db.installations.aggregate([
{ 
  $skip:50
},
{
  $limit:1
},
{
  $addFields:{
    tempField : {
      $cond : {
        if : {$ifNull : ["$openID", false]},
        then : "$openID",
        else : "invalidNumber"
      }
    }
  }
},
{
  $lookup:
  {
    from: 'users',
    localField:'tempField',
    foreignField: 'openid',
    as: 'worker'
  }
},
{
  $project:
  {
    tempField:0,
    openID:0,
    "worker._id":0,
    "worker.openid":0,
    "worker.group":0
  }
} 
]).pretty()