Mongo find documents where array contains x values of given array

Question

I have a collection which has documents like that one. The entity field is not set in each document and has different values:

{
    "_id" : ObjectId("5388cfbdec82ba7cd5438635"),
    "name" : "Name1",
    "entity" : [ 
        "Entity1", 
        "Entity2", 
        "Entity4",
        "Entity5"
    ]
}

Now I want to find all documents which contains exactly x values of the given array : ["Entity1","Entity2","Entity3","Entity4"]

Expected result

For the document posted above the values Entity1, Entity2, Entity4 are matching:

x = 4 the document should not be found (3 matching values but x is 4)
x = 3 document should be found (3 matching -> size = x)
x = 2 the document should not be found (3 matching values but x is 2)
x = 1 the document should not be found (3 matching values but x is 1)

Not really that clear. Are you looking for all documents that contain a sub-set of your required array or those that match all of it? Otherwise what exactly are you asking? — Neil Lunn
– Neil Lunn, Commented Jun 4, 2014 at 7:25
Excuse me if its not clear. I was asking for a sub-set of a fixed length. See the excepted answer. — SeeM
– SeeM, Commented Jun 4, 2014 at 15:50

Nicholas Smith · Accepted Answer · 2020-10-31 15:29:09Z

1

You can use .aggregate for this. This is probably what you're looking for:

var y = ["Entity1", "Entity2", "Entity3", "Entity4"];
db.col.aggregate([
    {
        $project :
        {
            _id : 1,
            name : 1,
            entity : 1,
            x : {
                $size : {
                    $ifNull: [{$setIntersection : ["$entity", y]}, []]
                }
            }
        } 
    },
    { $match : { x : 3 } }
]);

edited Oct 31, 2020 at 15:29

Nicholas Smith

7202 gold badges13 silver badges27 bronze badges

answered Jun 4, 2014 at 8:42

Christian P

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