Mongoose MongoDB find the one with smallest array-length of an array-field

Question

I need the document in a collection that has the least array size of a field among all. Document looks as follows:

{
    "_id": ObjectId("jsd273rsdjkvn847ckjnas74"),
    "associatedUsers": [
        ObjectId("jsd273rsdjkvn847ckjnas75"),
        ObjectId("jsd273rsdjkvn847ckjnas76")
    ]
}

Out of all the documents in MongoDB, I need to find the one with least number of associatedUsers in its array. Basically the one with shortest array length.

mickl · Accepted Answer · 2021-07-20 11:19:42Z

2

You can use $addFields to calculate new field based on array $size and then use $sort along with $limit to get the document with smallest array:

db.collection.aggregate([
    {
        $addFields: {
            size: { $size: "$associatedUsers" }
        }
    },
    {
        $sort: { size: 1 }
    },
    {
        $limit: 1
    }
])

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answered Jul 20, 2021 at 11:19

mickl

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