Django Design Patterns and Best Practices - Second Edition

Chaining multiple QuerySets

So far, we have been combining QuerySets of the same type belonging to the same base class. However, we might need to combine QuerySets from different models and perform operations on them.

For example, a user's activity timeline contains all their posts and comments in reverse chronological order. The previous methods of combining QuerySets won't work. A naïve solution would be to convert them to lists, concatenate, and sort them, as follows:

    >>>recent = list(posts)+list(comments)
    >>>sorted(recent, key=lambda e: e.modified, reverse=True)[:3]
    [<Post: user: Post1>, <Comment: user: Comment1>, <Post: user: Post0>]      

Unfortunately, this operation has evaluated both the lazy QuerySet objects. The combined memory usage ...