唯一无法在猫鼬模式下工作

Question

如何使objectId归档为唯一。在add_to_cart_user的架构中，我尝试过这样的unique=true，但它不起作用。

  schemaDefinition = {
    add_to_cart_user: {
      type: ObjectId,
      unique: true,
      required: [true, 'add_to_cart_user is required'],
    },
    add_to_cart_products: {
      type: [ProductSchema],
      validate: [
        {
          validator: NotEmptyArrayValidator,
          message: 'add_to_cart_products is required',
        },
      ],
    },
    add_to_cart_discount: {
      type: Number,
      default: 0,
      min: 0,
    },
    add_to_tax: {
      type: Number,
      default: 8,
    },
  };

谁能告诉我该怎么做？