Question

I’d like to add at runtime methods to my django model, so that it’s viewable via the admin-interface. Sense: the user should be able to add an CalculationRule and a “virtual” table occurs with the applied rule.

# model.py
from django.db import models
import types

def rule(self):
  return 12 * self.calorific_requirement

class BuildingTable(models.Model):
    building_nr = models.IntegerField()
    square_meter = models.FloatField()
    calorific_requirement = models.FloatField()

    @staticmethod
    def __new__(cls, *args, **kwargs):
      setattr(cls, "rule", types.MethodType(rule, cls))
      return models.Model.__new__(cls, *args, **kwargs)

    def reqirement_per_m2(self):  # this is also a rule!
      return self.calorific_requirement / self.square_meter

    def __unicode__(self):
        return "Building " + str(self.building_nr)

class CalculationRule(models.Model):
    name = models.CharField(max_length=200)
    rule = models.CharField(max_length=200)

    def __unicode__(self):
        return str(self.name)

But if I add “rule” to my admin.py it raises BuldingTableAdmin.list_display[4], 'rule' is not a callable or an attribute of 'BuldingTableAdmin' or found in the model 'BuildingTable'.

# admin.py
from calccols_app.models import BuildingTable, CalculationRule
from django.contrib import admin

class BuldingTableAdmin(admin.ModelAdmin):
  list_display = ["building_nr", "square_meter", "calorific_requirement", "reqirement_per_m2", "rule"]

admin.site.register(BuildingTable, BuldingTableAdmin)
admin.site.register(CalculationRule)

I’ve tested BuildingTable in django’s shell; the rule-method exists, but doesn’t work properly. Any ideas how I can inject methods, so that django can handle/register them?

Answer 1

You do not need to create fields in your model to display it in the admin. You can do it in ModelAdmin entity.

class UserAdmin(admin.ModelAdmin):
    list_display = ['username', 'fullname']

    def fullname(self, model):
        return  model.first_name + " " + model.second_name

Although we can add a method to your model dynamically. But you must take it as a method, not a property:

def _status(invite):
    if invite.is_activated:
        return "Activated"
    if invite.is_alive:
        return "Live"
    return "Expired"

_status.short_description = 'status'
Invite.status = _status # Invite is model class

This “property” is normally displayed in the admin and Django-templates.