Basic version

Author: Daan van der Kallen

binder/stored/__init__.py

@@ -0,0 +1,4 @@
+from .base import Stored  # noqa
+
+
+default_app_config = 'binder.stored.apps.StoredAppConfig'

binder/stored/apps.py

@@ -0,0 +1,11 @@
+from django.apps import AppConfig
+
+from .signal import apps_ready
+
+
+class StoredAppConfig(AppConfig):
+
+	name = 'binder.stored'
+
+	def ready(self):
+		apps_ready.send(sender=None)

binder/stored/base.py

@@ -0,0 +1,153 @@
+from collections import namedtuple
+
+from django.db.models import F, Aggregate
+from django.db.models.signals import post_save, class_prepared
+from django.db.models.expressions import BaseExpression
+from django.conf import settings
+
+from .signal import apps_ready
+
+
+Dep = namedtuple('Dep', ['model', 'fields', 'rev_path', 'rev_field'])
+
+
+def get_deps_base(model, expr):
+	"""
+	Given a model and an expr yield all changes that could affect the result
+	of this expr.
+
+	A change is defined as a 4-tuple of (model, changed, rev_path, rev_field).
+	"""
+	from ..plugins.loaded_values import LoadedValuesMixin
+
+	if not issubclass(model, LoadedValuesMixin):
+		raise ValueError(f'{model} should inherit from LoadedValuesMixin if you want to use it in a stored field')
+
+	if isinstance(expr, Aggregate):
+		expr, = expr.source_expressions
+
+	if isinstance(expr, F):
+		head, sep, tail = expr.name.partition('__')
+
+		field = model._meta.get_field(head)
+		if not sep and field.is_relation:
+			sep = '__'
+			tail = 'id'
+
+		if not sep:
+			if head != 'id':
+				yield Dep(model, {head}, 'id', 'id')
+			return
+
+		if not field.is_relation:
+			raise ValueError(f'expected {model.__name__}.{field} to be a relation')
+
+		if field.one_to_many:
+			yield Dep(field.related_model, {field.remote_field.name}, 'id', field.remote_field.column)
+		elif field.many_to_one:
+			yield Dep(model, {head}, 'id', 'id')
+		else:
+			raise ValueError('unsupported type of relation')
+
+		for dep in get_deps(field.related_model, F(tail)):
+			if dep.rev_path != 'id':
+				yield dep._replace(rev_path=f'{head}__{dep.rev_path}')
+			elif field.one_to_many:
+				yield dep._replace(rev_field=field.remote_field.column)
+			else:
+				yield dep._replace(rev_path=head)
+
+	else:
+		raise ValueError(f'cannot infer deps for {expr!r}')
+
+
+def get_deps(*args, **kwargs):
+	deps = {}
+	for dep in get_deps_base(*args, **kwargs):
+		key = dep._replace(fields=None)
+		try:
+			base_dep = deps[key]
+		except KeyError:
+			deps[key] = dep
+		else:
+			deps[key] = dep._replace(fields=base_dep.fields | dep.fields)
+	return deps.values()
+
+
+class Stored:
+
+	def __init__(self, expr):
+		self.expr = expr
+
+	def __set_name__(self, model, name):
+		from ..views import fix_output_field
+
+		if 'binder.stored' not in settings.INSTALLED_APPS:
+			raise ValueError('cannot use Stored if \'binder.stored\' is not in INSTALLED_APPS')
+
+		# We dont actually want this to be the attribute
+		delattr(model, name)
+
+		# Get field
+		fix_output_field(self.expr, model)
+		if isinstance(self.expr, F):
+			field = self.expr._output_field_or_none
+		elif isinstance(self.expr, BaseExpression):
+			field = self.expr.field
+		else:
+			raise ValueError(
+				'{}.{} is not a valid django query expression'
+				.format(model.__name__, name)
+			)
+
+		# Make blank & nullable copy of field
+		_, _, args, kwargs = field.deconstruct()
+		kwargs['blank'] = True
+		kwargs['null'] = True
+		field = type(field)(*args, **kwargs)
+		field.__binder_stored_expr = self.expr
+
+		# Add the field
+		def add_field(**kwargs):
+			class_prepared.disconnect(add_field, sender=model)
+
+			model.add_to_class(name, field)
+
+		class_prepared.connect(add_field, sender=model, weak=False)
+
+		# Add triggers for deps
+		def add_triggers(**kwargs):
+			apps_ready.disconnect(add_triggers)
+
+			register_init(model, name, self.expr)
+			for dep in get_deps(model, self.expr):
+				register_dep(model, name, self.expr, dep)
+
+		apps_ready.connect(add_triggers, weak=False)
+
+
+def update_queryset(queryset, name, expr):
+	updates = []
+	for obj in queryset.annotate(updated_value=expr):
+		setattr(obj, name, obj.updated_value)
+		updates.append(obj)
+	queryset.model.objects.bulk_update(updates, [name])
+
+
+def register_init(model, name, expr):
+	def update_values(instance, **kwargs):
+		if instance.field_changed('id'):
+			update_queryset(model.objects.filter(id=instance.id), name, expr)
+
+	post_save.connect(update_values, sender=model, weak=False)
+
+
+def register_dep(model, name, expr, dep):
+	def update_values(instance, **kwargs):
+		if instance.field_changed('id', *dep.fields):
+			ids = [getattr(instance, dep.rev_field)]
+			if instance.field_changed(dep.rev_field):
+				ids.append(instance.get_old_value(dep.rev_field))
+			update_queryset(model.objects.filter(id__in=ids), name, expr)
+
+	post_save.connect(update_values, sender=dep.model, weak=False)

binder/stored/signal.py

@@ -0,0 +1,4 @@
+from django.dispatch import Signal
+
+
+apps_ready = Signal()

tests/__init__.py

@@ -1,4 +1,4 @@
-from django import setup
+import django
 from django.conf import settings
 from django.core.management import call_command
 import os
@@ -55,6 +55,7 @@
 		'binder.plugins.token_auth',
 		'tests',
 		'tests.testapp',
+		'binder.stored',
 	],
 	'MIGRATION_MODULES': {
 		'testapp': None,
@@ -111,7 +112,7 @@
 	}
 })
 
-setup()
+django.setup()
 
 # Do the dance to ensure the models are synched to the DB.
 # This saves us from having to include migrations

tests/test_stored.py

@@ -0,0 +1,17 @@
+from django.test import TestCase
+
+from .testapp.models import Zoo, Animal
+
+
+class StoredTest(TestCase):
+
+	def test_deps(self):
+		zoo = Zoo.objects.create(name='Zoo')
+
+		zoo.refresh_from_db()
+		self.assertEqual(zoo.stored_animal_count, 0)
+
+		for n in range(1, 11):
+			Animal.objects.create(zoo=zoo, name=f'Animal {n}')
+			zoo.refresh_from_db()
+			self.assertEqual(zoo.stored_animal_count, n)

tests/testapp/models/zoo.py

@@ -1,9 +1,14 @@
 import os
 import datetime
+
 from django.core.exceptions import ValidationError
 from django.db import models
+from django.db.models import Count
 from django.db.models.signals import post_delete
+
 from binder.models import BinderModel, BinderImageField
+from binder.stored import Stored
+from binder.plugins.loaded_values import LoadedValuesMixin
 
 def delete_files(sender, instance=None, **kwargs):
 	for field in sender._meta.fields:
@@ -16,7 +21,7 @@ def delete_files(sender, instance=None, **kwargs):
 
 # From the api docs: a zoo with a name.  It also has a founding date,
 # which is nullable (representing "unknown").
-class Zoo(BinderModel):
+class Zoo(LoadedValuesMixin, BinderModel):
 	name = models.TextField()
 	founding_date = models.DateField(null=True, blank=True)
 	floor_plan = models.ImageField(upload_to='floor-plans', null=True, blank=True)
@@ -35,6 +40,8 @@ class Zoo(BinderModel):
 
 	binder_picture_custom_extensions = BinderImageField(allowed_extensions=['png'], blank=True, null=True)
 
+	stored_animal_count = Stored(Count('animals'))
+
 	def __str__(self):
 		return 'zoo %d: %s' % (self.pk, self.name)