Split complex migrations incrementally

Break complex schema changes into multiple, sequential migrations to ensure deployment safety and proper data handling. Each migration should represent a single, atomic change that can succeed or fail independently.

Why this matters:

• Multiple migrations in one PR can cause deployment failures if one succeeds and another fails, leaving the database out of sync with the codebase
• Complex changes are safer when split into logical steps that can be rolled back individually
• Incremental approach allows for safer data migration and validation at each step

Recommended approach:

1. Add nullable field - Introduce new columns as nullable first
2. Migrate data - Populate the new field with appropriate values
3. Add constraints - Apply NOT NULL constraints or other restrictions after data is migrated
4. Remove old fields - Deprecate or remove old columns in separate migration

Example: Instead of creating and altering in one migration:

# Avoid: Complex migration doing multiple operations
operations = [
    migrations.AddField(model_name="cohort", name="cohort_type", field=models.CharField(max_length=20, choices=CHOICES)),
    migrations.AlterField(model_name="cohort", name="cohort_type", field=models.CharField(max_length=20, choices=CHOICES, null=False)),
]

Prefer incremental steps across separate PRs:

# Step 1: Add nullable field
field=models.CharField(max_length=20, choices=CHOICES, null=True, blank=True)

# Step 2 (separate PR): Migrate existing data
# Step 3 (separate PR): Drop null constraint

One migration per PR - Keep each PR focused on a single migration to avoid deployment synchronization issues.