Notes about the Django Framework

django-notes.rst

Django ORM

pet_set is a lazy object that only makes a call to the dtabase when you begin to iterate over it. When the queryset is evaluated it will caches the results so latter calls to pet_set will not also call the database.

pet_set = Pet.objects.filter(species="Dog")
# The query is executed and cached.
for pet in pet_set:
    print(pet.first_name)
# The cache is used for subsequent iteration.
for pet in pet_set:
    print(pet.last_name)

The "if" statement will evaluate the queryset and cache the results so that only one call to the database is made.

You can avoid evaluating and potentially caching a largy queryset by calling queryset.exists() which just checks if at least one row in the database matches.

restaurant_set = Restaurant.objects.filter(cuisine="Indian")
if restaurant_set.exists():
    print("Found some resturaunts!")

# The `if` statement evaluates the queryset.
if restaurant_set:
    # The cache is used for subsequent iteration.
    for restaurant in restaurant_set:
        print(restaurant.name)

you can evaluate a queryset without caching the resutls by calling iterator()

iterate over large dataset example

pet_set = Pet.objects.all()
pet_iterator = pet_set.iterator()
#Look at first item in the iterator
try:
    pet_one = next(pet_iterator)
except:
    #No rows found, so no pets in the set
    pass
else:
from itertools import chain
    for pet in chain([pet_one], pet_set):
        print(pet.name)

via http://blog.etianen.com/blog/2013/06/08/django-querysets/

Model Forms with m2m Data

If a m2m field exists in the form, but is not rendered in the template then all m2m data for a given instance will be lost. For example if you fill out a DogForm then add toys via the DogToyForm then go back and edit DogForm then all his toys will be lost!

#in models.py
class DogToy(models.Model):
    name = models.CharField(max_length=60, blank=True)

class Dog(models.Model):
    color = models.CharField(max_length=60, blank=True)
    toys = models.ManyToManyField('DogToy', blank=True)

#in forms.py
class DogForm(forms.ModelForm):
    class Meta:
        model = Dog

class DogToyForm(forms.ModelForm):
    class Meta:
        model = DogToys

#in template
<form action="." method="post">
    {% csrf_token %}
    {{ form.color }}
    <input type="submit" />
</form>

So if you wish to allow a dogs information to be edited in a form that does not include photos it is import that you exclude forms that will not be rendered in the template. In the example above the DogToyForm should be changed to the following

class DogToyForm(forms.ModelForm):
    class Meta:
        model = DogToys
        exclude = ('toys')