Manage nested list of entities within entities in Google Cloud Datastore

Question

I am new to Datastore and I am trying to create a simple app that tracks books borrowing.

I would like the DB schema to be as follows:

books:
 book_id
  name:
  borrowing:
   timestamp:
    user_id
    firstname
    email

users:
 name:
 borrowed:
  timestamp:
   book_id

I am using the console to configure the DB.

I have defined two Kinds: Books and User

Business logic:

• 1 book can be borrowed by n users.
• 1 user can borrow n books

Basically, when a user borrows a book I want the two following inserts to occur:

• a new borrowing entry is appended to the book entity of type Book, with the current timestamp as key and user_id, firstname and email as properties
• a new borrowed entry is appended to the user entity of ty User, with the current timestamp as key and the book_id as property

How can I achieve this (managing nested lists children) with Datastore? Through embedded entities? Parent ref?

Here is an example of what I expect:

{
  "books": {
      "book1": {
          "name": "book number 1",
          "borrowing": {
              "1234567890": {
                  "user_id": "user1",
                  "firstname": "john",
                  "email": "john@example.com"
              },
              "2234567890": {
                  "user_id": "user2",
                  "firstname": "robin",
                  "email": "robin@example.com"
              }
          }
      }
      ...
  },
    "users": {
        "user1": {
            "firstname": "robin",
            "email": "robin@example.com",
            "borrowed": {
                "1234567890": {
                    "book_id": "book1"
                },
                "3247829398": {
                    "book_id": "book99"
                }
            }
        },
        ...
    }
}

Answer 1

In general this is not a scalable approach: every time a user borrows a book you'd have to re-write both the user and book entities, which will get progressively slower as both entities will keep growing.

I'd suggest a different approach: add a new type to your schema, let's call it borrowed_book, representing a book boorowed by a user:

borrowed_book:
  book_id
  user_id
  timestamp

Now every time a user borrows a book you'd simply create one such borrowed_book entity, pointing to both the book and the user. No changes to the user or the book entities. And no nesting required.

Side note: I'd place the firstname and email properties under the user entity type, they don't really belong to the borrowing event where they would be duplicated every time such event occurs for the same user.

Also try to not get confused by the ancestry - it is not required for establishing relationships, see E-commerce Product Categories in Google App Engine (Python)