Project nested array field values by their indexes in aggregation

Question

I'm doing an aggregation in MongoDB which should have an array fields projection in its $project stage . But I can't access array fields by their indexes :

{//projection stage
  $project: {
    'foo' : { '$ifNull' : ['$bar.0.baz.0.qux', '*'] }
  }
}

This sets foo to an empty array . bar is a multidimensional array field. my MongoDB version is 3.2 . What can I do for this without the hassle of $unwind/$group old heavy solution ?

Thanks for your help .

Answer 1

Use $slice, $map and $arrayElemAt:

{ "$project": {
    "foo": {
        "$ifNull": [
            { "$arrayElemAt": [
                { "$map": {
                    "input": { "$slice": [
                        { "$map": {
                            "input": { "$slice": [ "$bar", 0, 1 ] },
                            "as": "el",
                            "in": "$$el.baz"
                        }},
                        0, 1
                    ]},
                    "as": "el",
                    "in": { "$arrayElemAt": [ "$$el.qux", 0 ] }
                 }},
                 0
             ]},
            "*"
        ]
    }
}}

So the inner $map operators allow you to just select the particular fields out of each array, which you can $slice at the desired position to just return that element. i.e 0,1 is zero index and just one element.

The for the final resulting array you just use $arrayElemAt and retrieve the indexed element turning it into a single value.

Of course the $ifNull tests might need to be more involved depending on your structure, as if it's not consistent then you probably need to check each $map input and swap with a result accordingly.

But the general process is :

• $map to get fields
• $slice to reduce arrays from $map
• $arrayElemAt on the final array result.

---

On something like this:

  db.foo.insert({
    "bar": [
      { 
        "baz": [
          { "qux": 2 },
          { "qux": 5 }
        ]
      },
      {
        "baz": [
          { "qux": 3 },
          { "qux": 4 }
        ]
      }
    ]
  })

Produces:

{ "_id" : ObjectId("56e8c6b8ff2a05c0da90b31e"), "foo" : 2 }

Answer 2

Use multiple $project stage with $arrayElemAt operator with temporary fields.

{$project:{"tmp": {$arrayElemAt:["$bar",0]}}}, {$project:{"tmp2": {$arrayElemAt:["$tmp.baz",0]}}}

than your value is at $tmp2.qux.