NodeJS/Mongoose schema array of ObjectId
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Category Schema

const mongoose  = require('mongoose');
const timestamps    = require('mongoose-timestamp');
const cateorySchema = new mongoose.Schema({
  category_name: {
    type: String,
    trim: true,
    required: true,
  },
  active: {
        type: Boolean,
        default: true,
    }
});
cateorySchema.plugin(timestamps); // automatically adds createdAt and updatedAt timestamps
module.exports = mongoose.model('Category',cateorySchema);

SubCategory Schema

'use strict'
const mongoose    = require('mongoose');
const timestamps    = require('mongoose-timestamp');
const subCategorySchema = new mongoose.Schema({
    categories:{ type: mongoose.Schema.Types.ObjectId, ref: 'Category' },
    subcategorytitle:{
      type:String,
      trim:true,
      required: true
    },
    active: {
        type: Boolean,
        default: true
    }
});
subCategorySchema.plugin(timestamps); // automatically adds createdAt and updatedAt timestamps
module.exports = mongoose.model('Subcategory',subCategorySchema);