1👍
The Interaction objects will either have a postinteraction or newsinteraction attribute that will point to the object of the right type.
Alternatively you can use a library like django-model-utils and its InheritanceManager to allow you to do:
from model_utils.managers import InheritanceManager
class Interaction(models.Model):
...
objects = InheritanceManager()
...
Interaction.objects.all().select_subclasses()
That will give you the right object types.
👤Ben
0👍
You asked how to tell the subclass of an Interaction object without doing a query. The short answer is that you can’t. When you access Interaction.objects.all(), that just queries the myschema_interaction table, and that table doesn’t tell you whether there’s a matching record in the myschema_postinteraction table or the myschema_newsinteraction table.
To see what the subclass is, you can look at the postinteraction or newsinteraction attributes. To make one query instead of making a separate query for each record, you can use the prefetch_related() method.
Here’s a runnable example that shows the two techniques.
# Tested with Django 1.9.2
import logging
import sys
import django
from django.apps import apps
from django.apps.config import AppConfig
from django.conf import settings
from django.db import connections, models, DEFAULT_DB_ALIAS
from django.db.models.base import ModelBase
NAME = 'udjango'
def main():
setup()
class Interaction(models.Model):
# user = models.ForeignKey(
# User,
# verbose_name="Usuario"
# )
created = models.DateTimeField(
verbose_name="Criado em",
auto_now_add=True
)
value = models.FloatField(
verbose_name="Valor",
default=0
)
class PostInteraction(Interaction):
body = models.CharField(max_length=1000)
class NewsInteraction(Interaction):
headline = models.CharField(max_length=30)
syncdb(Interaction)
syncdb(PostInteraction)
syncdb(NewsInteraction)
PostInteraction.objects.create(body="I ate breakfast.", value=1)
NewsInteraction.objects.create(headline="No lunch today!", value=2)
PostInteraction.objects.create(body="I ate dinner.", value=3)
logging.info('All interaction values: {}'.format(
[interaction.value
for interaction in Interaction.objects.all()]))
logging.info('Interaction values and posts: {}'.format(
[(interaction.value, hasattr(interaction, 'postinteraction'))
for interaction in Interaction.objects.all()]))
logging.info('Interaction values and posts: {}'.format(
[(interaction.value, hasattr(interaction, 'postinteraction'))
for interaction in Interaction.objects.prefetch_related('postinteraction')]))
def setup():
DB_FILE = NAME + '.db'
with open(DB_FILE, 'w'):
pass # wipe the database
settings.configure(
DEBUG=True,
DATABASES={
DEFAULT_DB_ALIAS: {
'ENGINE': 'django.db.backends.sqlite3',
'NAME': DB_FILE}},
LOGGING={'version': 1,
'disable_existing_loggers': False,
'formatters': {
'debug': {
'format': '%(asctime)s[%(levelname)s]'
'%(name)s.%(funcName)s(): %(message)s',
'datefmt': '%Y-%m-%d %H:%M:%S'}},
'handlers': {
'console': {
'level': 'DEBUG',
'class': 'logging.StreamHandler',
'formatter': 'debug'}},
'root': {
'handlers': ['console'],
'level': 'INFO'},
'loggers': {
"django.db": {"level": "DEBUG"}}})
app_config = AppConfig(NAME, sys.modules['__main__'])
apps.populate([app_config])
django.setup()
original_new_func = ModelBase.__new__
@staticmethod
def patched_new(cls, name, bases, attrs):
if 'Meta' not in attrs:
class Meta:
app_label = NAME
attrs['Meta'] = Meta
return original_new_func(cls, name, bases, attrs)
ModelBase.__new__ = patched_new
def syncdb(model):
""" Standard syncdb expects models to be in reliable locations.
Based on https://github.com/django/django/blob/1.9.3
/django/core/management/commands/migrate.py#L285
"""
connection = connections[DEFAULT_DB_ALIAS]
with connection.schema_editor() as editor:
editor.create_model(model)
main()
Here’s the log output that includes the executed SQL:
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) PRAGMA foreign_keys; args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) PRAGMA foreign_keys = 0; args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) BEGIN; args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.schema.execute(): CREATE TABLE "udjango_interaction" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "created" datetime NOT NULL, "value" real NOT NULL); (params None)
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) CREATE TABLE "udjango_interaction" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "created" datetime NOT NULL, "value" real NOT NULL); args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) PRAGMA foreign_keys = 0; args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) PRAGMA foreign_keys; args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) PRAGMA foreign_keys = 0; args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) BEGIN; args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.schema.execute(): CREATE TABLE "udjango_postinteraction" ("interaction_ptr_id" integer NOT NULL PRIMARY KEY REFERENCES "udjango_interaction" ("id"), "body" varchar(1000) NOT NULL); (params None)
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) CREATE TABLE "udjango_postinteraction" ("interaction_ptr_id" integer NOT NULL PRIMARY KEY REFERENCES "udjango_interaction" ("id"), "body" varchar(1000) NOT NULL); args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) PRAGMA foreign_keys = 0; args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) PRAGMA foreign_keys; args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) PRAGMA foreign_keys = 0; args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) BEGIN; args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.schema.execute(): CREATE TABLE "udjango_newsinteraction" ("interaction_ptr_id" integer NOT NULL PRIMARY KEY REFERENCES "udjango_interaction" ("id"), "headline" varchar(30) NOT NULL); (params None)
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) CREATE TABLE "udjango_newsinteraction" ("interaction_ptr_id" integer NOT NULL PRIMARY KEY REFERENCES "udjango_interaction" ("id"), "headline" varchar(30) NOT NULL); args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) PRAGMA foreign_keys = 0; args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) BEGIN; args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) INSERT INTO "udjango_interaction" ("created", "value") VALUES ('2017-04-27 15:02:20.729660', 1.0); args=['2017-04-27 15:02:20.729660', 1.0]
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) INSERT INTO "udjango_postinteraction" ("interaction_ptr_id", "body") SELECT 1, 'I ate breakfast.'; args=(1, 'I ate breakfast.')
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) BEGIN; args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) INSERT INTO "udjango_interaction" ("created", "value") VALUES ('2017-04-27 15:02:20.754687', 2.0); args=['2017-04-27 15:02:20.754687', 2.0]
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) INSERT INTO "udjango_newsinteraction" ("interaction_ptr_id", "headline") SELECT 2, 'No lunch today!'; args=(2, 'No lunch today!')
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) BEGIN; args=None
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) INSERT INTO "udjango_interaction" ("created", "value") VALUES ('2017-04-27 15:02:20.778166', 3.0); args=['2017-04-27 15:02:20.778166', 3.0]
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) INSERT INTO "udjango_postinteraction" ("interaction_ptr_id", "body") SELECT 3, 'I ate dinner.'; args=(3, 'I ate dinner.')
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) SELECT "udjango_interaction"."id", "udjango_interaction"."created", "udjango_interaction"."value" FROM "udjango_interaction"; args=()
2017-04-27 15:02:20[INFO]root.main(): All interaction values: [1.0, 2.0, 3.0]
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) SELECT "udjango_interaction"."id", "udjango_interaction"."created", "udjango_interaction"."value" FROM "udjango_interaction"; args=()
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) SELECT "udjango_interaction"."id", "udjango_interaction"."created", "udjango_interaction"."value", "udjango_postinteraction"."interaction_ptr_id", "udjango_postinteraction"."body" FROM "udjango_postinteraction" INNER JOIN "udjango_interaction" ON ("udjango_postinteraction"."interaction_ptr_id" = "udjango_interaction"."id") WHERE "udjango_postinteraction"."interaction_ptr_id" = 1; args=(1,)
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) SELECT "udjango_interaction"."id", "udjango_interaction"."created", "udjango_interaction"."value", "udjango_postinteraction"."interaction_ptr_id", "udjango_postinteraction"."body" FROM "udjango_postinteraction" INNER JOIN "udjango_interaction" ON ("udjango_postinteraction"."interaction_ptr_id" = "udjango_interaction"."id") WHERE "udjango_postinteraction"."interaction_ptr_id" = 2; args=(2,)
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) SELECT "udjango_interaction"."id", "udjango_interaction"."created", "udjango_interaction"."value", "udjango_postinteraction"."interaction_ptr_id", "udjango_postinteraction"."body" FROM "udjango_postinteraction" INNER JOIN "udjango_interaction" ON ("udjango_postinteraction"."interaction_ptr_id" = "udjango_interaction"."id") WHERE "udjango_postinteraction"."interaction_ptr_id" = 3; args=(3,)
2017-04-27 15:02:20[INFO]root.main(): Interaction values and posts: [(1.0, True), (2.0, False), (3.0, True)]
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) SELECT "udjango_interaction"."id", "udjango_interaction"."created", "udjango_interaction"."value" FROM "udjango_interaction"; args=()
2017-04-27 15:02:20[DEBUG]django.db.backends.execute(): (0.000) SELECT "udjango_interaction"."id", "udjango_interaction"."created", "udjango_interaction"."value", "udjango_postinteraction"."interaction_ptr_id", "udjango_postinteraction"."body" FROM "udjango_postinteraction" INNER JOIN "udjango_interaction" ON ("udjango_postinteraction"."interaction_ptr_id" = "udjango_interaction"."id") WHERE "udjango_postinteraction"."interaction_ptr_id" IN (1, 2, 3); args=(1, 2, 3)
2017-04-27 15:02:20[INFO]root.main(): Interaction values and posts: [(1.0, True), (2.0, False), (3.0, True)]
Source:stackexchange.com