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.
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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