MongoDB: update dictionary in document

I wrote a function to convert a dictionary to a new dictionary which only updates fields in that dictionary:

def convert_2_dict_mongodb(obj):
    result = {}
    for key, val in obj.items():
        if not isinstance(val, dict):
            result[key] = val
            continue

        for sub_key, sub_val in val.items():
            new_key = '{}.{}'.format(key, sub_key)
            result[new_key] = sub_val
            if not isinstance(sub_val, dict):
                continue

            result.update(convert_2_dict_mongodb(result))
            if new_key in result:
                del result[new_key]

    return result

Example:

abc = {
    'a': 1,
    'b': {
        'x': 1,
        'y': 2,
        'z': {
            'g': 1,
            'h': {
                'e': [1, 2, 3]
            }
        }
    },
    'h': {
        'a': [1, 2, 3]
    }
}

To:

{'h.a': [1, 2, 3], 'b.x': 1, 'b.z.h.e': [1, 2, 3], 'b.z.g': 1, 'a': 1, 'b.y': 2}

from mongoDb website look at "Set Fields in Embedded Documents"

To specify a <field> in an embedded document or in an array, use dot notation.
For the document matching the criteria _id equal to 100, the following
operation updates the make field in the details document:


db.products.update(
{ _id: 100 },
{ $set: { "details.make": "zzz" } })

in your case

db.collection.update(
     {_id:ObjectId("1")},
     { $set: { "occurrences.12": "3", "occurrences.17": "2" }})