$addFields when no $match found

Question

Iam new Mongodb developer i wrote mongodb aggregation.one of my field lampStatus : \"OFF\" is 30 recors are there iam using \"{ $match: {lampStatus : \"OFF\"}}\"i got 30 rec

Answer 1

You can somewhat ridiculously do this with $facet and $ifNull aggregation

db.collection.aggregate([
  { "$facet": {
    "array": [
      { "$match": { "type": "L", "lampStatus": "ON" }},
      { "$group": {
        "_id": null,
        "TotalLights": { "$sum": 1 }
      }},
      { "$project": { "_id": 0, "TotalLights": 1 }}
    ]
  }},
  { "$project": {
    "TotalLights": {
      "$ifNull": [{ "$arrayElemAt": ["$array.TotalLights", 0] }, 0 ]
    }
  }}
])