title: add a unique field to a reference model
body: <pre><code>    //user schema
  const userSchema = new schema({
    name: { type: String, require: true },
    last_name: { type: String, require: true },
    password: { type: String, require: true },
    email: { type: String, require: true },
    phone: { type: Number, require: true },
    isAdmin: { type: Boolean, require: true, default: false },
  
    cart: [
      { type: mongoose.Types.ObjectId, ref: &quot;product&quot; },
    ],
  });
  </code></pre>
  <p>as you see this user model contains an array &quot; cart of products.</p>
  <p>I want to add another unique field &quot;quantity_addedToCart&quot; to each product without adding it to the product model.</p>
  <p>something like this.</p>
  <pre><code>cart: [
  { type: mongoose.Types.ObjectId, ref: &quot;product&quot; ,quantity_addedToCart:{type:Number,require:true},
  ],
  </code></pre>
  <p>I hope you got the idea guys and you can assist me to solve it.</p>