On Jul 24, 2017 4:09 AM, "johan de taeye" <johan.de.ta...@gmail.com> wrote:
I have a model that has a foreign key relation to a number of other objects.
When saving an instance of this model from the admin (or a ModelForm), I
see plenty of extra and redundant database calls.
For a single record it wouldn't make much of a difference, but when using
the same ModeForm to do some batch upload these become the bottleneck in
the process.
Has anyone bumped into the same performance bottleneck?
Has anyone developed some solution for this?
By logging all database queries and some digging in the code, here's my
analysis of what is happening:
1. Open the admin editing screen for a single record.
I leave all fields to the original value, except for a field (not one of
the foreign key fields)
2. When saving the record, the first query reads the existing record:
select field1, field2, field3, .... from mytable;
3. During the form/model validation, I get an extra database query for
each of the foreign key fields.
It is generated from the to_python method of django.forms.models.
ModelChoiceField:
select field_a, field_b, field_c, field, ... from related_table
where pk = 'id_from_first_query';
4. During the form/model validation, I get another database query for
each of the foreign key fields.
It verifies that the values actually exists in the database:
select (1) from related_table where pk = 'value from form';
The queries in step 3 and 4 are redundant if the field hasn't changed. The
first query gives enough data to allow us to verify that the new form value
and the foreign key field on the existing instance are equal. I am using
django 1.11.
The same queries, except 2, are executed when I create a new record. The
queries in step 4 are redundant then - we just retrieved the values from
the database.
Looking forward to any insights and hints...
You should look at modifying the query set that your view is using to pull
the main object to include select_related() calls. I don't know if you're
using function -based views or class-based views, so I can't comment
further on implementation.
https://docs.djangoproject.com/en/1.11/ref/models/querysets/#select-related
The extra calls are likely occurring when the related fields are being
accessed during validation, etc.
Also keep in mind that Django (or any other framework) has no idea whether
or not fields have "changed" in a form submission without pulling the
original set of values to compare against, so expect the object to be
pulled at least once on every request.
-James
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