Create ID with data of other fields

Question

I'm new to mongodb, and I'm using mongoose to validate and order the data (I'm open to change it to MySQL if this doesn't work). The app will be an e-shop, to buy merchandising related to movies, games, ext.

My schema is as follows:

var productSchema = {
    id: {
        type: String,
        required: true
    },
    name: {
        type: String,
        required: true
    },
    img: {
        type: String,
        required: true
    },
    price: {
        type: Number,
        required: true
    },
    stock: {
        type: Number,
        required: true
    },
    category: {
        object: {
            type: String,
            required: true
        },
        group: {
            type: String,
            required: true  
        },
        name: {
            type: String,
            required: true
        }
    }
};

This is what I would like to do:

If I have the following data in category:

• category.object = "ring"
• category.group = "movies"
• category.name= "lord of the rings"

I want the id to be made of the first letters of every field in category and a number (the number of the last item added plus 1). In this case, It would be RMLOTR1.

What I'm doing right now

I'm adding a lot of data at the same time, so every time I do it, I made a function that iterates through all the items added and does what I want but...

My question is

Is there a built-in way to do this with mongodb or mongoose, adding the data and creating the id at the same time? I know I can do a virtual, but I want the data to be stored.

Extras

• If it's not posible to do this with mongodb, is there a way to do this with MySQL?
• Is doing this kind of thing considered a correct/wrong approach?

Answer 1

You are basically looking for a "pre" middleware hook on the "save" event fired by creating new documents in the collection. This will inspect the current document content and extract the "strings" from values in order to create your "prefix" value for _id.

There is also another part, where the "prefix" needs the addition of the numeric counter when there is already a value present for that particular "prefix" to make it distinct. There is a common technique in MongoDB used to "Generate an auto-incrementing sequence field", which basically involves keeping a "counters" collection and incrementing the value each time you access it.

As a complete and self contained demonstration, you combine the techniques as follows:

var async = require('async'),
    mongoose = require('mongoose'),
    Schema = mongoose.Schema;

mongoose.connect('mongodb://localhost/warehouse');

var counterSchema = new Schema({
  "type": { "type": String, "required": true },
  "prefix": { "type": String, "required": true },
  "counter": Number
});

counterSchema.index({ "type": 1, "prefix": 1 },{ "unique": true });

counterSchema.virtual('nextId').get(function() {
  return this.prefix + this.counter;
});

var productSchema = new Schema({
  "_id": "String",
  "category": {
    "object": { "type": String, "required": true },
    "group": { "type": String, "required": true },
    "name": { "type": String, "required": true }
  }
},{ "_id": false });

productSchema.pre('save', function(next) {
  var self = this;

  if ( !self.hasOwnProperty("_id") ) {

    var prefix = self.category.object.substr(0,1).toUpperCase()
      + self.category.group.substr(0,1).toUpperCase()
      + self.category.name.split(" ").map(function(word) {
        return word.substr(0,1).toUpperCase();
      }).join("");

    mongoose.model('Counter').findOneAndUpdate(
      { "type": "product", "prefix": prefix },
      { "$inc": { "counter": 1 } },
      { "new": true, "upsert": true },
      function(err,counter) {
        self._id = counter.nextId;
        next(err);
      }
    );
  } else  {
    next();  // Just skip when _id is already there
  }
});

var Product = mongoose.model('Product',productSchema),
    Counter = mongoose.model('Counter', counterSchema);

async.series(
  [
    // Clean data
    function(callback) {
      async.each([Product,Counter],function(model,callback) {
        model.remove({},callback);
      },callback);
    },
    function(callback) {
      async.each(
        [
          {
            "category": {
              "object": "ring",
              "group": "movies",
              "name": "lord of the rings"
            }
          },
          {
            "category": {
              "object": "ring",
              "group": "movies",
              "name": "four weddings and a funeral"
            }
          },
          {
            "category": {
              "object": "ring",
              "group": "movies",
              "name": "lord of the rings"
            }
          }
        ],
        function(data,callback) {
          Product.create(data,callback)
        },
        callback
      )
    },
    function(callback) {
      Product.find().exec(function(err,products) {
        console.log(products);
        callback(err);
      });
    },
    function(callback) {
      Counter.find().exec(function(err,counters) {
        console.log(counters);
        callback(err);
      });
    }
  ],
  function(err) {
    if (err) throw err;
    mongoose.disconnect();
  }
)

This gives you output like:

[ { category: { name: 'lord of the rings', group: 'movies', object: 'ring' },
    __v: 0,
    _id: 'RMLOTR1' },
  { category:
     { name: 'four weddings and a funeral',
       group: 'movies',
       object: 'ring' },
    __v: 0,
    _id: 'RMFWAAF1' },
  { category: { name: 'lord of the rings', group: 'movies', object: 'ring' },
    __v: 0,
    _id: 'RMLOTR2' } ]
[ { __v: 0,
    counter: 2,
    type: 'product',
    prefix: 'RMLOTR',
    _id: 57104cdaa774fcc73c1df0e8 },
  { __v: 0,
    counter: 1,
    type: 'product',
    prefix: 'RMFWAAF',
    _id: 57104cdaa774fcc73c1df0e9 } ]

To first understand the Counter schema and model, you are basically defining something where you are going to look up a "unique" key and also attach a numeric field to "increment" on match. For convenience this just has a two fields making up the unique combination and a compound index defined. This could just also be a compound _id if so wanted.

The other convenience is the virtual method of nextId, which just does a concatenation of the "prefix" and "counter" values. It's also best practice here to include something like "type" here since your Counter model can be used to service "counters" for use in more than one collection source. So here we are using "product" whenever accessing in the context of the Product model to differentiate it from other models where you might also keep a similar sequence counter. Just a design point that is worthwhile following.

For the actual Product model itself, we want to attach "pre save" middleware hook in order to fill the _id content. So after determining the character portion of the "prefix", the operation then goes off and looks for that "prefix" with the "product" type data in combination in the Counter model collection.

The function of .findOneAndUpdate() is to look for a document matching the criteria in the "counters" collection and then where a document is found already it will "increment" the current counter value by use of the $inc update operator. If the document was not found, then the "upsert" option means that a new document will be created, and at any rate the same "increment" will happen in the new document as well.

The "new" option here means that we want the "modified" document to be returned ( either new or changed ) rather than what the document looked like before the $inc was applied. The result is that "counter" value will always increase on every access.

Once that is complete and a document for Counter is either incremented or created for it's matching keys, then you now have something you can use to assign to the _id in the Product model. As mentioned earlier you can use the virtual here for convenience to get the prefix with the appended counter value.

So as long as your documents are always created by either the .create() method from the model or by using new Product() and then the .save() method, then the methods attached to your "model" in your code are always executed.

Note here that since you want this in _id, then as a primary key this is "immutable" and cannot change. So even if the content in the fields referenced was later altered, the value in _id cannot be changed, and therefore why the code here makes no attempt when an _id value is already set.