Extract child objects into array inside MongoDB aggregation

Question

In my collection, all of my documents have an array called tags:

"tags" : [
        {
            "name" : "a", 
            "weight" : NumberInt(32)
        }, 
        {
            "name" : "b", 
            "weight" : NumberInt(26)
        }, 
        {
            "name" : "c", 
            "weight" : NumberInt(25)
        }, 
        ...
]

Each object in the array has keys name and weight. I need to remove the weight key, so that I'm just left with the names in an single-level array like ["a", "b", "c"...]

An explanation of why I need to do this:

Inside my aggregation, I want to calculate the number of matches between an array x = ["a", "b", "d"] and whatever tags each document has.

This is as far as I've gotten- inside a $project I create a new variable like so:

"$project": {
    ...
    "diff": {
       "$size": {
          "$setIntersection": [ x, "$tags" } ]
       }
    },
    ...
}

But of course this doesn't work as the $tags array is full of objects, and not strings. I need to strip the weight from the tags array and leave it just as an array of names like ["a", "b", "c" ...], then it should work.

How would I go about doing this?

Ashh · Accepted Answer · 2018-11-29 15:42:16Z

1

Just use . dot notation with the name inside tags. Something like

db.collection.aggregate([
  { "$project": {
    "matches": {
      "$size": {
        "$setIntersection": [
          [ "a", "b", "d"],
          "$tags.name"
        ]
      }
    }
  }}
])

edited Nov 29, 2018 at 15:42

answered Nov 29, 2018 at 12:20

Ashh

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1 Comment

mic Over a year ago

Thank you, it works perfectly. I didn't expect it to be so simple

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