I'm trying to make an aggregate with mongodb. My goal is that I want to get random names starting with letters A to Z. As a result, each word starts with letter must be only once in the response but I can't figure out how to do it. I used match condition with regex and sample condition to get random documents.

Here is my collection;

[
  {
    "name": "ahmet"
  },
  {
    "name": "barış"
  },
  {
    "name": "ceyhun"
  },
  {
    "name": "aslan"
  },
  {
    "name": "deniz"
  },
  ....
]

Here is my aggregate function;

db.collection.aggregate([
  {
    $match: {
      name: {
        $regex: "^a|^b|^c" // must be A to Z
      }
    }
  },
  {
    "$sample": {
      "size": 3 // Must be 26
    }
  }
])

I'm waiting response to be like this;

[
  {
    "name": "ahmet"
  },
  {
    "name": "barış"
  },
  {
    "name": "ceyhun"
  },
  .... // other words starting with d, e , f but only one word for each letter
]

But I'm getting;

[
  {
    "_id": ObjectId("5a934e000102030405000001"),
    "name": "barış"
  },
  {
    "_id": ObjectId("5a934e000102030405000003"),
    "name": "aslan"
  },
  {
    "_id": ObjectId("5a934e000102030405000000"),
    "name": "ahmet"
  },
  // name => aslan, name => ahmet (Two words starting with same letter)
]

I'm newbie at mongodb and if anyone can help me where I'm wrong, I'll be appreciate.

Mongo Playground

CodePudding user response：

You can do something like this:

Edit:

db.collection.aggregate([
  {
    $group: {
      _id: {$substr: ["$name", 0, 1]},
      name: {$push: "$name"}
    }
  },
  {
    $project: {_id: 0,
      name: {
        $arrayElemAt: [
            "$name", 
            {$toInt: {$multiply: [{$rand: {}}, {$size: "$name" }]}
          }
        ]
      }
    }
  }
])

As you can see on this playground example.

The $group will keep a list of names per each firstL, the $arryElemAt with the $rand will keep only a random item.