Django-tables2基于模型的表-具有计算的自定义列

Question

I have a model with various properties on, the most complex of which looks up the number of hours worked on a project, multiplies by the employee's hourly rate, and returns a sum total of the cost.

class DesignDetails(models.Model):

@property
def allowed_days(self):
    return BudgetItem.objects.get(budget__project=self.project, budget__current_marker=1, name=design_time_string).quantity

@property
def actual_cost(self):
    total = 0
    design_hours = self.project.designhours_set.exclude(hours_2=None)

    for hours in design_hours:
        dh2 = hours.hours_2
        if dh2:
            rate = hours.daily_record.employee.hourly_rate
            if not rate:
                rate = 30
            total += (dh2*rate)
    return total

@property
def cost_allowed(self):
    return self.allowed_days * design_rate * hours_in_day

@property
def cost_difference(self):
    return Decimal(self.actual_cost) - self.cost_allowed

I'm displaying this using Django-tables2. Ideally I'd like to calculate the cost_difference 'on the fly' in the table, and avoid having to recalculate the actual_cost, rather than having it as a property on the model.

How can I set up a column to use the value of other columns? Alternatively, should I be using something other than a property on the model itself?

I'm also considering abandoning Django-tables2, as I'm not sure it suits my needs here?

Thanks

Answer 1

You could use Django's cached_property decorator, and then the value will only be calculated once for each instance.

from django.utils.functional import cached_property

class DesignDetails(models.Model):

    @cached_propery
        def actual_cost(self):
            ...

I think that using a (cached) property on the model is an ok approach. I wouldn't recommend moving the logic into Django tables 2. It will be easier to reuse the code if it's on the model.