failed to generate unique invite code with randomlize library in mongodb

Question

I made a user schema based on mongoose or mongodb, in which there is a invitation code and this code is always unique in the database.

import randomize from 'randomatic';

const userSchema = new Schema<UserDataInterface>(
  {
    username: {
      type: String,
      required: [true, "Please username is mandatory."],
      unique: false,
    },
    inviteCode: { type: String, default: randomize('0', 8)  },
    ...
  }

The invite code is 8 length long numbers generated by a randomatic library, I expect it will be different for every user, so I didn't enforce inviteCode to be unique

but during the production, I found two latest registered users who are registered at different time have exact the same invitecode.

{
  "_id": {
    "$oid": "6656e341495d3ec1782baa7d"
  },
  "username": "ddd333",
  "inviteCode": "94770150",
  "createdAt": {
    "$date": "2024-05-29T08:11:45.240Z"
  }
}
{
  "_id": {
    "$oid": "66572d68495d3ec1782bab0f"
  },
  "username": "ddd233",
  "inviteCode": "94770150",
  "createdAt": {
     "$date": "2024-05-29T13:28:08.943Z"
  }
}

I found the problem is all user's inviteCode is the same value,

inviteCode: { type: String, default: randomize('0', 8) }.

It looks like randomize function only initalize only once

The problem is you are calling randomize() when the schema is defined instead of passing the function. That means when the script runs the default value will be the return value of the function invocation and therefore will always be the same as it's only called once. Pass a function instead. — jQueeny
– jQueeny, Commented May 30, 2024 at 8:02

WeDoTheBest4You · Accepted Answer · 2024-05-30 08:56:06Z

1

Please wrap the invocation within a function and pass it instead. @jQueeny has already commented the same. Please see the sample code below.

  const schema = new mongoose.Schema({
    _id: {
      type: String,
      default: () => randomize('0', 8),
    },
  });

It generated the following output while testing:

{ _id: '96726096', __v: 0 },
{ _id: '08571337', __v: 0 },
{ _id: '60216517', __v: 0 },
{ _id: '03736755', __v: 0 },
{ _id: '73289283', __v: 0 }

answered May 30, 2024 at 8:56

WeDoTheBest4You

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Comments

ray · Accepted Answer · 2024-05-30 12:10:00Z

Here is an alternative that does not rely on external library but use only MongoDB aggregation operators.

generate random number with $rand
$multiply the number with 100000000 and $toLong to make take only integer part
$toString and pad with 8 zeros in front
take substring for last 8 digits

db.collection.aggregate([
  {
    "$set": {
      // generate the inviteCode in long first; could be less than 8 digits
      "inviteCode": {
        "$toLong": {
          "$multiply": [
            {
              "$rand": {}
            },
            100000000
          ]
        }
      }
    }
  },
  {
    "$set": {
      // convert to string and pad zeros in front
      "inviteCode": {
        "$concat": [
          "00000000",
          {
            $toString: "$inviteCode"
          }
        ]
      }
    }
  },
  {
    "$set": {
      "inviteCode": {
        // get the last 8 digits
        "$substrCP": [
          "$inviteCode",
          {
            "$subtract": [
              {
                "$strLenCP": "$inviteCode"
              },
              8
            ]
          },
          8
        ]
      }
    }
  },
  {
    "$merge": {
      "into": "collection",
      "on": "_id"
    }
  }
])

Mongo Playground

To ensure the uniqueness of the field, add a unique index around it.

user824624 · Accepted Answer · 2024-05-29 23:30:35Z

0

I don't know why inviteCode: { type: String, default: randomize('0', 8) } only init once for all registered users.

so I tried

userSchema.pre("save", async function (next) {
  this.inviteCode = randomize('0', 8);
  next();
});

and it works out.

answered May 29, 2024 at 23:30

user824624

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