In my django site I have two apps, blog and links. blog has a model blogpost, and links has a model link. There should be a one to many relationship between these two things. There are many links per blogpost, but each link has one and only one blog post. The simple answer is to put a ForeignKey to blogpost in the link model.
That's all well and good, however there is a problem. I want to make the links app reusable. I don't want it to depend upon the blog app. I want to be able to use it again in other sites and perhaps associate links with other non-blogpost apps and models.
A generic foreign key seems like it might be the answer, but not really. I don't want links to be able to associate with any model in my site. Just the one that I explicitly specify. And I know from prior experience that there can be issues using generic foreign keys in terms of database usage because you can't do a select_related over a generic foreign key the way you can with a regular foreign key.
What is the "correct" way to model this relationship?
If you think the link app will always point to a single app then one approach would be to pass the name of the foreign model as a string containing the application label instead of a class reference (Django docs explanation).
In other words, instead of:
class Link(models.Model):
blog_post = models.ForeignKey(BlogPost)
do:
from django.conf import setings
class Link(models.Model):
link_model = models.ForeignKey(settings.LINK_MODEL)
and in your settings.py:
LINK_MODEL = 'someproject.somemodel'
I think TokenMacGuy is on the right track. I would look at how django-tagging handles a similar generic relationship using the content type, generic object_id, and generic.py. From models.py
class TaggedItem(models.Model):
"""
Holds the relationship between a tag and the item being tagged.
"""
tag = models.ForeignKey(Tag, verbose_name=_('tag'), related_name='items')
content_type = models.ForeignKey(ContentType, verbose_name=_('content type'))
object_id = models.PositiveIntegerField(_('object id'), db_index=True)
object = generic.GenericForeignKey('content_type', 'object_id')
objects = TaggedItemManager()
class Meta:
# Enforce unique tag association per object
unique_together = (('tag', 'content_type', 'object_id'),)
verbose_name = _('tagged item')
verbose_name_plural = _('tagged items')
Anoher way to solve this is how django-mptt does this: define only an abstract model in a reusable app(MPTTModel), and require to inherit it with defining some fields (parent=ForeignKey to self, or whatever your app usecase will require)
Probably you need to use the content types app to link to a model. You might then arrange for your app to check the settings to do some additional checking to limit which content types it will accept or suggest.
I'd go with generic relations. You can do something like select_related, it just require some extra work. But I think it's worth it.
One possible solution for generic select_related-like functionality:
http://bitbucket.org/kmike/django-generic-images/src/tip/generic_utils/managers.py
(look at GenericInjector manager and it's inject_to method)
This question and Van Gale's answer lead me to the question, how it could be possible, to limit contenttypes for GFK without the need of defining it via Q objects in the model, so it could be completly reuseable
the solution is based on
django.db.models.get_model
and the eval built-in, that evaluates a Q-Object from settings.TAGGING_ALLOWED. This is necessary for usage in the admin-interface
My code is quite rough and not fully tested
settings.py
TAGGING_ALLOWED=('myapp.modela', 'myapp.modelb')
models.py:
from django.db import models
from django.db.models import Q
from django.contrib.contenttypes.models import ContentType
from django.contrib.contenttypes import generic
from django.db.models import get_model
from django.conf import settings as s
from django.db import IntegrityError
TAGABLE = [get_model(i.split('.')[0],i.split('.')[1])
for i in s.TAGGING_ALLOWED if type(i) is type('')]
print TAGABLE
TAGABLE_Q = eval( '|'.join(
["Q(name='%s', app_label='%s')"%(
i.split('.')[1],i.split('.')[0]) for i in s.TAGGING_ALLOWED
]
))
class TaggedItem(models.Model):
content_type = models.ForeignKey(ContentType,
limit_choices_to = TAGABLE_Q)
object_id = models.PositiveIntegerField()
content_object = generic.GenericForeignKey('content_type', 'object_id')
def save(self, force_insert=False, force_update=False):
if self.content_object and not type(
self.content_object) in TAGABLE:
raise IntegrityError(
'ContentType %s not allowed'%(
type(kwargs['instance'].content_object)))
super(TaggedItem,self).save(force_insert, force_update)
from django.db.models.signals import post_init
def post_init_action(sender, **kwargs):
if kwargs['instance'].content_object and not type(
kwargs['instance'].content_object) in TAGABLE:
raise IntegrityError(
'ContentType %s not allowed'%(
type(kwargs['instance'].content_object)))
post_init.connect(post_init_action, sender= TaggedItem)
Of course the limitations of the contenttype-framework affect this solution
# This will fail
>>> TaggedItem.objects.filter(content_object=a)
# This will also fail
>>> TaggedItem.objects.get(content_object=a)
Related
I have implemented django generic relations to create relations between models. My models are,
class CessPoint(BaseModel):
....
title = models.CharField(max_length=100)
class BilledVehicle(BaseModel):
....
source = models.ForeignKey(CessPoint, on_delete=models.CASCADE)
class Bill(BaseModel):
.....
content_type = models.ForeignKey(
ContentType, on_delete=models.CASCADE, null=True)
object_id = models.UUIDField(null=True)
billed_item = GenericForeignKey()
class BillPayment(BaseModel):
.....
bill = models.ForeignKey(Bill, on_delete=models.CASCADE)
I would like to get the payments for BilledVehicle. This is how I would like to go about it.
BillPayment.objects.filter(bill__content_type=ContentType.objects.get_for_model(BilledVehicle)).values('bill__billed_item__source__title').annotate(total_paid=Sum('amount_paid')).order_by('-total_paid')
I am getting the error:
Field 'billed_item' does not generate an automatic reverse relation and therefore cannot be used for reverse querying. If it is a GenericForeignKey, consider adding a GenericRelation.
According to this answer, How to traverse a GenericForeignKey in Django?, defining a GenericRelation might solve my problem. But then again, I did not define a GenericRelation because adding one will cascade all relations as per the default behavior
Unlike ForeignKey, GenericForeignKey does not accept an on_delete argument to customize this behavior; if desired, you can avoid the cascade-deletion simply by not using GenericRelation, and alternate behavior can be provided via the pre_delete signal.
If you do not want to create a GenericRelation object in your BilledVehicle module, django will have no way of linking it directly to your Bill using values.
You can work around this by using a dictionary to store unique CessPoint titles and their total (do the annotation yourself) from the BillPayment queryset amount paid as follows.
qs = BillPayment.objects.filter(
bill__content_type=ContentType.objects.get_for_model(BilledVehicle))
ds = {}
for billPayment in qs:
title = billPayment.billed_item.source.title
ds[title] = ds.get(title, 0) + billPayment.amount_paid
Here are my simplified models :
from django.contrib.contenttypes.fields import (
GenericForeignKey, GenericRelation)
from django.db import models
from django.utils.translation import ugettext_lazy as _
class Thing(models.Model):
'''
Our 'Thing' class
with a link (generic relationship) to an abstract config
'''
name = models.CharField(
max_length=128, blank=True,
verbose_name=_(u'Name of my thing'))
# Link to our configs
config_content_type = models.ForeignKey(
ContentType,
null=True,
blank=True)
config_object_id = models.PositiveIntegerField(
null=True,
blank=True)
config_object = GenericForeignKey(
'config_content_type',
'config_object_id')
class Config(models.Model):
'''
Base class for custom Configs
'''
class Meta:
abstract = True
name = models.CharField(
max_length=128, blank=True,
verbose_name=_(u'Config Name'))
thing = GenericRelation(
Thing,
related_query_name='config')
class FirstConfig(Config):
pass
class SecondConfig(Config):
pass
And Here's the admin:
from django.contrib import admin
from .models import FirstConfig, SecondConfig, Thing
class FirstConfigInline(admin.StackedInline):
model = FirstConfig
class SecondConfigInline(admin.StackedInline):
model = SecondConfig
class ThingAdmin(admin.ModelAdmin):
model = Thing
def get_inline_instances(self, request, obj=None):
'''
Returns our Thing Config inline
'''
if obj is not None:
m_name = obj.config_object._meta.model_name
if m_name == "firstconfig":
return [FirstConfigInline(self.model, self.admin_site), ]
elif m_name == "secondconfig":
return [SecondConfigInline(self.model, self.admin_site), ]
return []
admin.site.register(Thing, ThingAdmin)
So far, I've a Thing object with a FirstConfig object linked together.
The code is simplified: in an unrelevant part I manage to create my abstract Config at a Thing creation and set the right content_type / object_id.
Now I'd like to see this FirstConfig instance as an inline (FirstConfigInline) in my ThingAdmin.
I tried with the django.contrib.contenttypes.admin.GenericStackedInline, though it does not work with my current models setup.
I tried to play around with the fk_name parameter of my FirstConfigInline.
Also, as you can see, I tried to play around with the 'thing' GenericRelation attribute on my Config Model, without success..
Any idea on how to proceed to correctly setup the admin?
According to the Django Docs you have to define the ct_fk_field and the ct_field if they were changed from the default values. So it may be enough to set ct_field to config_content_type.
Hope it works!
edit: Those values have to be declared in the Inline:
class SecondConfigInline(admin.StackedInline):
model = SecondConfig
ct_fk_field = "config_object_id"
ct_field = "config_content_type"
edit2:
I just realized an error in my assumption. Usually you should declare the Foreignkey on the Inline-model. Depending on the rest of your code you could just remove the generic Foreignkey on Thing+the genericRelation on Config and declare a normal Foreignkey on the Config-Basemodel.
This question is old, but I'll give it a try anyway.
I think the solution depends on what kind of relation you intend to create between Thing and your Config subclasses.
many-to-one/one-to-many
The way it is currently set up, it looks like a many-to-one relation: each Thing points to a single Config subclass, and many Things can point to the same Config subclass. Due to the generic relation, each Thing can point to a different model (not necessarily a Config subclass, unless you do some extra work).
In this case I guess it would make more sense to put the inline on the admin for the Config. That is, create a GenericStackedInline for Thing (which has the GenericForeignkey), and add the inline to a ConfigAdmin, which you can then use for all Config subclasses. Also see the example below. The generic inline will then automatically set the correct content_type and object_id.
many-to-many
On the other hand, if you are looking for a many-to-many relation between Thing and each Config subclass, then I would move the GenericForeignkey into a separate many-to-many table (lets call it ThingConfigRelation).
A bit of code says more than a thousand words, so let's split up your Thing class as follows:
class Thing(models.Model):
name = models.CharField(max_length=128)
class ThingConfigRelation(models.Model):
thing = models.ForeignKey(to=Thing, on_delete=models.CASCADE)
content_type = models.ForeignKey(ContentType, null=True, blank=True,
on_delete=models.CASCADE)
object_id = models.PositiveIntegerField(null=True, blank=True)
config_object = GenericForeignKey(ct_field='content_type',
fk_field='object_id')
Now it does make sense to add an inline to the ThingAdmin. The following is a bare-bones example of an admin that works for both sides of the relation:
from django.contrib import admin
from django.contrib.contenttypes.admin import GenericStackedInline
from .models import Thing, FirstConfig, SecondConfig, ThingConfigRelation
class ConventionalTCRInline(admin.StackedInline):
model = ThingConfigRelation
extra = 0
class GenericTCRInline(GenericStackedInline):
model = ThingConfigRelation
extra = 0
class ThingAdmin(admin.ModelAdmin):
inlines = [ConventionalTCRInline]
class ConfigAdmin(admin.ModelAdmin):
inlines = [GenericTCRInline]
admin.site.register(Thing, ThingAdmin)
admin.site.register(FirstConfig, ConfigAdmin)
admin.site.register(SecondConfig, ConfigAdmin)
Note that we use the conventional inline for the ForeignKey-side of the relation (i.e. in ThingAdmin), and we use the generic inline for the GenericForeignKey-side (in ConfigAdmin).
A tricky bit would be filtering the content_type and object_id fields on the ThingAdmin.
... something completely different:
Another option might be to get rid of the GenericForeignKey altogether and use some kind of single-table inheritance implementation with plain old ForeignKeys instead, a bit like this.
I'm trying to figure out the best way to set up the following django model (genericised for security reasons).
ThingA:
User(M2M through "UserRelation")
ThingB:
User(M2M through "UserRelation")
ThingC:
User(M2M through "UserRelation")
User:
Login_name
UserRelation:
User (foreginkey)
Thing (foreignkey) #is this generic to any of the above "things"
Privilege
I understand using "through" between two distinct models, but I'm not sure how to apply this to multiple models. Would I define a foreignkey for each of the "Thing" models in my UserRelation Model?
It looks like you are trying to setup a generic many-to-many relationship. There is a dedicated django app that you can be use for this purpose: django-gm2m
Here is how to use it in your generic case:
from django.db import models
from django.contrib.contenttypes.fields import GenericForeignKey
from gm2m import GM2MField
class ThingA(models.Model):
pass
class ThingB(models.Model):
pass
class ThingC(models.Model):
pass
class User(models.Model):
login_name = models.CharField(max_length=255)
things = GM2MField(through='UserRelation')
class UserRelation(models.Model):
user = models.ForeignKey(User)
thing = GenericForeignKey(ct_field='thing_ct', fk_field='thing_fk')
thing_ct = models.ForeignKey(ContentType)
thing_fk = models.CharField(max_length=255)
privilege = models.CharField(max_length=1)
You can now access all the things for a given user and all the User instances for a given 'thing', as well as the privilege attribute for each UserRelation instance.
This will additionally provide you with a handful of benefits (reverse relations, prefetching, etc.) you may need. A GM2MField basically behaves exactly like a django ManyToManyField.
Disclaimer: I am the author of django-gm2m
So I've set up my django site with the following admin.py:
import models
from django.contrib import admin
admin.site.register(models.Comment)
which uses this models.py:
from django.db import models
class Comment(models.Model):
text = models.CharField(max_length=400)
name = models.CharField(max_length=100)
date = models.DateTimeField(auto_now = True)
article = models.CharField(max_length=100)
However, when I go to the admin page, it shows the following:
Which generally is not very helpful. Clicking on each link gives me a page with that object's data, but I would like to be able to see the information for each object in this view. I've been looking at the ModelAdmin class at:
https://docs.djangoproject.com/en/dev/ref/contrib/admin/
but have not managed to wrap my head around it. Is it a separate model class that needs to be kept in sync with my "actual" model? Is it just an interface through which my Admin site accesses the actual model? Does it do what I want (allowing useful data to be shown in the admin interface) or does it do something else?
I'm thinking that the Django Admin page should be able to replace PHPMyAdmin for doing simple tasks, like browsing the DB and manually modifying individual objects. Is that the case?
The admin turns your object into a string so just put a def __str__ or def __unicode__
(As #Mandax has reminded me the docs recommend to define __unicode__ only.)
def __unicode__(self);
return u"%s (%s): %s" % (self.article, self.date, self.name)
Just as it says in the documentation, your model's ModelAdmin describes how the admin section will represent your model. It does need to be somewhat in sync with the actual model, it doesn't make sense for you to display fields that aren't present on your model, etc. You seem interested in the changelist view, which has numerous customization options (all described in the documentation, and in the tutorial). A simple start might be:
from django.contrib import admin
class CommentAdmin(admin.ModelAdmin):
# define which columns displayed in changelist
list_display = ('text', 'name', 'date', 'article')
# add filtering by date
list_filter = ('date',)
# add search field
search_fields = ['text', 'article']
admin.site.register(Comment, CommentAdmin)
There are a lot of options for customization, as always refer to the docs! Finally, you could certainly use it in lieu of PHPMyAdmin, it's very easy to setup admin for browsing, modifying object, etc, how much use you get out of it is up to you.
class Foo(models.Model):
title = models.CharField(max_length=20)
slug = models.SlugField()
Is there a built-in way to get the slug field to autopopulate based on the title? Perhaps in the Admin and outside of the Admin.
for Admin in Django 1.0 and up, you'd need to use
prepopulated_fields = {'slug': ('title',), }
in your admin.py
Your key in the prepopulated_fields dictionary is the field you want filled, and the value is a tuple of fields you want concatenated.
Outside of admin, you can use the slugify function in your views. In templates, you can use the |slugify filter.
There is also this package which will take care of this automatically: https://pypi.python.org/pypi/django-autoslug
Thought I would add a complete and up-to-date answer with gotchas mentioned:
1. Auto-populate forms in Django Admin
If you are only concerned about adding and updating data in the admin, you could simply use the prepopulated_fields attribute
class ArticleAdmin(admin.ModelAdmin):
prepopulated_fields = {"slug": ("title",)}
admin.site.register(Article, ArticleAdmin)
2. Auto-populate custom forms in templates
If you have built your own server-rendered interface with forms, you could auto-populate the fields by using either the |slugify tamplate filter or the slugify utility when saving the form (is_valid).
3. Auto-populating slugfields at model-level with django-autoslug
The above solutions will only auto-populate the slugfield (or any field) when data is manipulated through those interfaces (the admin or a custom form). If you have an API, management commands or anything else that also manipulates the data you need to drop down to model-level.
django-autoslug provides the AutoSlugField-fields which extends SlugField and allows you to set which field it should slugify neatly:
class Article(Model):
title = CharField(max_length=200)
slug = AutoSlugField(populate_from='title')
The field uses pre_save and post_save signals to achieve its functionality so please see the gotcha text at the bottom of this answer.
4. Auto-populating slugfields at model-level by overriding save()
The last option is to implement this yourself, which involves overriding the default save() method:
class Article(Model):
title = CharField(max_length=200)
slug = SlugField()
def save(self, *args, **kwargs):
self.slug = slugify(self.title)
super().save(*args, **kwargs)
NOTE: Bulk-updates will bypass your code (including signals)
This is a common miss-understanding by beginners to Django. First you should know that the pre_save and post_save signals are directly related to the save()-method. Secondly the different ways to do bulk-updates in Django all circumvent the save()-method to achieve high performance, by operating directly on the SQL-layer. This means that for the example model used in solution 3 or 4 above:
Article.objects.all().update(title='New post') will NOT update the slug of any article
Using bulk_create or bulk_update on the Article model will NOT update the slug of any article.
Since the save()-method is not called, no pre_save or post_save signals will be emitted.
To do bulk updates and still utilize code-level constraints the only solution is to iterate objects one by one and call its save()-method, which has drastically less performance than SQL-level bulk operations. You could of course use triggers in your database, though that is a totally different topic.
Outside the admin, see this django snippet. Put it in your .save(), and it'll work with objects created programmatically. Inside the admin, as the others have said, use prepopulated_fields.
For pre-1.0:
slug = models.SlugField(prepopulate_from=('title',))
should work just fine
For 1.0, use camflan's
You can also use pre_save django signal to populate slug outside of django admin code.
See Django signals documentation.
Ajax slug uniqueness validation will be useful too, see As-You-Type Slug Uniqueness Validation # Irrational Exuberance
Auto-populating slugfields at model-level with the built-in django slugify:
models.py
from django.db import models
class Place:
name = models.CharField(max_length=50)
slug_name = models.SlugField(max_length=50)
signals.py
from django.db.models.signals import pre_save
from django.dispatch import receiver
from django.template.defaultfilters import slugify as django_slugify
from v1 import models
#receiver(pre_save, sender=models.Place)
def validate_slug_name(sender, instance: models.Place, **kwargs):
instance.slug_name = django_slugify(instance.name)
Credits to: https://github.com/justinmayer/django-autoslug/blob/9e3992296544a4fd7417a833a9866112021daa82/autoslug/utils.py#L18
prepopulated_fields = {'slug': ('title',), }
autoslug has worked quite well for me in the past. Although I've never tried using it with the admin app.