Fetch Data from DB in base template Django Python

Question 1

You can either write a template context processor or a template tag.

Template context processor

settings.py

from django.conf import global_settings
TEMPLATE_CONTEXT_PROCESSORS = global_settings.TEMPLATE_CONTEXT_PROCESSORS + (
    "myapp.processor.foos",
)

myapp/processor.py

from myproject.myapp.models import Foo

def foos(request):
    return {'foos': Foo.objects.all()}

and then you can use {{ foos }} in any template

providing you pass the context_instance in your render_to_response

return render_to_response('index.html', {}, context_instance=RequestContext(request))

Template tags

Question 2

You want to use template context processors. I couldn't find a very clear description in the Django documentation, but template context processors are basically callables that are called with a request, and return a dictionary of context variables to be made available for all templates (not strictly all templates, but if you use django.shortcuts.render, you're covered).

Defining your active template context processors is a bit awkward, because you cannot just add your processor to the list of default processors, and you usually want to keep the default ones as well, so you need to dump the entire list of the default processors plus your processor in your settings file:

In your settings:

TEMPLATE_CONTEXT_PROCESSORS = (

    # the default processors:
    "django.contrib.auth.context_processors.auth",
    "django.core.context_processors.debug",
    "django.core.context_processors.i18n",
    "django.core.context_processors.media",
    "django.core.context_processors.static",
    "django.core.context_processors.tz",
    "django.contrib.messages.context_processors.messages",

    # your processor that will populate the context with a list of categories:
    "myapp.context_processors.categories",

)

In your myapp/context_processors.py:

def categories(request):
    return {
        'categories': Category.objects.all()
    }

Modify filenames and category retrieval as needed.

Question 3

Bringing this one up to date.

TEMPLATE_CONTEXT_PROCESSORS isn't available in the latest versions and is now within the TEMPLATES setting.

For example using the accepted answer:

project/settings.py

TEMPLATES = [
{
    'BACKEND': 'django.template.backends.django.DjangoTemplates',
    'DIRS': [BASE_DIR / 'templates'],
    'APP_DIRS': True,
    'OPTIONS': {
        'context_processors': [
            # Default processors
            'django.template.context_processors.debug',
            'django.template.context_processors.request',
            'django.contrib.auth.context_processors.auth',
            'django.contrib.messages.context_processors.messages',
            
            # Custom processor
            'myapp.context_processors.foos',
        ],
    },
},

]

myapp/context_processors.py

from .models import Foo

def foos(request):
    return {'foos': Foo.objects.all()}

Then use {{ foos }} in all templates.