Frequently Asked Questions¶
Triggers are scary, don’t you think?¶
Enforcing data integrity in application code is what you should be afraid of. Triggers, like uniqueness constraints, are a blessing that help solidify your data modeling.
The best way to ensure triggers are behaving correctly is to:
Write tests for them.
Run
python manage.py pgtrigger lsto verify they are installed in production.
Why not implement logic with signals or model methods?¶
Why can’t I join foreign keys in conditions?¶
Postgres only allows columns of the changed rows to be used in trigger conditions, and data cannot
be joined. That’s why, for example, one cannot write a condition like Q(old__foreign_key__field="value").
Conditional logic like this must be performed in the trigger function itself by manually joining data.
How do I test triggers?¶
Manipulate your models in your test suite and verify the expected result happens.
If you’ve turned off migrations for your test suite, call pgtrigger.install after the database is set up or set settings.PGTRIGGER_INSTALL_ON_MIGRATE to True to ensure triggers are installed for your tests.
Warning
Be sure the settings.PGTRIGGER_INSTALL_ON_MIGRATE setting is isolated to your test suite, otherwise it could
cause unexpected problems in production when reversing migrations.
When a failure-based trigger like pgtrigger.Protect fails, a django.db.utils.InternalError is raised and
can be caught in your test function. Keep in mind that this will place the current transaction in an errored
state. If you’d like to test triggers like this without needing to use a transaction test case, wrap the
assertion in transaction.atomic. This is illustrated below with pseudocode using
pytest-django.
from djagno.db import transaction
from django.db.utils import InternalError
import pytest
@pytest.mark.django_db
def test_protection_trigger():
with pytest.raises(InternalError, match="Cannot delete"), transaction.atomic():
# Try to delete protected model
# Since the above assertion is wrapped in transaction.atomic, we will still have
# a valid transaction in our test case here
How do I disable triggers?¶
Use pgtrigger.ignore if you need to temporarily ignore triggers in your application (see Ignoring Execution). Only use the core installation commands if you want to disable or uninstall triggers globally (see the Commands section). Never run the core
installation commands in application code.
How can I reference the table name in a custom function?¶
When writing a trigger in Meta, it’s not possible to access other model meta properties like db_table.
Use pgtrigger.Func to get around this. See Model properties in the func.