How to resolve Django migration errors from triggers.
Tags: webdev, django, python, postgres
Published: 2024-01-23 13:21:03

**Prerequisites**

- Intermediate-level Python.

- Command line interface knowledge.

- Django Framework knowledge.

- PostgreSQL and databases.

This error prevents you from executing schema changes on a table with enabled triggers. This post will explore why this error occurs and how to resolve it.

This is what the error looks like:
`django.db.utils.OperationalError: cannot ALTER TABLE "mytable" because it has pending trigger events`

**Understanding Django migration errors**
What's happening when we get this error?

In Django, makemigrations generates migration files that describe schema changes to your models—for example, adding a new field. migrate then applies these migrations by running ALTER TABLE statements.

However, PostgreSQL does not allow ALTER TABLE on tables that have enabled triggers. So if your table has triggers, the migrate step fails with the “pending trigger events” error.

**Why do triggers block alters**
Trigger’s fire in response to events like inserts, updates, or deletes. They allow you to perform actions like validating input or logging history.

Triggers also introduce some database locking issues. If PostgreSQL allowed an ALTER while triggers were enabled, it could lead to inconsistent data.

As a result, PostgreSQL completely blocks ALTER TABLE unless you disable or drop existing triggers first. Django migrations obey this constraint and error out.

Now that we understand the cause, what solutions exist for applying migrations?

1. **Temporarily disabling triggers**
One option is to temporarily disable existing triggers on the table during the migration.

```
ALTER TABLE mytable DISABLE TRIGGER ALL; 

# Run migrations with python manage.py migrate

ALTER TABLE mytable ENABLE TRIGGER ALL;
```
This unblocks Django from running ALTER TABLE, allowing the schema changes to apply.

Directly manipulating triggers can be tedious and error-prone. You need to remember to re-enable the trigger’s after.

2. **Manually drop and recreate triggers**
You can drop triggers before migrating.

```
DROP TRIGGER IF EXISTS my_insert_trigger on mytable;

# Run migrations  

CREATE TRIGGER ... 
```
This also gets Django migrations working. But again, it requires manually rebuilding your trigger logic.

3.**Use Djangos Schema editor**
Django offers a higher-level API for managing triggers and migrations cleanly.

The django.db.backends.base.schema.BaseDatabaseSchemaEditor class allows overriding schema modifying commands. This includes hooks to drop and recreate triggers automatically around a migration.

For example:

```
from django.db import migrations

class Migration(migrations.Migration):

    operations = [
        migrations.AddField( ... ) 
        ...
    ]

    def drop_triggers(self, schema_editor, schema_migration):
        schema_editor.execute("DROP TRIGGER...") 

    def recreate_triggers(self, schema_editor, schema_migration):
        schema_editor.execute("CREATE TRIGGER...")
```

Now triggers will automatically drop and recreate around the migration operation.

**A few other solutions**

- Create a replica PostgreSQL database for running migrations, without triggers enabled. Then sync the applied schema changes back to the primary database.

- Disable triggers directly in PostgreSQL by setting session_replication_role to replica during migration.

- ImproperlyConfigured the migration files to temporarily drop triggers by overriding SchemaEditor commands.

In most cases, the Django SchemaEditor hooks provide the best development experience for managing triggers. But in complex deployments, alternate solutions like migrations replicas may be necessary.

**Conclusion**

PostgreSQL triggers causing Django migration issues can be rather annoying. However, by comprehending the situation, you can identify a workaround that works.

The remainder of your trigger logic can still function when triggers are temporarily dropped, either manually or with SchemaEditor. Even with triggers enabled, you can continue to work productively on Django schema migrations if you have the correct approach.

Don't be shy, ask some questions in the comments.