My Journey to a Robust E-commerce Order API
Hey Dev.to community! 👋
Today marks a significant milestone in my ongoing journey to build a robust e-commerce API using Django REST Framework (DRF). As a passionate #Python and #Django developer, I've been diving deep into backend development, and today's session was all about strengthening the foundation of my orders, products, and customers applications.
The core focus? Extensive Unit Testing and refining API serialization to handle complex data relationships.
The Challenge: A Mysterious IntegrityError
While testing my order creation endpoint via Swagger, I ran into a seemingly cryptic error in the console:
sqlite3.IntegrityError: NOT NULL constraint failed: orders_order.total_amount
This traceback clearly pointed to my Order model's total_amount field. The database was refusing to save an Order because total_amount was NULL, but my model definition (implicitly) required a non-null value.
Internal Server Error: /api/orders/
Traceback (most recent call last):
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\backends\utils.py", line 105, in _execute
return self.cursor.execute(sql, params)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\backends\sqlite3\base.py", line 329, in execute
return super().execute(query, params)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
sqlite3.IntegrityError: NOT NULL constraint failed: orders_order.total_amount
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\core\handlers\exception.py", line 55, in inner
response = get_response(request)
^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\core\handlers\base.py", line 197, in _get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\views\decorators\csrf.py", line 65, in _view_wrapper
return view_func(request, *args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\rest_framework\viewsets.py", line 124, in view
return self.dispatch(request, *args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\rest_framework\views.py", line 509, in dispatch
response = self.handle_exception(exc)
^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\rest_framework\views.py", line 469, in handle_exception
self.raise_uncaught_exception(exc)
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\rest_framework\views.py", line 480, in raise_uncaught_exception
raise exc
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\rest_framework\views.py", line 506, in dispatch
response = handler(request, *args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\rest_framework\mixins.py", line 19, in create
self.perform_create(serializer)
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\rest_framework\mixins.py", line 24, in perform_create
serializer.save()
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\rest_framework\serializers.py", line 208, in save
self.instance = self.create(validated_data)
^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\orders\serializers.py", line 26, in create
order = Order.objects.create(**validated_data)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\models\manager.py", line 87, in manager_method
return getattr(self.get_queryset(), name)(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\models\query.py", line 679, in create
obj.save(force_insert=True, using=self.db)
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\models\base.py", line 822, in save
self.save_base(
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\models\base.py", line 909, in save_base
updated = self._save_table(
^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\models\base.py", line 1071, in _save_table
results = self._do_insert(
^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\models\base.py", line 1112, in _do_insert
return manager._insert(
^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\models\manager.py", line 87, in manager_method
return getattr(self.get_queryset(), name)(*args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\models\query.py", line 1847, in _insert
return query.get_compiler(using=using).execute_sql(returning_fields)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\models\sql\compiler.py", line 1823, in execute_sql
cursor.execute(sql, params)
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\backends\utils.py", line 122, in execute
return super().execute(sql, params)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\backends\utils.py", line 79, in execute
return self._execute_with_wrappers(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\backends\utils.py", line 92, in _execute_with_wrappers
return executor(sql, params, many, context)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\backends\utils.py", line 100, in _execute
with self.db.wrap_database_errors:
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\utils.py", line 91, in __exit__
raise dj_exc_value.with_traceback(traceback) from exc_value
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\backends\utils.py", line 105, in _execute
return self.cursor.execute(sql, params)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "C:\Users\Nicol\Aprendizaje\Udemy\Python\DjangoRestFramework\env\Lib\site-packages\django\db\backends\sqlite3\base.py", line 329, in execute
return super().execute(query, params)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
django.db.utils.IntegrityError: NOT NULL constraint failed: orders_order.total_amount
[19/Jun/2025 02:06:41] "POST /api/orders/ HTTP/1.1" 500 32475
The Diagnosis: Where Model Tests Meet API Logic
Interestingly, my dedicated unit tests for the Product and Customer models were passing perfectly. This confirmed that my models were correctly defined and behaved as expected in isolation.
For example, for the Product model, my tests covered creation, default values, __str__ representation, updates (including auto_now fields like updated_at which required a small time.sleep for accurate testing!), and deletion.
This led me to understand that the issue wasn't within the model itself, but rather in how the DRF serializer was processing the incoming API request data before saving it to the database. The total_amount is a calculated field (derived from order items), and my API wasn't providing an initial value.
The Solution: Smart Serialization and Initializing Values
To resolve the NOT NULL constraint violation and streamline the API's behavior, I implemented key changes in my orders/serializers.py:
1. Making Calculated Fields Read-Only
Fields like order_date, total_amount, status (for initial creation), and price_at_order are typically generated or calculated by the backend, not provided by the client. Marking them as read_only_fields in the serializer's Meta class tells DRF to ignore them during input (deserialization) but include them in the output (serialization).
# orders/serializers.py (snippet)
# ... (other imports) ...
class OrderItemSerializer(serializers.ModelSerializer):
# ...
class Meta:
model = OrderItem
fields = ['id', 'product', 'product_id', 'quantity', 'price_at_order']
read_only_fields = ['price_at_order'] # Client shouldn't provide this, it's calculated.
class OrderSerializer(serializers.ModelSerializer):
# ...
class Meta:
model = Order
fields = ['id', 'customer', 'order_date', 'total_amount', 'status', 'items']
# These fields are either auto-generated or have a default,
# so they should not be provided by the client on creation.
read_only_fields = ['order_date', 'total_amount', 'status']
2. Ensuring Initial total_amount During Order Creation
Even after making total_amount read-only, the database still required a non-null value during the Order object's instantiation. I explicitly passed a default of 0.00 when creating the Order in the serializer's create method.
# orders/serializers.py (inside OrderSerializer's create method)
import decimal # Make sure this is at the top of your file!
class OrderSerializer(...):
# ...
def create(self, validated_data):
items_data = validated_data.pop('items') # Crucial: Extract nested items data
# Initialize total_amount to 0.00 to satisfy the NOT NULL constraint.
# This is particularly important if the model itself doesn't have a default.
order = Order.objects.create(total_amount=decimal.Decimal('0.00'), **validated_data)
for item_data in items_data:
product_instance = item_data.pop('product_id')
OrderItem.objects.create(
order=order,
product=product_instance,
**item_data
)
# Note: I removed an explicit call to order.calculate_total_amount() here.
# My Django signals (post_save/post_delete on OrderItem) are already configured
# to automatically update the Order's total when its items are saved or deleted.
# This keeps the serializer lean and relies on the model's self-maintaining logic.
return order
3. Handling Nested Relationships for Read/Write
For OrderItems, I used a powerful DRF pattern:
For Reading (GET requests): I use product = ProductSerializer(read_only=True) to show detailed product information nested within the OrderItem.
For Writing (POST/PUT requests): I use product_id = serializers.PrimaryKeyRelatedField(queryset=Product.objects.all(), write_only=True) to expect just the product's ID from the client, simplifying the input payload.
# orders/serializers.py (inside OrderItemSerializer)
# Assuming ProductSerializer is correctly imported from products.serializers
from products.serializers import ProductSerializer
class OrderItemSerializer(serializers.ModelSerializer):
product = ProductSerializer(read_only=True) # Full product details on read
product_id = serializers.PrimaryKeyRelatedField( # Product ID on write
queryset=Product.objects.all(),
write_only=True
)
# ...
The Sweet Taste of Success!
After implementing these changes and restarting my server, the API calls from Swagger were finally successful!
My console now shows:
[19/Jun/2025 02:57:30] "POST /api/orders/ HTTP/1.1" 201 378
[19/Jun/2025 02:57:59] "GET /api/orders/ HTTP/1.1" 200 742
This journey reinforced the immense value of:
Thorough Unit Testing: Pinpointing where the issue truly lies (model vs. serializer).
Understanding DRF's Mechanics: Especially read_only_fields and custom create/update methods for nested writes.
Data Integrity: Ensuring fields meet database constraints.
Django Signals: Leveraging them for automated calculations and maintaining data consistency.
I'm incredibly grateful for the guidance received throughout this process. Every debugged error is a massive learning opportunity!
Seeking Opportunities!
I'm actively looking for junior to mid-level #Python / #Django / #BackendDeveloper roles. If you're building exciting projects and need someone passionate about clean, tested, and robust code, I'd love to connect!
Feel free to reach out and check out my work:
GitHub: https://github.com/NicolasAndresCL/API_con_DRF
LinkedIn: https://www.linkedin.com/in/nicolas-andres-cano-leal/
Thanks for reading! What are your go-to strategies for handling calculated fields in DRF, or debugging IntegrityErrors? Share your thoughts in the comments!