Django Queryset with an unusual operation "Merge"

Developers
Django Queryset with an unusual operation "Merge"

Consider, we have a StudentHistory model defined as:     


We can write two querysets on this model:   


Let's learn a bit about the Django queryset merging. We have three possible ways for merging the queryset.

1. If the querysets are of same model then we can use "|" (pipe) operator for merging querysets:

We can still perform operation on the merged queryset:


Limitation:
With | operator, we can only merge querysets of same model.


2. If the querysets are of different models, we can use "chain()" built-in function.


We also can sort the below merged_queryset on first_name using attrgetter function

Limitation: Chain function returns a list of objects instead of an queryset object. This can be problematic when the results are large in numbers and their pagination is needed.


3. Django queryset have in-built method "union()":

Union method can be used for merging querysets of same or different models.

Case1: If the querysets are of same model

Consider the StudentHistory model defined earlier in the post for below examples:


Case 2: If the querysets are of different models -

Now along with the already defined model StudentHistory we define another model Teacher below for this case:


Note - When models are different, union() function needs same named fields from the related model's querysets.

Example:

Thus we fetch only 3 same named fields i.e first_name, last_name, email. We can now perform union function on querysets:

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