Querying mongo array of embedded documents in aggregation pipeline

Question

I was looking into the different ways of querying on array of embedded documents in aggregation pipeline MongoDB. Looks like MongoDB has less support for this.

Let's say we have following documents in test collection:

/* 1 */
   {
    "_id" : ObjectId("59df2c39fbd406137d4290b3"),
    "a" : 1.0,
    "arr" : [ 
        {
           "key": 1,
           "sn" : "a",
           "org": "A"
        }
    ]
   }

/* 2 */
{
    "_id" : ObjectId("59df2c47fbd406137d4290b4"),
    "a" : 2.0,
    "arr" : [ 
        {
            "sn" : "b",
            "key": 2,
            "org": "B"
        }
    ]
}

/* 3 */
{
    "_id" : ObjectId("59df2c50fbd406137d4290b5"),
    "a" : 3.0,
    "arr" : [ 
        {

           "key": 3,
           "sn" : "c", 
           "org": "C"
        }
    ]
}

/* 4 */
{
    "_id" : ObjectId("59df2c85fbd406137d4290b6"),
    "a" : 1.0,
    "arr" : [ 
        {
           "key": 1,
           "sn" : "a",
           "org": " A"
        }
    ]
}

/* 5 */
{
    "_id" : ObjectId("59df2c9bfbd406137d4290b7"),
    "a" : 3.0,
    "arr" : [ 
        {
            "sn" : "b",
            "key": 2,
        }
    ]
}

/* 6 */
{
    "_id" : ObjectId("59df2e41fbd406137d4290b8"),
    "a" : 4.0,
    "arr" : [ 
        {
            "sn" : "b",
            "key" : 2
        }
    ]
}

/* 7 */
{
    "_id" : ObjectId("59df2e5ffbd406137d4290b9"),
    "a" : 5.0,
    "arr" : [ 
        {
           "key" : 2,
            "sn" : "b"
        }, 
        {
            "sn" : "a",
            "key" : 1
        }
    ]
}

And I wanted to categorize the above documents based on "arr.sn" field value using below query:

db.test.aggregate([{"$addFields": {"Category" : { $switch: {
     branches : [
       { case : { $eq : [ "$arr.nm", "a" ] }, then : "Category 1"}
    ],
    default : "No Category"
    }}}}])

but $eq operator is not giving correct result, if I use the same $eq in find method, it works:

db.test.find({"arr.sn" : "a"})

I am looking at the way to do it with only single field, here in case "arr.sn" field. Is there any way to project the field from embedded documents from the array?

Any help would be appreciated.

Answer 1

$eq(aggregation) compares both value and type different from query eq opeator which can compare values for any type.

You need $in(aggregation) to verify value in a array.

Something like

[
  {
    "$addFields": {
      "Category": {
        "$switch": {
          "branches": [
            {
              "case": {
                "$in": [
                  "a",
                  "$arr.sn"
                ]
              },
              "then": "Category 1"
            }
          ],
          "default": "No Category"
        }
      }
    }
  }
]