MongoDB: Counting how many of each distinct values there are?

Question

I have a collection of documents holding a list of feedbacks for different items. It looks something like this:

{
           


        

Answer 1

You were very close, but of course $eq just returns a true/false value, so to make that numeric you need $cond:

db.collection(collectionName).aggregate([
  { "$group" : {
       "_id": "$item",
       "good_count": { 
           "$sum": { 
               "$cond": [ { "$eq": [ "$rating",  "good" ] }, 1, 0] 
           }
       },
       "neutral_count":{
           "$sum": { 
               "$cond": [ { "$eq": [ "$rating", "neutral" ] }, 1, 0 ]
            }
       },
       "bad_count": { 
           "$sum": { 
               "$cond": [ { "$eq": [ "$rating",  "bad" ] }, 1, 0 ]
           }
       }
  }}
])

As a "ternary" operator $cond takes a logical condition as it's first argument (if) and then returns the second argument where the evaluation is true (then) or the third argument where false (else). This makes true/false returns into 1 and 0 to feed to $sum respectively.

Also note that "case" is sensitive for $eq. If you have varing case then you likely want $toLower in the expressions:

               "$cond": [ { "$eq": [ { "$toLower": "$rating" },  "bad" ] }, 1, 0 ]

---

On a slightly different note, the following aggregation is usually more flexible to different possible values and runs rings around the conditional sums in terms of performance:

db.collection(collectionName).aggregate([
    { "$group": {
        "_id": { 
            "item": "$item",
            "rating": { "$toLower": "$rating" }
        },
        "count": { "$sum": 1 }
    }},
    { "$group": {
        "_id": "$_id.item",
        "results": {
            "$push": {
                "rating": "$_id.rating",
                "count": "$count"
            }
        }
    }}
])

That would instead give output like this:

{
    "_id": "item_1"
    "results":[
        { "rating": "good", "count": 12 },
        { "rating": "neutral", "count": 10 }
        { "rating": "bad", "count": 67 }
    ]
}

It's all the same information, but you did not have to explicitly match the values and it does execute much faster this way.