How to use NOT IN array condition inside mongodb $lookup aggregate

Question

I have two collections:

Users:
{
    _id: ObjectId(\'5e11d2d8ad9c4b6e05e55b82\'),
    name:"vijay"
}

Followers :
{
    _id:ObjectId(\'5ed0c8faac47af698         


        

Answer 1

You should use $not $in with $expr expression, Because $nin is a query operator not for aggregation expression,

• one more fix you need to create variable using let: { following: "$following"} and use inside pipeline $$following, because lookup pipeline will not allow to access fields without reference,

  {
    $lookup: {
      from: "Users",
      let: {
        following: "$following"
      },
      pipeline: [
        {
          $match: {
            $expr: {
              $not: {
                $in: [
                  "$_id",
                  "$$following"
                ]
              }
            }
          }
        }
      ],
      as: "result"
    }
  }

Working Playground: https://mongoplayground.net/p/08OT6NnuYHx