How to set _id to db document in Mongoose?

I'm trying to dynamically create _id's for my Mongoose models by counting the documents in the db, and using that number to create the _id (assuming the first _id is 0). However, I can't get the _id to set from my values. Here's my code:

//Schemas
var Post = new mongoose.Schema({
    //_id: Number,
    title: String,
    content: String,
    tags: [ String ]
});

var count = 16;

//Models
var PostModel = mongoose.model( 'Post', Post );

app.post( '/', function( request, response ) {

    var post = new PostModel({
        _id: count,
        title: request.body.title,
        content: request.body.content,
        tags: request.body.tags
    });

    post.save( function( err ) {
        if( !err ) {
            return console.log( 'Post saved');
        } else {
            console.log( err );
        }
    });

    count++;

    return response.send(post);
});

I've tried to set the _id a number of different ways, but it's not working for me. Here's the latest error:

{ message: 'Cast to ObjectId failed for value "16" at path "_id"',
  name: 'CastError',
  type: 'ObjectId',
  value: 16,
  path: '_id' }

If you know what's going on, please let me know.

Answers:

Answer

The first piece of @robertklep's code doesn't work for me (mongoose 4), also need to disabled _id

var Post = new mongoose.Schema({
  _id: Number,
  title: String,
  content: String,
  tags: [ String ]
}, { _id: false });

and this works for me

Answer

Create custom _id in mongoose and save that id as a mongo _id. Use mongo _id before saving documents like this.

const mongoose = require('mongoose');
    const Post = new mongoose.Schema({
          title: String,
          content: String,
          tags: [ String ]
        }, { _id: false });

// request body to save

let post = new PostModel({
        _id: new mongoose.Types.ObjectId().toHexString(), //5cd5308e695db945d3cc81a9
        title: request.body.title,
        content: request.body.content,
        tags: request.body.tags
    });


post.save();
Answer

You either need to declare the _id property as part of your schema (you commented it out), or use the _id option and set it to false (you're using the id option, which creates a virtual getter to cast _id to a string but still created an _id ObjectID property, hence the casting error you get).

So either this:

var Post = new mongoose.Schema({
    _id: Number,
    title: String,
    content: String,
    tags: [ String ]
});

Or this:

var Post = new mongoose.Schema({
    title: String,
    content: String,
    tags: [ String ]
}, { _id: false });

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