How to limit records in tables with n to n relation before joining them , to avoid duplicates in django ORM?

问题

Extending my previous question on stack-overflow. I have four tables:

A <--- Relation ---> B ---> Category

(So the relation between A and B is n to n, where the relation between B and Category is n to 1) Relation stores 'Intensity' of A in B. I need to calculate the intensity of A in each Category and find the Maximum result. It is achievable using:

A.objects.values(
    'id', 'Relation_set__B__Category_id'
).annotate(
     AcIntensity=Sum(F('Relation_set__Intensity'))
).aggregate(
     Max(F('AcIntensity'))
)['AcIntensity__max']

Now I need to filter the intensities based on some fields in B beforhand:

A.objects.values(
    'id', 'Relation_set__B__Category_id'
).filter(
    Relation_set__B__BType=type_filter
).annotate(
    AcIntensity=Sum(F('Relation_set__Intensity'))
).aggregate(
    Max(F('AcIntensity'))
)['AcIntensity__max']

However I need to avoid duplication resulted due to table join which messes the calculation up.(beside those field to define filtration, I do not need any fields in B)
Is there a way to achieve this using Django ORM?

Update
I think what I need is to limit the records in Relation table (based on B filters) before querying the database. How can I do that?
(Maybe using Prefetch_related and Prefetch?)

回答1:

Finally I've done it using conditional aggregation.
You could find more details in this stack-overflow post. So the final code would be:

result = A.objects.values(
    'id', 'Relation_set__B__Category_id'
).annotate(
    AcIntensity=Sum(
        Case(
            When(
               q_statement_for_filter,then=F('Relation_set__Intensity'),
            ),
            default=0,
            output_field=FloatField()
        )
    )
).exclude(
    AcIntensity=0
).aggregate(
    Max(F('AcIntensity'))
)['AcIntensity__max']

Notice that 'q_statement_for_filter' cannot be an empty Q().

来源：https://stackoverflow.com/questions/48742045/how-to-limit-records-in-tables-with-n-to-n-relation-before-joining-them-to-avo

标签

django

filter

orm

jointable

prefetch