Understanding ListProperty backend behavior in GAE

Question

I'm trying to understand how you're supposed to access items in a GAE db.ListProperty(db.Key).

Example:

A Magazine db.Model entity has a db.ListProperty(db.Key) that contains 10 Article entities. I want to get the Magazine object and display the Article names and dates. Do I make 10 queries for the actual article objects? Do I do a batch query? What if there's 50 articles? (Don't batch queries rely on the IN operator, which is limited to 30 or fewer elements?)

Answer 1

So you are describing something like this:

class Magazine(db.Model):   
    ArticleList = db.ListProperty(db.Key)

class Article(db.Model):
    ArticleName = db.StringProperty()
    ArticleDate = db.DateProperty()

In this case the simplest way to grab the listed articles is to use the Model.get() method, which looks for a key list.

m = Magazine.get() #grab the first record

articles = Article.get(m.ArticleList) #get Articles using key list

for a in articles:
    name = a.ArticleName
    date = a.ArticleDate
    #do something with this data

Depending on how you plan on working with the data you may be better off adding a Magazine reference property to your Article entities instead.