Tutorial

Overview

Postgres triggers provide the ability to specify database actions that should occur when operations happen in the database (INSERT, UPDATE, DELETE, TRUNCATE) on certain conditions of the affected rows.

The pgtrigger.Trigger object is the base class for all triggers. Attributes of this class mirror the syntax required for making a Postgres trigger, and one has the ability to input the exact PL/pgSQL code that is executed by Postgres in the trigger. pgtrigger also has several helper classes, like pgtrigger.Protect, that implement some core triggers you can configure without having to write PL/pgSQL syntax.

When declaring a trigger, one can provide the following core attributes:

• when

  When the trigger should happen. Can be one of pgtrigger.Before or pgtrigger.After to execute the trigger before or after an operation. One can use pgtrigger.InsteadOf for row-level operations of a view.

  Note

  pgtrigger.Before and pgtrigger.After can be used on SQL views as well as tables under some circumstances. See the docs for a breakdown of when these constructs can be used for various types of operations.

• operation

  The operation which triggers execution of the trigger function. This can be one of pgtrigger.Update, pgtrigger.Insert, pgtrigger.Delete, pgtrigger.Truncate, or pgtrigger.UpdateOf. All of these can be OR ed together (e.g. pgtrigger.Insert | pgtrigger.Update) to configure triggering on a combination of operations.

  Note

  pgtrigger.UpdateOf is triggered when columns appear in an UPDATE statement. It will not be triggered if other triggers edit columns. See the notes in the Postgres docs for more information about this use case.

  Note

  Some conditions cannot be combined together for a valid trigger. For example, pgtrigger.UpdateOf cannot be combined with other operations.

• condition (optional)

  An optional condition on which the trigger is executed based on the OLD and NEW row variables that are part of the trigger.

  One can use the pgtrigger.Condition class to write a free-form clause (e.g. OLD.value = "value"). The pgtrigger.Q condition also mimics Django’s Q object to specify a filter clause on the affected rows. For example, a condition of pgtrigger.Q(old__value='hello') will only trigger when the old row’s value field is hello.

  Note

  Be sure to familiarize yourself with OLD and NEW when writing conditions by consulting the Postgres docs. For example, the OLD row in pgtrigger.Insert triggers is always NULL and the NEW row in pgtrigger.Delete triggers is always NULL. OLD and NEW is always NULL for pgtrigger.Statement triggers as well. One must keep these caveats in mind when constructing triggers to avoid unexpected behavior.

• name (optional)

  Registers the trigger with a human-readable name so that it can be referenced in other django-pgtrigger functionality such as pgtrigger.ignore.

• level (optional, default=pgtrigger.Row)

  Declares if the trigger fires for every row (pgtrigger.Row) or every statement (pgtrigger.Statement). Defaults to pgtrigger.Row.

• referencing (optional)

  When constructing a statement-level trigger, allows one to reference the OLD and NEW rows as transition tables using the pgtrigger.Referencing construct. For example, pgtrigger.Referencing(old='old_table_name', new='new_table_name') will make an old_table_name and new_table_name table available as transition tables in the statement-level trigger. See this for an additional explanation on the referencing construct and read the trigger cookbook later for an example.

  Note

  The REFERENCING construct for statement-level triggers is only available in Postgres10 and up.

Installing triggers for models

Similar to the Django admin, pgtrigger triggers are registered to models using the pgtrigger.register decorator. The decorator takes a variable amount of pgtrigger.Trigger objects that should be installed for the model.

For example, this trigger definition protects this model from being deleted:

from django.db import models
import pgtrigger


@pgtrigger.register(
    pgtrigger.Protect(operation=pgtrigger.Delete)
)
class CannotDelete(models.Model):
    field = models.CharField(max_length=16)

The triggers are installed automatically when running manage.py migrate. If a trigger definition is removed from the project, the triggers will be removed in the database. If the trigger changes, the new one will be created and the old one will be dropped during migrations.

To turn off creating triggers in migrations, configure the PGTRIGGER_INSTALL_ON_MIGRATE setting to False. Triggers can manually be configured with the following code:

• pgtrigger.install: Install triggers

• pgtrigger.uninstall: Uninstall triggers

• pgtrigger.enable: Enable triggers

• pgtrigger.disable: Disable triggers

Note

If triggers are disabled (as opposed to uninstalled), they have to be re-enabled again and will not be re-enabled automatically during migrations.

Warning

Installing, uninstalling, enabling, and disabling are table-level operations that call ALTER on the table. This is a global operation and will affect all running processing. Do not use these methods in application code. If you want to ignore a trigger dynamically in an application, using pgtrigger.ignore, which is covered in the next section.

Ignoring trigger execution dynamically

As mentioned previously, one should avoid using pgtrigger.install, pgtrigger.uninstall, pgtrigger.enable, and pgtrigger.disable in application code. Use pgtrigger.ignore to dynamically ignore a trigger in a single thread of execution.

When using pgtrigger.ignore, django-pgconnection is used to dynamically set a Postgres variable that trigger objects parse and determine if they should proceed.

Because of this, the user needs to additionally set up their project with django-pgconnection to use this feature. To do this, make sure settings.DATABASES is wrapped in pgconnection.configure() in settings.py like so:

import pgconnection

DATABASES = pgconnection.configure({
    'default': {
        # default database config..
    }
})

To ignore a trigger, first be sure that a name has been provided to the trigger, and then reference the model and the trigger name with the pgtrigger.ignore context manager. Here’s an example of a model that is protected against deletes and uses pgtrigger.ignore to ignore the trigger:

from django.db import models
import pgtrigger


@pgtrigger.register(
    pgtrigger.Protect(name='protect_deletes', operation=pgtrigger.Delete)
)
class CannotDelete(models.Model):
    pass


# Bypass deletion protection
with pgtrigger.ignore('my_app.CannotDelete:protect_deletes'):
    CannotDelete.objects.all().delete()

As shown above, pgtrigger.ignore takes a trigger URI that is formatted as {app_label}.{model_name}:{trigger_name}. Multiple trigger URIs can be given to pgtrigger.ignore, and pgtrigger.ignore can be nested. If no triggers are provided, all triggers are ignored.

Although one should strive to create triggers that produce a consistent database (and thus use pgtrigger.ignore sparingly), one practical use case of pgtrigger.ignore is making an “official” interface for doing an operation. See Official interfaces in the trigger cookbook for an example.

Trigger cookbook

Here are a few more examples of how you can configure triggers using the utilities in pgtrigger.

Keeping a field in-sync with another

We can register a pgtrigger.Trigger before an update or insert to ensure that two fields remain in sync.

import pgtrigger


@pgtrigger.register(
    pgtrigger.Trigger(
        operation=pgtrigger.Update | pgtrigger.Insert,
        when=pgtrigger.Before,
        func='NEW.in_sync_int = NEW.int_field; RETURN NEW;',
    )
)
class MyModel(models.Model):
    int_field = models.IntField()
    in_sync_int = models.IntField(help_text='Stays the same as "int_field"')

Note

When writing a pgtrigger.Before trigger, be sure to return the row over which the operation should be applied. Returning no row will prevent the operation from happening (which can be useful for certain behavior). See the docs here for more information about this behavior.

Soft-delete models

A soft-delete model is one that sets a field on the model to False instead of deleting the model from the database. For example, it is common is set an is_active field on a model to False to soft delete it.

The pgtrigger.SoftDelete trigger takes the field as an argument and sets it to False whenever a deletion happens on the model. For example:

import pgtrigger


@pgtrigger.register(pgtrigger.SoftDelete(field='is_active'))
class SoftDeleteModel(models.Model):
    # This field is set to false when the model is deleted
    is_active = models.BooleanField(default=True)

m = SoftDeleteModel.objects.create()
m.delete()

# The model will still exist, but it is no longer active
assert not SoftDeleteModel.objects.get().is_active

The pgtrigger.SoftDelete trigger allows one to do soft deletes at the database level with no instrumentation in code at the application level. This reduces the possibility for holes in the application that can accidentally delete the model when not going through the appropriate interface.

Note

When using pgtrigger.SoftDelete, keep in mind that Django will still perform cascading operations to models that reference the soft-delete model. For example, if one has a model that foreign keys to SoftDeleteModel in the example with on_delete=models.CASCADE, that model will be deleted by Django when the parent model is soft deleted. One can use models.DO_NOTHING if they wish for Django to not delete references to soft-deleted models.

Append-only models

Create an append-only model using the pgtrigger.Protect utility and registering it for the UPDATE and DELETE operations:

import pgtrigger
from django.db import models


@pgtrigger.register(
    pgtrigger.Protect(
        operation=(pgtrigger.Update | pgtrigger.Delete)
    )
)
class AppendOnlyModel(models.Model):
    my_field = models.IntField()

Note

This table can still be truncated, although this is not an operation supported by Django. One can still protect against this by adding the pgtrigger.Truncate operation.

Official interfaces

pgtrigger.Protect triggers can be combined with pgtrigger.ignore to create “official” interfaces for doing database operations in your application.

For example, let’s protect inserts on our custom User model and force all engineers to use one common interface to create users:

from django.db import models


@pgtrigger.ignore('my_app.User:protect_inserts')
def create_user(**kwargs):
    """
    This is the "official" interface for creating users. Any code
    that tries to create users and does not go through this interface
    will fail
    """
    return User.objects.create(**kwargs)


@pgtrigger.register(
    pgtrigger.Protect(name='protect_inserts', operation=pgtrigger.Insert)
)
class User(models.Model):
    pass

Users of this application must call create_user to create users. Any other pieces of the application that bypass this interface when creating users will have errors.

Dynamic deletion protection

Only allow models with a deletable flag to be deleted:

import pgtrigger
from django.db import models


@pgtrigger.register(
    pgtrigger.Protect(
        operation=pgtrigger.Delete,
        condition=pgtrigger.Q(old__is_deletable=False)
    )
)
class DynamicDeletionModel(models.Model):
    is_deletable = models.BooleanField(default=False)

Redundant update protection

Want to error every time someone tries to update a row with the exact same values? Here’s how:

import pgtrigger
from django.db import models


@pgtrigger.register(
    pgtrigger.Protect(
        operation=pgtrigger.Delete,
        condition=pgtrigger.Condition(
            'OLD.* IS NOT DISTINCT FROM NEW.*'
        )
    )
)
class RedundantUpdateModel(models.Model):
    redundant_field1 = models.BooleanField(default=False)
    redundant_field2 = models.BooleanField(default=False)

Configuring triggers on external models

Triggers can be registered for models that are part of third party apps. This can be done by manually calling the pgtrigger.register decorator:

from django.contrib.auth.models import User
import pgtrigger

# Register a protection trigger for the User model
pgtrigger.register(pgtrigger.Protect(...))(User)

Note

Be sure that triggers are registered via an app config’s ready() method so that the registration happens! More information on this here.

Statement-level triggers and transition tables

Most of the terminology and examples around Postgres triggers have been centered around “row-level” triggers, i.e. triggers that fire on events for every row. However, row-level triggers can be expensive in some circumstances. For example, imagine we are doing a bulk Django update over a table with 10,000 rows:

MyLargeModel.objects.update(is_active=False)

If we had any row-level triggers configured for MyLargeModel, they would fire 10,000 times for every updated row even though the query above is only issuing one single update statement.

Although triggers are issued at the database level and will not induce expensive round trips to the database, it can still be unnecessarily expensive to do row-level triggers for certain situations.

Statement-level triggers, in contrast to row-level triggers, are executed once per statement. One only needs to provide level=pgtrigger.Statement to the trigger to configure this. However, keep in mind that trigger conditions and are largely not applicable to statement-level triggers since the OLD and NEW row variables are always NULL.

Postgres10 introduced the notion of “transition tables” to allow users to access old and new rows in a statement-level trigger (see this for an example). One can use the pgtrigger.Referencing construct to write a statement-level trigger that references the old and new rows.

For example, imagine we have a log model that logs changes to a table and keeps track of an old field and new field for every update. We can create a statement-level trigger that logs the old and new fields from a transition table to this persisted log model like so:

from django.db import models
import pgtrigger


class LogModel(models.Model):
    old_field = models.CharField(max_length=32)
    new_field = models.CharField(max_length=32)


@pgtrigger.register(
    pgtrigger.Trigger(
        level=pgtrigger.Statement,
        when=pgtrigger.After,
        operation=pgtrigger.Update,
        referencing=pgtrigger.Referencing(old='old_values', new='new_values'),
        func=f'''
            INSERT INTO {LogModel._meta.db_table}(old_field, new_field)
            SELECT
                old_values.field AS old_field,
                new_values.field AS new_field
            FROM old_values
                JOIN new_values ON old_values.id = new_values.id;
            RETURN NULL;
        ''',
    )
)
class LoggedModel(models.Model):
    field = models.CharField(max_length=32)

With this trigger definition, we’ll now have the following happen with only one additional query in the trigger:

LoggedModel.objects.bulk_create([LoggedModel(field='old'), LoggedModel(field='old')])

# Update all fields to "new"
LoggedModel.objects.update(field='new')

# The trigger should have persisted these updates
print(LogModel.values('old_field', 'new_field'))

>>> [{
  'old_field': 'old',
  'new_field': 'new'
}, {
  'old_field': 'old',
  'new_field': 'new'
}]

Note

Check out django-pghistory if you want automated history tracking built on top of django-pgtrigger.