models helpers

codeforlife/models/__init__.py

@@ -1,3 +1,7 @@
+"""Helpers for module "django.db.models".
+https://docs.djangoproject.com/en/3.2/ref/models/
+"""
+
 import typing as t
 
 from django.db.models import Model as _Model

codeforlife/models/signals/__init__.py

@@ -0,0 +1,16 @@
+"""Helpers for module "django.db.models.signals".
+https://docs.djangoproject.com/en/3.2/ref/signals/#module-django.db.models.signals
+"""
+
+import typing as t
+
+UpdateFields = t.Optional[t.FrozenSet[str]]
+
+
+def _has_update_fields(actual: UpdateFields, expected: UpdateFields):
+    if expected is None:
+        return actual is None
+    if actual is None:
+        return False
+
+    return all(update_field in actual for update_field in expected)

codeforlife/models/signals/post_save.py

@@ -0,0 +1,21 @@
+"""Helpers for module "django.db.models.signals.post_save".
+https://docs.djangoproject.com/en/3.2/ref/signals/#post-save
+"""
+
+from . import UpdateFields, _has_update_fields
+
+
+def has_update_fields(actual: UpdateFields, expected: UpdateFields):
+    """Check if the expected fields were updated.
+
+    Args:
+        actual: The fields that were updated.
+        expected: A subset of the fields that were expected to be updated. If no
+            fields were expected to be updated, set to None.
+
+    Returns:
+        If the fields that were expected to be updated are a subset of the
+        fields that were updated.
+    """
+
+    return _has_update_fields(actual, expected)

codeforlife/models/signals/pre_save.py

@@ -0,0 +1,66 @@
+"""Helpers for module "django.db.models.signals.pre_save".
+https://docs.djangoproject.com/en/3.2/ref/signals/#pre-save
+"""
+
+import typing as t
+
+from .. import AnyModel
+from . import UpdateFields, _has_update_fields
+
+
+def was_created(instance: AnyModel):
+    """Check if the instance was created.
+
+    Args:
+        instance: The current instance.
+
+    Returns:
+        If the instance was created.
+    """
+
+    return instance.pk is not None
+
+
+def has_update_fields(actual: UpdateFields, expected: UpdateFields):
+    """Check if the expected fields are going to be updated.
+
+    Args:
+        actual: The fields that are going to be updated.
+        expected: A subset of the fields that are expected to be updated. If no
+            fields are expected to be updated, set to None.
+
+    Returns:
+        If the fields that are expected to be updated are a subset of the
+        fields that are going to be updated.
+    """
+
+    return _has_update_fields(actual, expected)
+
+
+def has_previous_values(
+    instance: AnyModel,
+    predicates: t.Dict[str, t.Callable[[t.Any, t.Any], bool]],
+):
+    """Check if the previous values are as expected.
+
+    Args:
+        instance: The current instance.
+        predicates: A predicate for each field. It accepts the arguments
+        (previous_value, value) and returns True if the values are as expected.
+
+    Raises:
+        ValueError: If arg 'instance' has not been created yet.
+
+    Returns:
+        If all the previous values are as expected.
+    """
+
+    if not was_created(instance):
+        raise ValueError("Arg 'instance' has not been created yet.")
+
+    previous_instance = instance.__class__.objects.get(pk=instance.pk)
+
+    return all(
+        predicate(previous_instance[field], instance[field])
+        for field, predicate in predicates.items()
+    )