I'm trying to perform a tricky aggregation to return the size of a nested array within a document in the collection.

Here is how to re-create my sample data:

```
db.test.insert({
    projects: [
        {
            _id: 1,
            comments: [
                'a',
                'b',
                'c'
            ]
        },
        {
            _id: 2,
            comments: [
                'a',
                'b'
            ]
        },
        {
            _id: 3,
            comments: []
        }
    ]
})

```

The aggregation I would perform goes here:

```
db.test.aggregate([
    // enter aggregation here
])

```

Here is the desired output:

```
[{
    projects: [
        {
            _id: 1,
            comment_count: 3
        },
        {
            _id: 2,
            comment_count: 2
        },
        {
            _id: 3,
            comment_count: 0
        }
    ]
}]

```

I'm struggling with how to write this. If I try the following:

`"projects.comment_count": {"$size": }`

The result returns the size of the resulting array:

```
[{
    projects: [
        {
            _id: 1,
            comment_count: 3
        },
        {
            _id: 2,
            comment_count: 3
        },
        {
            _id: 3,
            comment_count: 3
        }
    ]
}]

```

If I try to use the $map method like this:

```
"projects.comment_count": { 
    "$map": { 
        "input": "$projects", 
        "as": "project", 
        "in": {
            "$size": "$$project.comments"
        } 
    } 
}

```

It will return an array that looks like this for each object in the array:

```
[{
    projects: [
        {
            _id: 1,
            comment_count: [3, 2, 0]
        },
        {
            _id: 2,
            comment_count: [3, 2, 0]
        },
        {
            _id: 3,
            comment_count: [3, 2, 0]
        }
    ]
}]

```

Thanks in advance!