How to remove duplicates from the database table, alter the model with unique=True and makemigration-CodePudding

class CommonInfo(models.Model):
    name = models.CharField(max_length = 100)
    age = models.PositiveIntegerField()
    class Meta:
        abstract=True
        ordering=['name']
class Student(CommonInfo):
    home_group =models.CharField(max_length=5)
    class Meta(CommonInfo.Meta):
        db_table='student_info'

I have a similar database model with existing data. I want to add a uique=True on the field name. Is there any way I could remove the existing duplicate data before I alter the field name as unique?

CodePudding user response：

You can include the remove duplicates logic in your migration, for example:

Assuming that you have an ExampleModel with an example_field already migrated and with data, first, you need to include the unique constraint into that field:

class TestModel(models.Model):
    ...
    example_field = models.CharField(max_length=32, unique=True)
    ...

Then make the migration to that model with makemigrations, once you have a migration script ready, you can modify it including an additional operation before the AlterField operation, something like:

...

def remove_duplicates(apps, schema_editor):
    ExampleModel = apps.get_model("example_app", "ExampleModel")
    db_alias = schema_editor.connection.alias

    # Get the unique values for the "example_field"
    unique_value_list = ExampleModel.objects.using(db_alias).values_list('example_field', flat=True).distinct()

    for unique_value in unique_value_list:
        # Get ids for a specific value except the first one
        pks = (
            ExampleModel.objects.using(db_alias)
            .filter(example_field=unique_value)
            .values_list('id', flat=True)[1:]
        )

        # Remove selected ids
        ExampleModel.objects.using(db_alias).filter(id__in=pks).delete()


class Migration(migrations.Migration):
    ...
    operations = [
        migrations.RunPython(remove_duplicates), # Include the remove duplicates operation
        migrations.AlterField(
            ...

Now, your migration should work normally.

This is a very simple example, you might need to do some additional things like lock the table while running the migration to avoid possible errors during the migration, but hope it is clear.

CodePudding user response：

First check whether other field depend on the row and then you can -- During development, open the database, go to the table find the row and delete.