MongoDB: top-level array conditions

Question

I have an array of arrays in MongoDB 4.2.2:

db.foo.insertOne({'foo': [[1, 3], [2], [3]]})

I'd like to remove elements of foo, first elements of which are greater than 1. But I cannot figure out how.

I tried something like this (and many more) but it does not pull anything:

db.foo.update({}, {$pull: {foo: {'0': {$gt: 1}}}})

Is it possible?

EDIT: Expected result:

db.foo.find({})
{ "_id": ObjectId("..."), "foo": [ [1, 3] ] }

Can you show what you expect the document to look like after this operation? I think it will help make clear your intent. — James Wahlin
– James Wahlin, Commented Feb 13, 2020 at 16:28
How do you apply "which are greater than 1" on an array? Do you consider the Array [1,3] greater than 1? If yes, what are the exact conditions for that? — Wernfried Domscheit
– Wernfried Domscheit, Commented Feb 13, 2020 at 16:45
@JamesWahlin sorry, I've edited my answer. @WernfriedDomscheit "first elements of which are greater than 1" i.e. I compare the first elements, so [1, 3] does not hold the condition. — pallly
– pallly, Commented Feb 14, 2020 at 9:11

pallly · Accepted Answer · 2020-03-10 20:16:39Z

1

If you are using MongoDB 4.2, you can make use of a pipeline in the new update. This allows you to pass an aggregation pipeline as the update argument:

db.foo.update({},[
    {$addFields:{
        foo:{
            $filter:{
                input:"$foo",
                cond:{
                    $lte: [{$arrayElemAt: ["$$this", 0]}, 1]
                }
            }
        }
    }}
  ])

edited Mar 10, 2020 at 20:16

pallly

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answered Feb 13, 2020 at 17:00

Joe

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4 Comments

pallly Over a year ago

Great! So the "$$this" keyword can address the top level. Btw, after "$$this" there should be comma instead of period (I cannot edit it because it's only 1 character and I need 6). @Joe, maybe you can?

pallly Over a year ago

wait... it behaves weirdly on some inputs, I need to test it more... Ok, got it, editing your answer.

Joe Over a year ago

and if you want to use a name other than this, you can specify an as field to the filter with the name you want.

pallly Over a year ago

There are few more incompatibilities with my question, the line should be $lte: [{$arrayElemAt: ["$$this", 0]}, 1]. My edit was not accepted for some reason, @Joe, could you please fix it so that I could mark your answer again as accepted?

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