How to check if sub-field exists and is not null in mongodb aggregation?

Question

I got quite an annoying problem, and I can't seem to find any solution for my specific case. In my mongo agggregation pipeline, I'd like to add a field, and depending on the existence of another field's subfield, I'd like to assign the value of that other field's subfield, or 1 if that other field's subfield does not exist.

Here is what I tried:

pipelineStages.push(
{$addFields: 
    {field: 
        {$cond: [
             {$ne: ["$otherField.subField", null]}, 
                    "$otherField.subField", 1]
             }
        }
     }
 );

However, it only works fine for the case that the field exists. In the other, that new field is not added to the pipeline at all. Any ideas what I'm doing wrong?

Something like this `{$addFields: {field: { $ifNull: [ "$otherField.subField", 1 ] } } }` — s7vr, Mar 23 '17 at 15:14
Possible duplicate of [MongoDB aggregate by field exists](http://stackoverflow.com/questions/25497150/mongodb-aggregate-by-field-exists) — s7vr, Mar 23 '17 at 15:18
No. Like I said, I need the value of a subfield, not a field. The answer below doesn't cover that. — Michał Szydłowski, Mar 23 '17 at 15:20
Imagine you have two docs with `{"otherField":{"subField":2, "extraField":3}}, {"otherField":{"oneField":3}}`. If you run the above query you should get output as `{"otherField" : { "subField" : 2, "extraField" : 3}, "field" : 2} {"otherField" : { "oneField" : 3 }, "field" : 1 } }`. Isn't that what you need ? If not please consider adding some sample document to the post. — s7vr, Mar 23 '17 at 15:56

score 12 · Accepted Answer · answered Nov 06 '18 at 17:46

Wrapping $ifNull in $cond does the trick, even with the subfield problem.

pipelineStages.push({
    $addFields: {
        newField: {
            $cond: [
                { 
                    "$ifNull": [ 
                        "$otherField.subField", 
                        false 
                    ] 
                },
                "$otherField.subField",
                1
            ]
        }
     }
 );

score 4 · Answer 2 · answered Oct 21 '19 at 22:20

To check if the field exists and is not null, use:

pipelineStages.push({ 
  $addFields: {
    field: {
      $cond: [
        {"$gt": [$otherField, null]}, "$otherField.subField", 1
      ]
    }
  }
})

On the contrary, to check if the field doesn't exist or is null, use:

{"$lte": [$otherField, null]}

Vitalii Andrusishyn · Answer 3 · 2017-03-23T15:21:34.497

2

Did you try this version of condition:

pipelineStages.push({ $addFields: 
    {field: 
        {$cond: [{$and: [{"$otherField": {"$exists": true}}, {"$otherField.subField": {"$exists": true}}]}, "$otherField.subField", 1]
        }
    }
 });

edited Mar 23 '17 at 15:21

answered Mar 23 '17 at 15:15

Vitalii Andrusishyn

3,984
1
25
31

Yes, and it wouldn't let me specify a field with a '.' sign. – Michał Szydłowski Mar 23 '17 at 15:21
And if try like i edit code at this time? Also non effect? – Vitalii Andrusishyn Mar 23 '17 at 15:25

How to check if sub-field exists and is not null in mongodb aggregation?

3 Answers3