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

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

{
  {
    item: "item_1"
    rating: "neutral"
    comment: "some comment"
  },
  {
    item: "item_2"
    rating: "good"
    comment: "some comment"
  },
  {
    item: "item_1"
    rating: "good"
    comment: "some comment"
  },
  {
    item: "item_1"
    rating: "bad"
    comment: "some comment"
  },
  {
    item: "item_3"
    rating: "good"
    comment: "some comment"
  },
}

I want a way to find out how many different ratings each item got.

so the output should look something like this:

{
  {
    item: "item_1"
    good: 12
    neutral: 10
    bad: 67
  },
  {
    item: "item_2"
    good: 2
    neutral: 45
    bad: 8
  },
  {
    item: "item_3"
    good: 1
    neutral: 31
    bad: 10
  }

}

This is what I've done

db.collection(collectionName).aggregate(
          [
             {
               $group:
                 {
                   _id: "$item",
                   good_count: {$sum: {$eq: ["$rating",  "Good"]}},
                   neutral_count:{$sum: {$eq: ["$rating",  "Neutral"]}},
                   bad_count:{$sum: {$eq: ["$rating",  "Bad"]}},
                 }
             }
           ]
)

The format of the output looks right, but the counts are always 0.

I'm wondering what's the properway of summing things up by looking at the distinct values of the same field?

Thanks!