Check If field exists in an sub-document of an Array

Question

I have a schema that is similar to this.

{id: Number,
  line_items: [{ 
    id: String,
    quantity: Number,
    revi         


        

Answer 1

You can find this another way without $map and $unwind used $redact like this :

db.collectionName.aggregate({
  "$match": {
    "line_items.review_request_sent": {
      "$exists": true
    }
  }
}, {
  "$redact": {
    "$cond": {
      "if": {
        "$eq": [{
          "$ifNull": ["$review_request_sent", "null"]
        }, "null"]
      },
      "then": "$$DESCEND",
      "else": "$$PRUNE"
    }
  }
}, {
  "$match": {
    "line_items": {
      "$size": 1
    }
  }
}).pretty()

In above query first match check whether review_request_sent is presents or not in redact $ifNull use to check whether review_request_sent presents or not if not then it replaced to "null" and $eq to check "null" this will return documents like

{ "_id" : ObjectId("55e6ca322c33bb07ff2c163e"), "id" : 1, "line_items" : [ ] }
{ "_id" : ObjectId("55e6ca322c33bb07ff2c163f"), "id" : 2, "line_items" : [ { "id" : 39 } ] }
{ "_id" : ObjectId("55e6ca322c33bb07ff2c1640"), "id" : 3, "line_items" : [ ] }

and after that last match check only those documents whose line_items array contains value i.e $size to 1