Aggregate two arrays where both elements not null in MongoDB

Question

Problem

I'd like to get two equally long arrays where both don't contain null elements.

Approach

I can successfully return two arrays, but they contain null values. When I exclude null values they're of course not equally long.

aggregate([
  {
    $unwind: '$row'
  }, {
    $match: {
      $or: [{
        'row.identifier': 'fah'
      }, {
        'row.identifier': 'agr'
      }]
    }
  }, {
    $group: {
      _id: '$row.identifier',
      rows: {
        $push: '$row.value'
      }
    }
  }
]

Result

[[5, null, 64, 34, 1], [53, 31, null, null, 7]]

null values still present.

Wanted result:

[[5, 1], [53, 7]]

null values and values at the same index are removed.

---

1. Update

Here are two example documents as requested:

[{ // 1st
  row: [{
    value: 53,
    identifier: 'agj'
  }, {
    value: 51,
    identifier: 'hrw'
  }, {
    value: null,
    identifier: 'rgs'
  }]
}, { // 2nd
  row: [{
    value: null,
    identifier: 'agj'
  }, {
    value: 72,
    identifier: 'hrw'
  }, {
    value: 11,
    identifier: 'rgs'
  }]
}]

Answer 1

There may be a way of doing this filtering in mongodb, I'm not that familiar with it. But in javascript you could write a filter function like the following:

var notNullFilter = function(inputs){
    var outputs = [];
    for (var i=0; i<inputs.length; i++){
        outputs.push([]);
    }
    for (var k=0; k<inputs[0].length; k++){
        var isNull = false;
        for (var j=0; j<inputs.length; j++){
            if (inputs[k][j] == null){
                isNull = true;
                break;
            }
        }
        if (!isNull){
            for (var l=0; l<inputs.length; l++){
                outputs[l].push(inputs[l][k]);
            }
        }
    }
};

If you were using a library like lodash this would be much easier but this function should work.