pymongo - Invalid document cannot encode object

Question

I am trying to find an image with the closest colour in the database given an image using the euclidean algorithm (since I do not really care about the distance, I omit the sqrt). Each item in the database contains the fields r, g, b.

Heres my code to retrieve the result:

r, g, b = utilities.get_avg_rgb(match_image.convert('RGB'))

res = self.database_collection.aggregate([
    {'$set': {
        'diff': {
            '$sum': [{'$pow': [{'$sub': ['$r', r]}, 2]},
                     {'$pow': [{'$sub': ['$g', g]}, 2]},
                     {'$pow': [{'$sub': ['$b', b]}, 2]}]
        }
    }},
    {'$sort': {'diff', 1}},
    {'$limit': 1}
])

However, I get the following error:

<class 'tuple'>: (<class 'bson.errors.InvalidDocument'>, InvalidDocument("cannot encode object: {1, 'diff'}, of type: <class 'set'>"), <traceback object at 0x0F5483C8>)

I think it says I am trying to encode an object with class set which it cannot do so (?) but I could not find the reason.

May I know why this happens and how to fix it? Thanks

I believe the $sort operator takes <field1>: <sort order>, where you have <field1>, <sort order>. Try {'$sort': {'diff': 1}} — It'sNotMe
– It'sNotMe, Commented Jul 30, 2019 at 15:44
@It'sNotMe Yup that solved the problem, but now I am getting a new exception <class 'tuple'>: (<class 'pymongo.errors.OperationFailure'>, OperationFailure("Unrecognized pipeline stage name: '$set'"), <traceback object at 0x0EBFFC60>) — weilueluo
– weilueluo, Commented Jul 30, 2019 at 15:47
@It'sNotMe Thanks alot for seeing that for me! I will try to work on from here, do you mind making it as an answer so I can accept it? — weilueluo
– weilueluo, Commented Jul 30, 2019 at 15:53

It'sNotMe · Accepted Answer · 2019-07-30 15:57:52Z

1

I believe the $sort operator takes <field1>: <sort order>, where you have <field1>, <sort order>. Try {'$sort': {'diff': 1}}.

Also, $set is an update operator and does not appear to be supported in the aggregation pipeline.

answered Jul 30, 2019 at 15:57

It'sNotMe

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