CODEWITHSUNDEEP
androidgoogle-app-enginegoogle-cloud-datastore
Riyaz Ahamed
Riyaz Ahamed

Reputation: 802

Data Model - How to structure entities in GAE

I am an android developer. I have an app which connects to MySql DB in server. Now i am planning to migrate them to Google app engine. But i am stuck with how to structure my entities.

There are tables 'users' and 'books'. Users can own multiple books and a book can be owned by multiple people. So there is a reference table called 'book-owners'. There is a column in this table called as the number of pages read. So each user to book relationship has a this special property 'no. of pages read'.

With MySql I could perform join queries to get all the info i want.Now with GAE we can not perform join queries. All I want is these,

  1. All books owned by a user along with 'no. of pages read' property
  2. Number of owners for a particular book

When I query for user i need the following response:

{"user_id":"123","user_name":"John Doe","books":[{"book_id":"123456", "book_name":"Some book name","pages_read":126},{"book_id":"123457","book_name":"Some book name","pages_read":26},{"book_id":"123458","book_name":"Some book name","pages_read":274},{"book_id":"123459","book_name":"Some book name","pages_read":48}]}

So how to structure my entities. And how to query them back? Kindly give me some advice / pointers. Thanks already.

Upvotes: 0

Views: 65

Answers (1)

minou
minou

Reputation: 16553

Here is one way:

class User(ndb.Model):
    [any data here you want]

class Book(ndb.Model):
    [any data here you want]

class OwnedBook(ndb.Model):
    user = ndb.KeyProperty(User)
    book = ndb.KeyProperty(Book)
    pages_read = ndb.IntegerProperty()

To get books owned by a user with pages read, do this:

books_owned_by_user = OwnedBook.query(OwnedBook.user = user1)

After getting the OwnedBook entities, you'll need to do further queries to get more information about the books.

for owned_book in OwnedBook.query(OwnedBook.user = user1):
    book = owned_book.book.get()

You'll need to think about how to do this efficiently for your particular application.

To get the number of owners of a book, do this:

num_owners = OwnedBook.query(OwnedBook.book = book1).count()

Upvotes: 2

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