MongoDB aggregate by field exists

Question

I have a hard time believing this question hasn\'t been asked and answered somewhere already, but I can\'t find any trace of it.

I have a MongoDB aggregation query t

Answer 1

My answer is:

{$match:{
    $and:[{
        name:{
            $exists:true
        }
    }, {
        $expr:{
            $eq:["$$id", "$_id"]
        }
    }]
}}

I use this in lookup, on my pipeline stage. This post 2 rules the first one, name must exist. And the second thing is the relation between these 2 collection. I Am sure u can modify this for your question.