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.

The django-pgtrigger library is designed so that users only need to use Python and Django idioms for registering common triggers on models. More advanced users, however, can always directly write the raw PL/pgSQL dialect used for Postgres triggers.

Here are all of basic attributes of triggers that you will use when using just about any of the triggers in this library:

• name

  The identifying name of trigger. Is unique for every model and must be <= 47 characters. The trigger name is used for performing trigger management operations and must be provided.

• 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.

Here are the more advanced attributes of triggers that you will want to know about when writing more complex triggers or writing your own trigger functions.

• func

  The raw PL/pgSQL function that is executed.

  Note

  This is not the entire declared trigger function, but rather the snippet of PL/pgSQL that is nested in the `DECLARE ... BEGIN ... END` portion of the trigger.

• declare (optional)

  If the trigger requires additional variable declarations, they can be defined as a list of (variable_name, variable_type) tuples. For example declare=[('my_var_1', 'BOOLEAN'), ('my_var_2', 'JSONB')]

• 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.

Defining and installing triggers

Triggers are defined in the triggers attribute of the model Meta class.

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

from django.db import models
import pgtrigger


class CannotDelete(models.Model):
    class Meta:
        triggers = [
            pgtrigger.Protect(name='protect_deletes', operation=pgtrigger.Delete)
        ]

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.

If you want to register triggers on external models, install them manually, or disable them, see the Advanced trigger installation guide section for more details.

Trigger cookbook

Here are examples using built-in trigger classes and raw SQL.

Validating field transitions

Similar to how one can configure a finite state machine on a model field with django-fsm, the pgtrigger.FSM ensures that a field can only do configured transitions on update.

For example, this trigger ensures that the status field of a model can only transition from “unpublished” to “published” and from “published” to “inactive”. Any other updates on the status field will result in an exception:

class MyModel(models.Model):
    """Enforce valid transitions of the 'status' field"""
    status = models.CharField(max_length=32, default='unpublished')

    class Meta:
        triggers = [
            pgtrigger.FSM(
                name='status_fsm',
                field='status',
                transitions=[
                    ('unpublished', 'published'),
                    ('published', 'inactive'),
                ]
            )
        ]

Note

Similar to other triggers, pgtrigger.FSM can be supplied with a condition to only enforce the state transitions when a condition is met.

Note

The pgtrigger.FSM trigger currently only works for non-null CharField fields.

Keeping a field in-sync with another

Here we create a pgtrigger.Trigger that runs before an update or insert to ensure that two fields remain in sync.

import pgtrigger

class MyModel(models.Model):
    int_field = models.IntField()
    in_sync_int = models.IntField(help_text='Stays the same as "int_field"')

    class Meta:
        triggers = [
            pgtrigger.Trigger(
                name='keep_in_sync',
                operation=pgtrigger.Update | pgtrigger.Insert,
                when=pgtrigger.Before,
                func='NEW.in_sync_int = NEW.int_field; RETURN NEW;',
            )
        ]

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 a value upon deletion 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 a value to set on delete. The value defaults to False. For example:

import pgtrigger


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

    class Meta:
        triggers = [
            pgtrigger.SoftDelete(name='soft_delete', field='is_active')
        ]


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

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

In the above example, the boolean field “is_active” is set to False upon deletion. pgtrigger.SoftDelete works with nullable CharField, IntField, and BooleanField fields.

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 of application error.

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

Here we create an append-only model using the pgtrigger.Protect trigger for the UPDATE and DELETE operations:

import pgtrigger
from django.db import models


class AppendOnlyModel(models.Model):
    my_field = models.IntField()

    class Meta:
        triggers = [
            pgtrigger.Protect(
                name='protect_updates_and_deletes',
                operation=(pgtrigger.Update | pgtrigger.Delete)
            )
        ]

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.

Note

Ignoring triggers requires additional conifguration. See the Ignoring trigger execution dynamically section to learn more.

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)


class User(models.Model):
    class Meta:
        triggers = [
            pgtrigger.Protect(name='protect_inserts', operation=pgtrigger.Insert)
        ]

Users of this application must call create_user to create users. Any other code that creates users will fail.

Dynamic deletion protection

Here we only allow models with a deletable flag to be deleted:

import pgtrigger
from django.db import models


class DynamicDeletionModel(models.Model):
    is_deletable = models.BooleanField(default=False)

    class Meta:
        triggers = [
            pgtrigger.Protect(
                name='protect_deletes',
                operation=pgtrigger.Delete,
                condition=pgtrigger.Q(old__is_deletable=False)
            )
        ]

Redundant update protection

Here we raise an error when someone makes a redundant update to the database:

import pgtrigger
from django.db import models


class RedundantUpdateModel(models.Model):
    redundant_field1 = models.BooleanField(default=False)
    redundant_field2 = models.BooleanField(default=False)

    class Meta:
        triggers = [
            pgtrigger.Protect(
                name='protect_redundant_updates',
                operation=pgtrigger.Update,
                condition=pgtrigger.Condition(
                    'OLD.* IS NOT DISTINCT FROM NEW.*'
                )
            )
        ]

Freezing published models

Here we have a Post model with a status field. We only allow edits to this model when it’s not published.

import pgtrigger
from django.db import models


class Post(models.Model):
    status = models.CharField(default='unpublished')
    content = models.TextField()

    class Meta:
        triggers = [
            pgtrigger.Protect(
                name='freeze_published_model',
                operation=pgtrigger.Update,
                condition=pgtrigger.Q(old__status='published')
            )
        ]

With the above, we’ve set a condition so that the Post model can no longer be updated once the status field is “published”.

We extend this example by allowing a published model to be able to be edited, but only if that status is “inactive”

import pgtrigger
from django.db import models


class Post(models.Model):
    status = models.CharField(default='unpublished')
    content = models.TextField()

    class Meta:
        triggers = [
            pgtrigger.Protect(
                name='freeze_published_model_allow_deactivation',
                operation=pgtrigger.Update,
                condition=(
                  pgtrigger.Q(old__status='published')
                  & ~pgtrigger.Q(new__status='inactive')
            )
        ]

In the above, we protect updates on any published posts unless the update is transitioning the published post into an inactive state.

Versioned models

Here we write a pgtrigger.Trigger that dynamically increments a model version before an update is applied:

class Versioned(models.Model):
    """
    This model is versioned. The "version" field is incremented on every
    update, and users cannot directly update the "version" field.
    """
    version = models.IntegerField(default=0)
    char_field = models.CharField(max_length=32)

    class Meta:
        triggers = [
            # Protect anyone editing the version field directly
            pgtrigger.Protect(
                name='protect_updates',
                operation=pgtrigger.Update,
                condition=pgtrigger.Q(old__version__df=pgtrigger.F('new__version'))
            ),
            # Increment the version field on changes
            pgtrigger.Trigger(
                name='versioning',
                when=pgtrigger.Before,
                operation=pgtrigger.Update,
                func='NEW.version = NEW.version + 1; RETURN NEW;',
                # Don't increment version on redundant updates.
                condition=pgtrigger.Condition('OLD.* IS DISTINCT FROM NEW.*')
            )
        ]

In the above, we’ve added two triggers:

1. One that protects updating the version field of the model. We don’t want people tampering with this field.

2. A trigger that increments the version of the NEW row before an update is applied.

We return the NEW row in the second trigger definition. Postgres uses this return value for the update operation. We’ve also ensured that the versioning trigger only fires when anything in the row has changed.

Note

The return value from pgtrigger.Before triggers is very important. If you return NULL, it will tell Postgres to ignore the operation.

Statement-level triggers and transition tables

Most of the terminology and examples around Postgres triggers have been for “row-level” triggers, i.e. triggers that fire on events for every row. However, row-level triggers can be expensive in some circumstances if doing large bulk operations.

Statement-level triggers can be used to mitigate these scenarios. Triggers are executed once per statement and can be configured with level=pgtrigger.Statement in the trigger definition.

In statement level triggers, the OLD and NEW row variables are always NULL. We instead use “transition tables” to access old and new rows. One can use the pgtrigger.Referencing construct to write a statement-level trigger that references the old and new rows. See this example for more explanations about these constructs.

Note

Transition tables are only available in Postgres 10 and up.

Here we have a history model that keeps track of changes to a field for every update of the tracked table. We 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 HistoryModel(models.Model):
    old_field = models.CharField(max_length=32)
    new_field = models.CharField(max_length=32)


class TrackedModel(models.Model):
    field = models.CharField(max_length=32)

    class Meta:
        triggers = [
            pgtrigger.Trigger(
                name='statement_level_log',
                level=pgtrigger.Statement,
                when=pgtrigger.After,
                operation=pgtrigger.Update,
                referencing=pgtrigger.Referencing(old='old_values', new='new_values'),
                func=f'''
                    INSERT INTO {HistoryModel._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;
                ''',
            )
        ]

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

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

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

# The trigger should have tracked these updates
print(HistoryModel.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.

Tracking model history and changes

django-pgtrigger can be used to snapshot all model changes, snapshot changes whenever a particular change happens, and even attach context from your application (e.g. the authenticated user) to the triggered event.

Historical tracking and auditing is a problem that is going to be different for every organization’s needs. Because of the scope of this problem, we have created a history tracking library called django-pghistory that solves common needs for doing model change tracking. It is implemented using django-pgtrigger. Check out the docs here.

Ignoring trigger execution dynamically

Use pgtrigger.ignore to dynamically ignore a trigger in a single thread of execution. Below we ignore deletion protection:

from django.db import models
import pgtrigger


class CannotDelete(models.Model):
    class Meta:
        triggers = [
            pgtrigger.Protect(name='protect_deletes', operation=pgtrigger.Delete)
        ]


# 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.

When used, django-pgconnection dynamically sets a Postgres variable that the trigger understands. This allows us to ignore a trigger’s execution for a single thread rather than disabling it globally.

To use this feature, you will need to wrap settings.DATABASES with pgconnection.configure() in settings.py like so:

import pgconnection

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

Advanced trigger installation guide

Manual installation and disabling

Warning

Installing, uninstalling, enabling, and disabling triggers 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 Ignoring trigger execution dynamically section.

There are circumstances when it is undesirable to always install triggers after migrations, especially when performing complex multi-step migrations where installing a trigger midway could result in errors. To turn off creating triggers in migrations, configure the PGTRIGGER_INSTALL_ON_MIGRATE setting to False.

Triggers can be programmatically configured with the following code:

• pgtrigger.install: Install triggers

• pgtrigger.uninstall: Uninstall triggers

• pgtrigger.enable: Enable triggers

• pgtrigger.disable: Disable triggers

• pgtrigger.prune: Uninstall triggers created by django-pgtrigger that are no longer in the codebase.

Triggers can also be configured with similar management commands. See the Commands section for more details.

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.

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.

More trigger examples

The fun doesn’t stop here. There is an entire tutorial repository for using django-pgtrigger at https://wesleykendall.github.io/django-pgtrigger-tutorial/. This tutorial covers many of the examples we’ve already covered, and it has interactive code examples you can run locally. Go check it out!