How to make a light query for a many to many relationship in Google Apps Engine?

Question

How to make a light query for a many to many relationship?

Users has many Lists

the ListUser is the model that links them

Currently I'm doing like this but there are a lot of get queries to get all this data.

lists = []
for list in user.lists:
    lists.append(list.list)

Now I got this:

list_users = user.lists.fetch(1000)
# problem is here: "list.list" will retrive data from database, but I just want that list's key, how to do?
list_keys = [list.list for list in list_users]
lists = List.get_by_key_name(list_keys)

Answer 1

You should use get_value_for_datastore.

list_keys = [ListUser.list.get_value_for_datastore(list_user)
              for list_user in list_users]
lists = db.get(list_keys)

If you have not already, you might want to take a look at some of the 'mastering the datastore' articles. Specifically the one on modeling relationships. Perhaps you can accomplish what you need using a list of List keys instead, maybe it will save you a query.