Check if all element in an array have the value for a certain field in mongodb

Question

I need to verify that for each document it has the same value for the baz field.

The structure looks like this.

{
    _id : 111111,
    foo : 123123,
    bar : [
        {
            subId : 121212,
            baz : abc123
        },
        {
            subId : 131313,
            baz : abc123
        },
    ]
},
{
    _id : 222222,
    foo : 234234,
    bar : [
        {
            subId : 212121,
            baz : def456
        },
        {
            subId : 313131,
            baz : def456
        },
    ]
},

Basically I just need to make sure that for each document, baz should be always the same. How can I query it?

Can you elaborate by explaining the actual business need? if it's to validate input coming from a single source then it should be done on insert/update else can you just save the baz field as a root level field? — Tom Slabbaert
– Tom Slabbaert, Commented Feb 28, 2021 at 16:14
I need to check all the documents in the collection to have same baz for each document. Maybe I can get all the _id for the documents with different baz. — aedan
– aedan, Commented Mar 1, 2021 at 0:38

Dheemanth Bhat · Accepted Answer · 2021-02-28 16:27:45Z

1

Try This:

db.allSame.aggregate([
    {
        $addFields: {
            "uniqueBazCount": {
                $sum: {
                    $map: {
                        input: "$bar",
                        as: "item",
                        in: {
                            $cond: [{ $eq: ["$$item.baz", { $arrayElemAt: ["$bar.baz", 0] }] }, 0, 1]
                        }
                    }
                }
            }
        }
    },
    {
        $match: { uniqueBazCount: 0 }
    }
]);

To get the docs with different values change $match as below:

{
    $match: {
        uniqueBazCount: { $gt: 0 }
    }
}

edited Feb 28, 2021 at 16:27

answered Feb 28, 2021 at 16:16

Dheemanth Bhat

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