Call to model "__str__" when rendering a table with custom column

[jxrossel]

Hi,

When rendering a table associated with a model, the model "str" method is called for each row. In my case, this leads to many unnecessary queries (or the need to use "select_related" for undisplayed data). Can this be avoided ?

[jieter]

@jxrossel Can you share a minimal example for the models/table to reproduce this?

[jxrossel]

from django.db import models
import django_tables2 as dt

class Device(models.Model):
     
    name = models.CharField( _('name'), max_length=200, unique=True )
    
    def __str__(self):
        print( '__str__ called' )
        return self.name

class DeviceTable(dt.Table):
    class Meta:
        model = Device
        orderable = False
        fields = ['name']
    
    edit = dt.Column( verbose_name=_('Edit'), orderable=False, empty_values=[] )
    render_edit = lambda self: 'lala'
    

The problem occurs only if edit is defined as a custom column with custom rendering. If edit is removed and render_edit is replaced by render_name, it doesn't occur

[jieter]

This is a failing test without relying on print statements:
https://github.com/jieter/django-tables2/pull/514/files#diff-327b130c82fa4e83e97f6c1a50a0da1cR444

Stack trace leads to

django-tables2/django_tables2/rows.py

Line 234 in 8344601

column.current_value = self.get_cell(column.name)

[jieter]

The problem is: __repr__ is called in

django-tables2/django_tables2/utils.py

Line 333 in 8344601

raise ValueError('Failed lookup for key [%s] in %r'

, which in turn calls six.text_type(self) in the default implementation.

While I think this should be fixed, I also think your model's __str__ method should be relatively cheap to execute, or if it is, it should cache it's return value.

[jxrossel]

Thanks! Well, in my case, the str is not really expensive, but it requires multiple joins that I didn't need for this particular table. Thanks again for your work on django-tables2!

[jieter]

released 1.17.0