I am working to figure out the model for a Django project: an app to track Books.
Among other fields, every Book has either/both a Printer and a Publisher, which are basically identical. So, here's how it stands:
class Book(models.Model):
title = models.CharField(max_length=100)
printer = models.ForeignKey('Printer')
publisher = models.ForeignKey('Publisher')
class Printer(models.Model):
name = models.CharField(max_length=100)
location = models.CharField(max_length=100)
class Publisher(models.Model):
name = models.CharField(max_length=100)
location = models.CharField(max_length=100)
It seems to me this is bad database form: it's not DRY. In addition, quite often, a Book might be printed by a firm which publishes the same or another book: in other words, the tables can overlap. So, the two models Printer and Publisher should really be combined, while they need to remain distinct in the admin.
My question: how best to do this? Should I create another model, Firm, and create one-to-one relationships between it and Printer/Publisher?
The Django way to handle that is to create an Abstract Base Model. This is the DRY way to create your models. Here is the code:
class BaseModel(models.Model):
name = models.CharField(max_length=100)
location = models.CharField(max_length=100)
class Meta:
abstract = True
class Printer(BaseModel):
pass
class Publisher(BaseModel):
pass
This will allow you to specify redundant fields only once. Also, if you need to add any extra fields to one model, just add them instead of using pass.
Related
I need extend a model from another model.
Case:
core/models.py
class Master(models.Model):
code = models.CharField(max_length=30, unique=True)
name = models.CharField(max_length=100, blank=False, null=False)
class Meta:
abstract = True
class City(Master):
zipcode = models.IntegerField()
custom/models.py
from core.models import City
class City(City)
newfield = models.CharField(max_length=20)
custom is an app.
I have tried with proxy model but it is not what I need, since proxy model adds a new table. https://docs.djangoproject.com/en/2.2/topics/db/models/#proxy-models
I need is that when I migrate add the new field to City.
More info.
In core the table is created and in custom you can add new fields that the client needs. The idea is that core is only maintained as standard.
Proxy models don't add new tables. From the docs link you mentioned:
The MyPerson class operates on the same database table as its parent Person class.
If you want one table called core_city, and another called custom_city, the second one having an extra field, you simply subclass it. Perhaps it would be easier to use an alias:
from core.models import City as CoreCity
class City(CoreCity):
newfield = models.CharField(max_length=20)
custom_city will have all fields from core_city, plus a newfield. The description of how this works (and an example) is covered in the docs section Multi-table inheritance.
If what you want is to have one single database table, then you should use a proxy Model, however they really don't allow you to create new fields. The field should be created in the parent model, or otherwise exist in the database and not be handled by Django migrations at all.
You are looking for Abstract base classes models:
Abstract base classes are useful when you want to put some common information into a number of other models. You write your base class and put abstract=True in the Meta class.
This is the base class:
#core/models.py
class City(Master):
zipcode = models.IntegerField()
class Meta:
abstract = True # <--- here the trick
Here your model:
#custom/models.py
from core.models import City as CoreCity
class City(CoreCity):
newfield = models.CharField(max_length=20)
For many uses, this type of model inheritance will be exactly what you want. It provides a way to factor out common information at the Python level, while still only creating one database table per child model at the database level.
You can update or create your class constants after its defined like this
from core.models import City
City.newfield = models.CharField(max_length=20)
You may need to use swappable models, using them you can define a City class and change it with whichever model you need later,
but that way you can't import and use the base City model directly, you may need to provide a method like get_city_model for that, as your public API.
class City(Master):
zipcode = models.IntegerField()
class Meta:
swappable = 'CORE_CITY_MODEL'
and maybe replace it later with some other model, then just set CORE_CITY_MODEL to that model in the form of 'app_name.model_name'.
The django.contrib.auth is a good example of this, you may consider checking User model and get_user_model method. Although I think you may face problems if you change your city model after you did run migrate, it may not move your data to the new table, but I'm not sure about this.
So, in our database we had a Car model, but there could be different kinds of cars, with different fields and different processes associated to them, so I made some model classes that inherit from that Car, it's something like this:
class Car(models.Model):
field = models.CharField(max_length=300)
class CarA(Car):
field_a = models.CharField(max_length=300)
class CarA1(CarA):
field_a1 = models.CharField(max_length=300)
class CarA2(CarA):
field_a2 = models.CharField(max_length=300)
class CarB(Car):
field_b = models.CharField(max_length=300)
Also, there are trips, and each trip has a car associated to them:
class Trip(BaseModel):
car = models.ForeignKey(Car)
If I have a CarA1 instance, car_a1, I can save it in a Trip with car_a1.car_ptr, but it would be saved as a Car, and from there I don't think there's a way to get the CarA1 instance again except searching by the Car id in all the subclasses.
So, if I want to know which kind of car the trip has, or get a specific subclass field, it would be really complicated.
What do you suggest to do? I can just keep only the Car class with the specific fields nullable (field_a, field_b, etc), and a field to specify the kind of car, but that doesn't seem right, I would have to check on the kind to validate the fields, etc. But right now it seems like the better solution.
I'm using Django 1.11 and Python 3.6 btw.
Sorry I didn't use real code, I thought it would be way more complicated, also sorry if I didn't explain it very well, I'm sleep deprived and stressed.
Thanks!
The problem in your model is when a new type of car introduced say car c you need to create another table for that. It is not ideal and what you need is one table for store the car details(commonly used) and separate table say Properties to store other details and give a foreign-key relation to Car model.
First, you can get to the child model from the parent model (see model based inheritance).
If you want to stick with your inheritance based design, you could define a field in Car called car_type which would correspond to the type of the child table. For example:
class Car(models.Model):
CAR_A = 'car_a'
CAR_B = 'car_b'
CAR_TYPE_CHOICES = (
(CAR_A, 'Car_A'),
(CAR_B, 'Car_b'),
)
field = models.CharField(max_length=300)
car_type = models.CharField(max_length=20, choices=CAR_TYPE_CHOICES)
And then you could override the save method of the child models so that their car_type field is the one that corresponds to their model.
class CarA(Car):
field_a = models.CharField(max_length=300)
def save(self, *args, **kwargs):
self.car_type = Car.CAR_A
super(CarA, self).save(*args, **kwargs)
# ... similarly for CarB
Now you can get to the child model from Car based on its car_type.
if car.car_type === Car.CAR_A:
print(car.cara.field_a)
Note that this will start to get cumbersome if you have many levels of inheritance, so I would encourage you to reconsider your design.
I would like to be able to let users create locations "on-the-fly" when they create a report. Using the following models if possible.
models:
class Report(models.Model):
...
location = forms.ManyToManyField(Location)
class Location(models.Model):
name = forms.CharField(max_length=255)
...
The behavior I am searching for is close to the one given by the django formsets.
Indeed I am currently able to somewhat do that using instead a foreignkey relationship and relying on formsets. Using this technic,django-extra-views and django-dynamic-formset, I am even able to let users choose how many locations they want to add.
A strip down version of the actual setup :
models:
class Report(models.Model):
...
class Location(models.Model):
name = forms.CharField(max_length=255)
report = forms.ForeignKey('Report')
forms:
class LocationForm(forms.ModelForm):
class Meta:
model = Location
fields = '__all__'
class LocationFormSet(extra_views.InlineFormSet):
model = TestLocation
form_class = TestLocationForm
views:
class ReportCreateView(extra_views.CreateWithInlinesView):
model = Report
inlines = [LocationFormSet, ]
Unfortunately using this setup, a location can not belong to more than one report which bring a lot of duplicated location. That's why I think the ManyToMany relationship would be better suited. I stumbled upon some answers related to this question that talked about intermediary model but I wasn't able to go anywhere on my own to make it work.
Is their a way to tweak my working setup to make it work with a manytomany relationship? Or does it exists a completely different approach to let user create "on-the-fly" the locations during the report creation?
Thank's!
For create "on-the-fly" I had changed ManyToMany by ForeingKey.
class Report(models.Model):
...
location = forms.ManyToManyField(Location) # REMOVE
class Location(models.Model):
report = models.ForeignKey(Report) # ADD
name = forms.CharField(max_length=255)
...
And then, InlineForms will work.
I'm designing a new Django app and due to several possibilities, I'm not sure which would be the best, thus I'd like opinions, and hopefully improve what I got so far.
This question comes close but not quite. This one touches the flat/nested subject which is helpful, while still not answering the question.
There are many others on the same subject, and yet none tell me what I want to know.
Background
The models have each unique properties with some shared attributes, and I need to reference them in another model, optimally with a single entry point rather than having a field for each possible model.
I want to be able to do complex Django ORM queries involving the Base class and filter by SubClass when needed. E.g Event.objects.all() to return all events. I'm aware of Django model utils Inheritance Manager and intend to use it if possible.
Also, I'll be using django admin to create and manage the objects, so an easy integration is a must. I want to be able to create a new SubEvent directly, without having first to create a Event instance.
Example
To illustrate, let's say I have the following models for app A.
class Event(models.Model):
commom_field = models.BooleanField()
class Meta:
abstract = True
class SubEventA(Event):
email = models.EmailField(unique=True)
class SubEventB(Event):
title = models.TextField()
class SubEventC(Event):
number = models.IntegerField(default=10)
# and so on
And also an app B, where I want to be able to reference a event which can be of any type, like:
class OtherModel(models.Model):
event = models.ForeignKey('A.Event')
# This won't work, because `A.Event` is abstract.
Possible solutions
Use a GenericForeignKey.
# B.models.py
class OtherModel(models.Model):
content_type = models.ForeignKey(ContentType)
object_id = models.PositiveIntegerField()
event = GenericForeignKey('content_type', 'object_id')
What I don't like about this is that I'll lose the querying capabilities Django ORM has, and I might need to do additional fiddling to get it working on admin. Not sure, never dealt with this before
Flatten Event
I can bring it all up to the base class and have flags or checks outside the model definition, something like:
class Event(models.Model):
commom_field = models.BooleanField()
email = models.EmailField(blank=True)
title = models.TextField(blank=True)
number = models.IntegerField(default=10)
This might seem like the best idea at first, but of course there are other kind of fields, and that forces me to allow nulls/blanks for most of them (like the email field), losing the db level integrity check.
OneToOne relationships
Rather than abstract like on 1 or flatten on 2 it is possible to have a db table for each, where the models will look like:
class Event(models.Model):
commom_field = models.BooleanField()
class SubEventA(models.Model):
event = models.OneToOneField(Event)
email = models.EmailField(unique=True)
class SubEventB(models.Model):
event = models.OneToOneField(Event)
title = models.TextField(blank=True)
class SubEventC(models.Model):
event = models.OneToOneField(Event)
number = models.IntegerField(default=10)
So far it solved the two initial problems, but now when I get to the admin interface, I'll have to customize each form to create the base Event before saving a SubEvent instance.
Questions
Is there a better approach?
Can any of the choices I present be improved in any direction (ORM query, DB constraints, admin interface)?
I've pondered about both answers and came up with something based off of those suggestions. Thus I'm adding this answer of my own.
I've chosen to use django-polymorphic, quite nice tool suggested by #professorDante. Since this is a multi-table inheritance, #albar's answer is also somewhat correct.
tl;dr
django-polymorphic attends the 3 main requirements:
Allow django ORM querying style
Keep db level constraints by having a multi-table inheritance and one table for each sub class
Easy django admin integration
Longer version
Django-polymorphic allows me to query all different event instances from the base class, like:
# assuming the objects where previously created
>>> Event.objects.all()
[<SubEventA object>, <SubEventB object>, <SubEventC object>]
It also has great django admin integration, allowing seamless objects creation and editing.
The models using django-polymorphic would look like:
# A.models.py
from polymorphic import PolymorphicModel
class Event(PolymorphicModel):
commom_field = models.BooleanField()
# no longer abstract
class SubEventA(Event):
email = models.EmailField(unique=True)
class SubEventB(Event):
title = models.TextField()
class SubEventC(Event):
number = models.IntegerField(default=10)
# B.models.py
# it doesnt have to be polymorphic to reference polymorphic models
class OtherModel(models.Model):
event = models.ForeignKey('A.Event')
Besides, I can reference only the base model from another class and I can assign any of the subclasses directly, such as:
>>> sub_event_b = SubEventB.objects.create(title='what a lovely day')
>>> other_model = OtherModel()
>>> other_model.event = sub_event_b
My .2c on this. Not sure about your design in #3. Each SubEvent subclasses Event, and has a one-to-one to Event? Isn't that the same thing?
Your proposal on the Generic Key is exactly what it is designed for.
Another possibility - Polymorphism with Mixins. Use something like Django-polymorphic, so querying returns you the subclass you want. I use this all the time and its super useful. Then make Mixins for attributes that will be reused across many classes. So a simple example, making an email Mixin
class EmailMixin(models.Model):
email = models.EmailField(unique=True)
class Meta:
abstract = True
Then use it
class MySubEvent(EmailMixin, models.Model):
<do stuff>
This way you dont have redundant attributes on subclasses, as you would if they were all in the parent.
Why not a multi-table inheritance?
class Event(models.Model):
commom_field = models.BooleanField()
class SubEventA(Event):
email = models.EmailField(unique=True)
class SubEventB(Event):
title = models.TextField(blank=True)
class SubEventC(Event):
number = models.IntegerField(default=10)
I have an entity in my application which has some attribute like auth.User but it has also some extra attributes. So I created a OneToOne relationship with auth.User
class UserEntity(models.Model):
user = models.OneToOneField(User)
... other fields ...
I also have a Person model which is UserEntity so created it as this:
class Person (models.Model):
userEntity = models.OneToOneField(UserEntity)
... other fields ...
There are many different attributes like addresses,experiences,education and other details that I want to associate with my Person. A Person can have many addresses,experiences,education,speciality
I have a Speciality Model like this.
class Speciality(models.Model):
name = models.CharField()
code = models.CharField()
... other fields ...
The problem is how should I my data model be designed so that I can retrieve a person/user with all addresses,expereiences,specialities etc. I can associate each Person/user with Speciality since Speciality is an independent model and can exist without a user. Currently I have created another model for each e.g for Speciality I have created
class PersonSpeciality(models.Model):
person = models.ForeignKey(Person)
specialities = models.ForeignKey(Speciality)
Can I design it in a better way so I can have fast searching and retrievals and no mess.
Thank you
If a Person can have multiple Specialities, then you can implement a oneToMany relationship, just by adding a foreignKey:
class Speciality(models.Model):
name = models.CharField()
code = models.CharField()
person = models.ForeignKey(Person)
However, if you have several Persons with the same specialities (let's say 10 Persons are Painters), you would have 10 different Painters entries.
To avoid duplication, you can implement a ManyToMany relationship :
class Speciality(models.Model):
name = models.CharField()
code = models.CharField()
person = models.ManyToManyField(Person)
Django will create a third table in your DB to handle that (you don't need to write your PersonSpeciality class).