped down to this as an example:

{
    "date":"2019-08-15",
    "status":"5345",
    "foo":
    {
        "bar":
        {
            "years":
            {
                "2018":
                {
                    "const":1234
                },
                "2019":
                {
                    "const":4321
                }
            }
        }
    }
}

I am trying to get the "const" values from this document using pyMongo.

The keys in "years" is varying with the "date" of the document.

I have attempted to use this pipeline where I try to use the year of "date" to get the "const" from this year:

pipeline=[
    {'$match':{'status':{'$exists': True}}},
    {'$project':
        'const_thisYear':{
            '$let':{
                'vars':{
                    'yr':{ '$year': {'$convert':{'input': '$date','to': 'date'}}},
                    'res': '$foo.bar.years'
                },
                'in': '$$res.$$yr.const'
            }
        }
    }
]

When aggregating I get the following python exception:

OperationFailure: FieldPath field names may not start with '$'.

How do I do this correctly?

Python 3.7.7

Answer 1

You should revise your collection structure to not store data as keys; but irrespective, just using regular python dict manipulation can get you out of the hole:

for doc in db.mycollection.find({'status': {'$exists': True}}, {'foo.bar.years': 1}):
    for year, year_value in doc['foo']['bar']['years'].items():
        print(year, year_value.get('const'))