I'm creating a todos app with MERN stack, and in an attempt to create personal todos for each user, I found this error:

MongoServerError: E11000 duplicate key error collection: Todos-APP.todos index: _id_ dup key: { _id: ObjectId('6201a648189af9a06ac75ca1') }
    at C:\Users\Administrador\Documents\GitHub\todos-app\server\node_modules\mongodb\lib\operations\insert.js:51:33
    at C:\Users\Administrador\Documents\GitHub\todos-app\server\node_modules\mongodb\lib\cmap\connection_pool.js:272:25
    at handleOperationResult (C:\Users\Administrador\Documents\GitHub\todos-app\server\node_modules\mongodb\lib\sdam\server.js:370:9)
    at MessageStream.messageHandler (C:\Users\Administrador\Documents\GitHub\todos-app\server\node_modules\mongodb\lib\cmap\connection.js:479:9)
    at MessageStream.emit (node:events:390:28)
    at processIncomingData (C:\Users\Administrador\Documents\GitHub\todos-app\server\node_modules\mongodb\lib\cmap\message_stream.js:108:16)
    at MessageStream._write (C:\Users\Administrador\Documents\GitHub\todos-app\server\node_modules\mongodb\lib\cmap\message_stream.js:28:9)
    at writeOrBuffer (node:internal/streams/writable:389:12)
    at _write (node:internal/streams/writable:330:10)
    at MessageStream.Writable.write (node:internal/streams/writable:334:10) {
  index: 0,
  code: 11000,
  keyPattern: { _id: 1 },
  keyValue: {
    _id: ObjectId {
      [Symbol(id)]: Buffer(12) [Uint8Array] [
         98,   1, 166,  72,  24,
        154, 249, 160, 106, 199,
         92, 161
      ]
    }
  }
}

I have well-founded suspicions that the problem arises because in my Todo Schema, I have a field 'owner' which gets populated with the client's _id from MongoDB (the client that sent the request to create the new todo):

const mongoose = require('mongoose');

const TodoSchema = new mongoose.Schema({

    _id: {
        type: mongoose.Types.ObjectId,
        default: mongoose.Types.ObjectId()
    },
    owner: {
        type: mongoose.Types.ObjectId,
        ref: 'User'
    },
    title: {
        type: String,
        required: true
    },
    description: {
        type: String, 
        required: true
    },
    isCompleted: {
        type: Boolean, 
        required: true
    }

});

module.exports = mongoose.model('Todo', TodoSchema);

Now, this is how the server is handling the request:

// Create Todo
router.post('/createTodo', async (req, res) => {
    const { ownerName, title, description, isCompleted } = req.body;

    if(!title || !description || isCompleted === null || ownerName === null) return res.status(400).send();

    const owner = await User.findOne({ username: ownerName }).exec();

    const existingTodo = await Todo.findOne({ owner: owner._id, title: title }).exec();

    if(existingTodo) return res.status(409).send();

    try {
        const todo = new Todo({
            owner: owner._id,
            title, 
            description,
            isCompleted
        })

        todo.save();
        res.status(200).send();

    } catch (error) {
        console.error(`ERROR: ${error}`);
        res.status(409).end();
    }
});

I'd like to add that I've noticed that once I log in, I can successfully create a new todo, but if I try to create another one within the same session, it throws that error, but if the session expires and I log in again, then I can create another new todo; just one todo per session.

CodePudding user response：

Try to remove explicitly adding _id to schema with value of mongoose.Types.ObjectId(). Remove these lines from schema definition.

_id: {
  type: mongoose.Types.ObjectId,
  default: mongoose.Types.ObjectId()
},

Mongo will create unique _id by default. To create ObjectId with Mongoose, I think you should use new mongoose.Types.ObjectId(). So maybe you are missing new keyword.