Quick Start¶
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 must provide the following attributes:
when
When the trigger should happen. Can be one of
pgtrigger.Before
orpgtrigger.After
to execute the trigger before or after an operation.operation
The operation which triggers execution of the trigger function. This can be one of
pgtrigger.Update
,pgtrigger.Insert
,pgtrigger.Delete
,pgtrigger.Truncate
, orpgtrigger.UpdateOf
. All of these can beOR
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 anUPDATE
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
An optional condition on which the trigger is executed based on the
OLD
andNEW
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"
). Thepgtrigger.Q
condition also mimics Django’sQ
object to specify a filter clause on the affected rows. For example, a condition ofpgtrigger.Q(old__value='hello')
will only trigger when the old row’svalue
field ishello
.
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
: Installs all triggerspgtrigger.uninstall
: Uninstalls all triggerspgtrigger.enable
: Enables all triggerspgtrigger.disable
: Disables all 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.
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 “BEFORE” trigger, be sure to return the row over which the operation should be applied. Returning no row will prevent the operation from happening.
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.
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.