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 na?ve 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 of the two lists can be overwhelming. Besides, it can be quite slow to convert large QuerySets into lists.

A much better solution uses iterators to reduce the memory consumption. Use the itertools.chain method to combine multiple QuerySets as follows:

    >>> from itertools import chain
    >>> recent = chain(posts, comments)
    >>> sorted(recent, key=lambda e: e.modified, reverse=True)[:3]

Once you evaluate a QuerySet, the cost of hitting the database can be quite high.
So, it is important to delay it as long as possible by performing only operations that will return QuerySets unevaluated.

Keep QuerySets unevaluated as long as possible.