How can I limit output of MongoDB query in Python?

I have this example MongoDB entry in a users DB:

{
     _id: xxx,
     name: "name",
     files:[{
          fileName: "test",
          size: 654
          },
          {fileName: "test2",
          size: 50
     }]
}

And I’m using Pymongo to query the file named “test”:

users.find_one({'files.filename':"test"})

How can I limit the output from Pymongo, so it only gives me the dict where “test” is contained? In other words, I want it to output this:

{
     fileName: "test",
     size: 654
}

Answer

Here’s one way you could do it.

db.users.aggregate([
  {
    "$match": {
      "files.fileName": "test"
    }
  },
  {
    "$replaceWith": {
      "$first": {
        "$filter": {
          "input": "$files",
          "as": "file",
          "cond": {
            "$eq": ["$$file.fileName", "test"]
          }
        }
      }
    }
  }
])

Try it on mongoplayground.net.