Limiting the number of records - django

Question

I have one question, the answer I cant find in Google.
So, I have the model of cars (only char fields) and model of pictures of that cars:
class Pictures(models.Model):
car = models.ForeignKey(Car,related_name='pictures')
path = models.CharField(max_length=255)
about = models.CharField(max_length=32)
main = models.BooleanField('Main Picture')
def __unicode__(self):
return str(self.id)
Its possible to do the following: I want only 4 pictures for one car. For example, BMW X5 - has only 4 pictures, and I cant add 5 pictures (from the admin interface). One car - 4 pictures (max). Its possible?
As you can see, I have field main, picture with this field=1 will apear in gallery, like a start picture in slide show. But I can add main=1 for all pictures, and its wrong. So, its possible do the following:
1 car = 3 pictures(main=0) + 1 picture(main=1)
One car has 4 positions (max) in pictures table (4 pictures), and only one of that pictures has main=1. In admin interface I can add many pictures for one car and add main=1 for all pictures. How it can be limited?
Thanks

Answer 1

class MyModelAdmin(admin.ModelAdmin):
...........
def has_add_permission(self, request):
count = Pictures.objects.filter(main=True).count()
if count <= 4:
return True
return False

Answer 2

Validate your model with a custom form and clean method.
model.py:
class Image(models.Model):
image = models.ImageField(upload_to='images/%Y/%m/%d', ...)
main = models.BooleanField()
# Distinguish between main and normal images
def __unicode__(self):
if self.main:
return "%s (main)" %self.image
else:
return self.image
class Car(models.Model):
...
images = models.ManyToManyField(Image, blank=True, null=True,
related_name="%(app_label)s_%(class)s_related",
help_text="Max 1 main image and max 4 images total.")
Then in your admin.py create a ModelForm with custom clean method:
from django import forms
from django.contrib import admin
from models import Image, Car
class CarAdminForm(forms.ModelForm):
class Meta:
model = Car
def clean_images(self):
data = self.cleaned_data['images']
if data.count() > 4:
raise forms.ValidationError("Max 4 images.")
if data.filter(main=True).count() > 1:
raise forms.ValidationError("Max 1 main image.")
return data
class CarAdmin(admin.ModelAdmin):
form = CarAdminForm
filter_horizontal = ['images', ]
...
admin.site.register(Car, CarAdmin)
Bonus: to make a main image required:
if not data.filter(main=True).count() == 1:
raise forms.ValidationError("Exact 1 main image required!")