MRO and mixin in Python Django

What Happens When Fields Are Defined in Both the Mixin and the Model?

If a field (or method) is defined in both the mixin and the model:

✅ The model’s definition takes precedence.

Python’s method resolution order (MRO) ensures that the field or method defined in the subclass (your model) overrides the one from the mixin.

🧨 If You Remove Fields from the Model (but they exist in the mixin)

✅ What Happens in Python

• Your model still has those fields — because it inherits them from the mixin.

• You can still access obj.failed_scrapy_times, etc.

• No runtime errors.

⚠️ What Happens in Django Migrations

This is where things get tricky.

• Django tracks field definitions per model class, not per inheritance chain.

• If you remove a field from the model and it’s only defined in the mixin:

  • Django will think the field was deleted.

  • It will generate a migration to drop the column from the database.

🛡️ How to Prevent Unintended Schema Changes

✅ Option 1: Keep the fields in the model

Even if they’re inherited, explicitly define them in the model to keep Django’s migration system happy.

✅ Option 2: Use the mixin only during initial model creation

If you want the mixin to define fields, use it before the first migration — and don’t override those fields in the model later.

✅ Option 3: Manually edit migrations

If you know what you're doing, you can manually adjust the migration file to prevent dropping columns.

🧠 Summary

Action	Result in Python	Result in DB
Field defined in mixin only	Works fine	May be dropped by migration
Field defined in model	Overrides mixin	Safely tracked by Django
Field removed from model	Django may drop it	⚠️ Risk of data loss

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The best way is to only define methods inside mixin, fields should be defined inside the model

Defining only methods in mixins and keeping fields in the model ensures:

✅ Advantages of Method-Only Mixins

1. Migration Safety

• Django’s migration system only tracks fields defined directly in the model.

• No risk of fields being dropped or missed due to inheritance ambiguity.

2. Explicit Schema

• Your model’s fields are clearly visible and self-contained.

• Easier for other developers (or future you!) to understand the database structure.

3. Flexible Reuse

• Mixins can be reused across models with different field configurations.

• You can customize field behavior per model without touching the mixin.

4. Cleaner Debugging

• No surprises from inherited fields during introspection or admin customization.

🧠 Summary

Approach	Pros	Cons
Fields in mixin	DRY, reusable	Risky migrations, hidden schema
Fields in model, methods in mixin	Explicit, safe, flexible	Slight duplication across models