How to connect related documents?

Question

Node.js

there are two
artist.js files in shemas folder

'use strict';

var Mongoose  = require('mongoose');
var categoryShema = require('category.js')

var artistSchema = new Mongoose.Schema({
    id:{type: Number,required:true},
    name:{type: String,required:true},
    description: String,
    created_at:{type:Date, default: Date.now},
    category:{type:Schema.Types.ObjectId, ref: 'categoryModel'}
});

var artistModel = Mongoose.model('artist', artistSchema);
module.exports = artistModel;

and categories.js

'use strict';
var Mongoose  = require('mongoose');

var categorySchema = new Mongoose.Schema({
    id:{type: Number},
    name:{type: String,required:true},
    description:{type: String},
    created_at:{type:Date, default: Date.now},
    title:{type:String},
    keywords:{type:String},

});

var categoryModel = Mongoose.model('category', categorySchema);
module.exports = categoryModel;

when restarting pm2. pm2 log gives error

Error: Cannot find module 'category.js'

The question is how to connect correctly.