Query

[
  {
    "$match": {
       "programCode": { "$in": ["", null] } 
    }
  },
  {
    $lookup: {
      from: "subscription",
      localField: "participantId",
      foreignField: "userId",
      as: "result"
    }
  },
 {
    $match: {
      $expr: { $eq: [ { $size: "$result" }, 0 ] }
    }
  },
  {
      $lookup:{
          from:"participant",
          localField:"participantId",
          foreignField:"_id",
          as:"participantData"
      }
  },

  {
  "$project": {
    _id:0,
    "size": { "$size": "$result" },
    "participantId":1,
    email:"$participantData.email",
    planName:"$result.subscriptionPlan.name"
  }
}
  
]