Django-tables2: how to order_by accessor - django

Question

Say we have a column like:
num_member = tables.Column(accessor = 'members.count', verbose_name = 'number of members' )
When I tried to sort this in the template, it raises:
Field Error: Cannot resolve keyword u'count' into field
I read the document and it says we can use order_by by passing in some sort of accessor, but how exactly do we do this please?

Answer 1

For function like Model's property method, you can access it directly using accessor. For example:
Class MyModel(models.Model):
data= models.CharField(max_length=255)
#property
def print_function(self):
return 'hello world'
#Table class
class MyTable(tables.Table):
data= tables.Column(accessor='print_function')
class Meta:
model = MyModel
fields = ('data')
Using the above method, you can show different kinds of data in table using accessor:
Class SomeModel(models.Model):
some_data= models.CharField(max_length=255)
data= models.ManyToManyField(MyModel)
#property
def count_function(self):
some_data= self.data.objects.count() #returns count of the objects
return some_data
#Table class
class SomeTable(tables.Table):
data= tables.Column(accessor='count_function')
class Meta:
model = SomeModel
fields = ('data')
And accessor can be used for directly accessing related foreignkey model's field value like:
Class SomeModel(models.Model):
somedata= models.ForeignKey(MyModel)
#Table class
class MyTable(tables.Table):
data= tables.Column(accessor='somedata.data')
class Meta:
model = SomeModel
fields = ('data')
EDIT
Lets give an example of how order_by can be used:
#Class Based view, I am subclassing ListView and SingleTableView here for example
class MyView(ListView, SingleTableView):
def get_context_data(self, **kwargs):
context = super().get_context_data(**kwargs)
context['table'].order_by = '-last_updated' #last_updated is a datetimefield in model
return context
In the above code, what I have done is that, I am changing the order of the table data in context which will later be rendered in template.

Answer 2

Fairly old question, however, I have been confronted with the same problem today: I couldn't order my tables if the accessor was a property (or 0-argument-method) and not a model field.
After not finding anything in the docs and inspecting the source code, it turned out that tables2 will pass the ordering to the database if its data is a QuerySet, but otherwise it will do a Python list sort with an appropriate key:
# django_tables2/tables.py -> class TableData
def order_by(self, aliases):
# ...
if hasattr(self, "queryset"):
translate = lambda accessor: accessor.replace(Accessor.SEPARATOR, QUERYSET_ACCESSOR_SEPARATOR)
if accessors:
self.queryset = self.queryset.order_by(*(translate(a) for a in accessors))
else:
self.list.sort(key=OrderByTuple(accessors).key)
I assume that this can not be trivially solved by using a try-except instead of the if-else because an exception would only be raised once the queryset is evaluated which only happens later.
Solution: whenever your sort-parameter is not a model field, turn the QuerySet into a list before handing it to the table. For many cases in django, this will be as simple as overriding get_queryset:
def get_queryset(self):
qs = super(ViewName, self).get_queryset()
return list(qs)
This should work best if your accessor is a cached_property on the model of your table, e.g.:
from django.utils.functional import cached_property
#cached_property
def member_count(self):
# do the heavy stuff here in the model
return whatever
Then, in the table:
num_member = tables.Column(
accessor='members_count',
verbose_name='number of members'
)