Question

Let’s take this models:

User
  - name

Product 
  - name
  - category

List 
  - name
  - creation_date
  - user (reference)

Product_List
  - list ( reference)
  - product ( reference)

How can I retrieve a list of the products that remain out of the list?

• Should I retrieve them all and then delete them programmatically ( doesn’t this make the request slower? )

  1. Get all the products of a certain list of a certain user
  2. Get all the products
  3. Extract the difference ( a nested for? )

Sorry, I’m kind of newbie on this, suggestions and comments are welcome!

Thanks!

Answer 1

If you structure your data like this:

class Product(db.Model):
  # ...

class UserInfo(db.Model):
  # ...

class ProductList(db.Model):
  owner = db.ReferenceProperty(UserInfo)
  products = db.ListProperty(db.Key)

Then you can retrieve the products not in a list like this:

product_keys = set(Product.all(keys_only=True).fetch(1000))
product_list = ProductList.get_by_id(product_list_id)
missing_products = product_keys - set(product_list.products)

missing_products is a set of keys, which you can pass to db.get to retrieve the corresponding product entities.

This will, of course, require retrieving the entire list of products, but this is exactly what a relational database would have to do to satisfy the query too.