Adding a `has_rated` Field to Your API Response with Ninja

This post explores how to enhance your API responses in a Django application using Ninja, a fast, asynchronous-friendly web framework. We’ll focus on adding a boolean field, has_rated, to indicate whether a user has already rated a particular company associated with a transaction.

Scenario

Imagine you have a system where users can make transactions and then rate the companies involved in those transactions. When a user requests details about a specific transaction, you want to include information about whether they have already rated the company linked to that transaction.

The Code

Here’s the relevant code snippet, broken down for clarity:

try:
    user_transaction = UserTransactions.objects.select_related("company", "user").get(transaction_id=transaction_id)
except UserTransactions.DoesNotExist:
    return JsonResponse({"msg": "transaction does not exist"}, safe=True, status=400)

if user_transaction.user_id != user_id:
    return JsonResponse({"msg": "invalid transaction (not yours)"}, safe=True, status=400)

if hasattr(user_transaction, "company"):
    if user_transaction.company is not None:
        company_id = user_transaction.company.id
        has_rated = BrandRating.objects.filter(user_id=user_id, company_id=company_id).exists()

        user_transaction.has_rated = has_rated

return user_transaction

Explanation

1. Transaction Retrieval: The code first attempts to retrieve a UserTransactions object based on the provided transaction_id. select_related("company", "user") optimizes the query by fetching related company and user objects in the same database query, preventing multiple database hits (improving performance).
2. Error Handling: If the transaction doesn’t exist, a JsonResponse with a 400 status code is returned, indicating a bad request.
3. Authorization Check: It then verifies that the transaction belongs to the requesting user (user_id). If not, another error response is returned.
4. Company and Rating Check: The core logic resides within the if hasattr(user_transaction, "company") block. This checks if the user_transaction object has a company attribute. Inside this block, it checks if the company is not None. If both conditions are met:
   • The company_id is extracted.
   • A query is performed on the BrandRating model to check if a rating exists for the given user_id and company_id. The .exists() method is used for efficiency; it returns True if any matching records are found, and False otherwise, without fetching the actual rating objects.
   • Finally, the has_rated attribute is dynamically added to the user_transaction object and set to the boolean result of the rating check.
5. Return Value: The modified user_transaction object, now including the has_rated attribute, is returned.

Integration with Ninja

In a Ninja API, you’d typically serialize the user_transaction object using a Pydantic schema. Make sure your Pydantic schema for UserTransaction includes has_rated: bool as an optional field. This allows Ninja to automatically include the has_rated field in the JSON response.

Benefits

• Clear API Response: Provides clients with immediate information on whether a user has rated a company, improving user experience.
• Efficient Database Queries: Uses select_related and exists() for optimized database access.
• Dynamic Data Enrichment: Demonstrates how to dynamically add information to existing objects before returning them in an API response.

Considerations

• Performance: For very large datasets, consider caching the rating information to avoid repeated database queries.
• Data Integrity: Ensure proper indexing on the BrandRating table for user_id and company_id to optimize the rating check query.

By following this pattern, you can effectively enrich your API responses with calculated data, providing more context and value to your API consumers.