Django ORM access User table through multiple models - django

Question

views.py
I'm creating a queryset that I want to serialize and return as JSON. The queryset looks like this:
all_objects = Program.objects.all()
test_data = serializers.serialize("json", all_objects, use_natural_keys=True)
This pulls back everything except for the 'User' model (which is linked across two models).
models.py
from django.db import models
from django.contrib.auth.models import User
class Time(models.Model):
user = models.ForeignKey(User)
...
class CostCode(models.Model):
program_name = models.TextField()
...
class Program(models.Model):
time = models.ForeignKey(Time)
program_select = models.ForeignKey(CostCode)
...
Question
My returned data has Time, Program, and CostCode information, but I'm unable to query back the 'User' table. How can I get back say the 'username' (from User Table) in the same queryset?
Note: I've changed my queryset to all_objects = Time.objects.all() and this gets User info, but then it doesn't pull in 'CostCode'. My models also have ModelManagers that return the get_by_natural_key so the relevant fields appear in my JSON.
Ultimately, I want data from all four models to appear in my serialized JSON fields, I'm just missing 'username'.
Here's a picture of how the JSON object currently appears in Firebug:
Thanks for any help!

Answer 1

It seems a bit heavyweight at first glance but you could look at using Django REST Framework:
http://www.django-rest-framework.org/api-guide/serializers#modelserializer
You can define and use the serializer classes without having to do anything else with the framework. The serializer returns a python dict which can then be easily dumped to JSON.
To get all fields from each related model as nested dicts you could do:
class ProgramSerializer(serializers.ModelSerializer):
class Meta:
model = Program
depth = 2
all_objects = Program.objects.all()
serializer = ProgramSerializer(all_objects, many=True)
json_str = json.dumps(serializer.data)
To customise which fields are included for each model you will need to define a ModelSerializer class for each of your models, for example to output only the username for the time.user:
class UserSerializer(serializers.ModelSerializer):
class Meta:
model = User
fields = ('username', )
class TimeSerializer(serializers.ModelSerializer):
"""
specifying the field here rather than relying on `depth` to automatically
render nested relations allows us to specify a custom serializer class
"""
user = UserSerializer()
class Meta:
model = Time
class ProgramSerializer(serializers.ModelSerializer):
time = TimeSerializer()
class Meta:
model = Program
depth = 1 # render nested CostCode with default output
all_objects = Program.objects.all()
serializer = ProgramSerializer(all_objects, many=True)
json_str = json.dumps(serializer.data)

Answer 2

What you really want is a "deep" serialization of objects which Django does not natively support. This is a common problem, and it is discussed in detail here: Serializing Foreign Key objects in Django. See that question for some alternatives.
Normally Django expects you to serialize the Time, CostCode, Program, and User objects separately (i.e. a separate JSON array for each) and to refer to them by IDs. The IDs can either be the numeric primary keys (PKs) or a "natural" key defined with natural_key.
You could use natural_key to return any fields you want, including user.username. Alternatively, you could define a custom serializer output whatever you want there. Either of these approaches will probably make it impossible to load the data back into a Django database, which may not be a problem for you.