User has many Shopping Lists

A User has many Shopping Lists, to create a has-many relation between User and Shopping List model, we will need to do a similar exercise like we did for Shopping List and Item model. Each ShoppingList object would need to keep an ID to the user it belongs to so that we can query for all Shopping Lists for that user. To add this, we will need to follow these steps:

1. Open the ShoppingList.swift file in your Vapor app and add the following new property as an instance variable inside the ShoppingList class to store userId:

var userId: Identifier

2. Next, create a user-computed property, which will allow us to get the user to a Shopping List:

var user: Parent<ShoppingList, User> {
  return parent(id: userId)
}

3. Now, add the userId column name that will be used by MongoDB to store the user ID inside of the database collection:

static let userId = "user__id"

4. Now, we will need to update our initializer so that it can take the userId as a parameter and assign it to its userId...