When and in which cases is it recommended to create custom migrations instead of using standard ones?

In Python, when working with migrations (for example, in Django), standard migrations are generated automatically based on model changes. However, there are situations where it is recommended to create custom migrations manually:

• When you need to perform complex data operations that cannot be expressed through standard schema changes (e.g., data migration or format transformation).
• When you need to optimize migrations or add custom SQL code.
• If automatic migration generation does not correctly reflect changes or leads to errors.
• To create migrations that depend on external conditions or require executing specific code.

Example of creating a custom migration in Django:

from django.db import migrations

def forwards_func(apps, schema_editor):
    MyModel = apps.get_model('myapp', 'MyModel')
    for obj in MyModel.objects.all():
        obj.field = transform(obj.field)
        obj.save()

class Migration(migrations.Migration):
    dependencies = [
        ('myapp', '0001_initial'),
    ]

    operations = [
        migrations.RunPython(forwards_func),
    ]

Thus, custom migrations are needed for more precise control and to perform non-standard operations when changing the database.