Counting occurrence of elements in an array in document

[Steven_Meyer1 (Steven Meyer)]

Is there a way of counting the occurrence of elements in an array within a document without using $unwind?

To put it another way, is there an equivalent of Python list count?

[steevej (Steeve Juneau)]

You will need $reduce and potentially $addToSet, $filter or some other operators.

With sample documents and expected result the recommendation can be more specific.

[Steven_Meyer1 (Steven Meyer)]

Thanks for your response.

I’ll be more specific:

Within a document we have an array field that looks like this:

“grades”:[“A”, “E”, “D”, “A”, “E”, 'C", “A”, “D”, “C”, “B”]

I need to extract documents that have two or more “A” grades.

[Steven_Meyer1 (Steven Meyer)]

You pointed me in the right direction. Thanks. This query did it.

q1 = [{
    "$project": {
      "countA": {
        "$reduce": {
          "input": "$grade_list",
          "initialValue": 0,
          "in": {
            "$cond": [
              { "$eq": ["$$this", "A"] },
              { "$add": ["$$value", 1] },
              "$$value"
            ]
          }
        }
      }
    }
}]