How to combine multiple QuerySets in Django?

Criteria to chain or union

In the Django universe, multiple ways exist to combine QuerySets based on your requirements and constraints. Here's how you make the choice:

Union operator for similar models

For QuerySets returning the same model instances:

• Union (|): Preserves the QuerySets' laziness (they just refuse to work until absolutely necessary 🤷). Allows further queryset methods like .order_by() without any hitches.

Everyone for themselves with itertools.chain

When combining QuerySets from varied models or need a custom sorting:

• chain(): It constructs an iterable by chaining together multiple QuerySets. Perfect for those moments when you're dealing with QuerySets of different models.

Single database query with union method

When given QuerySets from the same model and really want to execute a single trip to the database:

• union(): Combines QuerySets using the SQL UNION operator. Remember, with great power comes great restrictions.

Tackling sorting and pagination

What do we do when combined QuerySets cry out for sorting or pagination?

Sorting by attribute

To order the results by a specific attribute:

• sorted(): Use Python's sorted() with attrgetter to flexibly sort on specific attributes and even reverse the order.

  from operator import attrgetter
  # arrange first by beauty, then by humor.
  pageant_order = sorted(combined_iterable, key=attrgetter('beauty', 'humor'), reverse=True)  # Miss Universe style! 👸
  

Pagination over combined results

For displaying the combined results with pagination:

• QuerySetChain: Use this to simulate a single QuerySet that allows pagination.

  # Industrial strength queryset chaining, handle with caution
  combined_queryset = QuerySetChain(queryset1, queryset2)  # feels like one big happy family 👨‍👩‍👧‍👦
  

• The list way: In situations where compatibility with generic views is a must, the chained iterables should be converted to a list.

Handling concerns and tips

Performance thoughts

• Lazy-loading and memory: QuerySets are inherently lazy and more memory efficient. But, like a good Netflix binge, avoid going overboard. Don't convert to a list unless necessary.

Spot the error

• The infamous 'clone attribute' error: Keep this bug at bay by using itertools.chain and not calling list-specific methods on a QuerySet.

Our friend, itertools.chain

• Chaining with list conversion: This may sometimes be necessary for compatibility with certain class-based generic views.

Optimizing the usage

Rely on itertools.chain

• Fast and memory-efficient: Avoids the premise of converting QuerySets to lists and hangs on to the lazy evaluation of QuerySets.

Be mindful of memory usage

• QuerySets over lists: While converting to lists can be light on memory, it loses lazy evaluation.

Limit the database hits

• Combine the trips: Combining querysets with union() saves us from making multiple trips to the database. Basically, carpooling for QuerySets.

Improving compatibility with Django abstractions

Merging views and QuerySet chains

For best compatibility with Django's class-based views:

• Add template_name: In case of using classes like QuerySetChain, providing template_name attribute ensures seamless compatibility with generic views.

Cloning in custom QuerySet chains

When making a custom chain class:

• Cloning enabled: It's crucial to bake the clone() method into your QuerySetChain to have it behave like a Django QuerySet.

Counting entries

For getting the total joined entries despite all the union and chaining madness:

• Good old count(): Make use of count() method on QuerySetChain to get the mutual headcount.