Django three way join using Foreign Keys - django

Question

I recently started evaluating Django for migrating our archaic web application written 10 years ago. I have been reading up Django documentation for the last few days, but haven't been able to figure out the best way to achieve a multi table database join in my case:
Model:
class Product(models.Model):
productid = models.IntegerField(primary_key=True, db_column='ProductId')
productname = models.CharField(max_length=120, db_column='ProductName')
class Testcases(models.Model):
testcaseid = models.IntegerField(primary_key=True, db_column='TestCaseId')
testcasename = models.CharField(max_length=240, db_column='TestCaseName')
class Testmatrix(models.Model):
testmatrixid = models.IntegerField(primary_key=True, db_column='TestMatrixId')
productid = models.ForeignKey(Product, db_column='ProductId')
testcaseid = models.ForeignKey(Testcases, db_column='TestCaseId')
class Status(models.Model):
testmatrixid = models.ForeignKey(Testmatrix, db_column='TestMatrixId')
title = models.CharField(max_length=240, db_column='Title', blank=True)
(Note that model was generated by inspectdb and I'd prefer not to modify it at this point in time)
View:
from django.shortcuts import render_to_response
from mysite.testmatrix.models import Product, Testcases, Testmatrix, Status
def get_products(request):
tm = list(Testmatrix.objects.filter(productid='abc'))
return render_to_response('products.html', {'tm': tm})
template is designed to be minimal at this point to help focus on the real issue in (views/model).
Template: (products.html)
{% extends "main.html" %}
{% block body %}
<table>
{% for tm in tm %}
<tr>
<td>{{ tm.testmatrixid }}</td>
<td>{{ tm.testcaseid.testcasename }}</td>
</tr>
{% endfor %}
</table>
{% endblock %}
Problem:
Although I'm able to join Testmatrix and Testcase models, I am unable to generate an equivalent queryset by joining all of TestMatrix, TestCase, Status records on say productid='abc'
I tried the following:
1) Use select_related between Testmatrix and Testcases and Product tables and was able to access attributes across all three models (testmatrixid, productid, productname, testcaseid, testcasename). However I'm not sure how to extend this auto foreign key referencing to Status model. This would have been easier if all Foreign Keys were defined within Testmatrix itself. But Status has a Foreign Key to TestMatrix.
2) I tried using something like: entries = Status.objects.filter(testmatrixid__productid=pid). This again gave me a queryset as a result of joining Testmatrix and Status, but not Testcases.
Pardon any blaring mistakes or bloopers. This is my very first post!

Answer 1

So you need to access a related_object. It is very simple.
First, add related_name here:
class Status(models.Model):
testmatrixid = models.ForeignKey(Testmatrix, db_column='TestMatrixId', related_name='statuses')
Now you can get all the statuses for desired Testmatrix like
test_matrix.statuses.all()
If you don't want to hit DB, when you access statuses, don't forget to use select_related.

Answer 2

Without any specific error messages, it is hard to diagnose the cause of said errors. However, in your example, in views.get_products: tm = list(Testmatrix.objects.filter(productid='abc')) will not work, because 'abc' is a string and your productid is actually a Product object (not just an integer, even though the field is an integer foreign key to your reference table's pk); you can do tm = list(Testmatrix.objects.filter(productid=Product.objects.get(product_name='abc')), assuming that 'abc' is the product name of the product record. When you set a field to models.ForeignKey(...), you address that reference record as an object, not an id.
Other than that, nothing blaring, your template looks solid and your models look fine to me. I would suggest creating some test cases to see where the errors lie: Django Testing; Also, this is also a great tutorial to understand TDD and unit testing with Django. Using unittests, you can verify every step of your application and make future updates with assurance.