mongodb $cond with $arrayElemAt as false condition inside aggregation

July 3, 2023

I’m inside the $project step.

{
  'res' : {$cond: [
    {$or: [{'$lte' :['$list', null]},
    {$eq: ['$list', []]}]
    }, 
    '$root_field', 
    {'$arrayElemAt': 
    ['$list.obj.field', 
    0]]}}
}

I want to check if a root list is null or empty. If it is, I want to take another root field, otherwise I need to take the first element of the list’s object’s value. But this is not written well, as compass tells me:

I cannot find anything online relative to my case. How to manage the $arrayElemAt inside the $cond? I suppose that’s the problem here

>Solution :

You misplaced on closing brackte:

db.collection.aggregate([
  {
    $set: {
      res: {
        $cond: [
          {
            $or: [
              { $lte: [ "$list", null ] },
              { $eq: [ "$list", [] ] }
            ]
          },
          "$root_field",
          { $arrayElemAt: [ "$list.obj.field", 0 ] }
        ]
      }
    }
  }
])