How do you ensure that the ShortId used in Mongoose won't be duplicated? Or if duplication happens then?

[Kaijun]

As we see in the doc

Mongoose Unique Id

_id: {
    type: String,
    unique: true,
    'default': shortid.generate
},

I consider if shortid.generate will accidentally generate a duplicated id for the record.

If it really happens, will mongoose throw an error something like duplicated index error by saving the new record, or will it regenerate a new id automatically until it's not duplicated anymore?

[manuel-di-iorio]

Yes, mongoose will give you the error E11000 duplicate key error index but it will not automatically retry the query. You have to handle this part yourself or with a wrapper module.

There is https://github.com/jjwchoy/mongoose-shortid but this actually uses an internal shortid generation system. If there aren't yet modules, you could look at the source code of this one and build your own module!

[Kaijun]

@manuel-di-iorio thanks! i just implemented something by using the pre() hook in mongoose, here is my code if anyone is interested 😄

import mongoose from 'mongoose'
import shortid from 'shortid'

const xxxSchema = mongoose.Schema({
  code: {
    type: String,
    unique: true,
  },
  ...
})

// generating a non-duplicate Code
xxxSchema.pre('save', function(next){  // can't use arror function, or this will be undefinded. fat arrow is lexically scoped.
  let ctx = this
  attempToGenerate(ctx, next)
})

function attempToGenerate(ctx, callback) {
    let newCode = shortid.generate()
    ctx.constructor.findOne({'code': newCode}).then((course) => {
      if(course){
        attempToGenerate(ctx, callback)
      }
      else{
        ctx.code = newCode
        next();
      }
    }, (err) => {
      next(err)
    })
  }

export default mongoose.model('XXX', xxxSchema)

Note: ctx.constructor.findOne is kind of tricky way to access the model. Solution found on StackOverflow

[manuel-di-iorio]

👍

Module wrapper:

const shortid = require("shortid");

exports.inject = (schema) => {
    schema.pre('save', kaijun code here..
};

Use it with:

const mongooseShortid = require("chooseModuleName")   
mongooseShortid.inject(yourSchema);

PS: I would also place the attempToGenerate private function outside of the pre save hook

[Kaijun]

@manuel-di-iorio thanks a lot!

[doomedramen]

Just some notes for anyone reading this in the future.

In the Kaijun code, I believe next() is supposed to be callback() inside of attempToGenerate.
Also, if you move this code from pre('save') to pre('validate') you can add required: true to the code param in the schema.