[PYTHON] Django Polymorphic Associations Tutorial

Introduction

My article Django behaves differently from other Polymorphic I wrote about how Django's Polymorphic Model is different from the so-called polymorphic model. This time, I'll explain how to implement ** polymorphic related ** in Django.

environment

• Python 3.7.5
• Django 2.2.1
• SQLite

goal

In the Rails Guide's Polymorphic Associations chapter, polymorphic related is implemented as follows.

class Picture < ApplicationRecord
  belongs_to :imageable, polymorphic: true
end
 
class Employee < ApplicationRecord
  has_many :pictures, as: :imageable
end
 
class Product < ApplicationRecord
  has_many :pictures, as: :imageable
end

The goal is to implement a Model that has the following ER-like attributes.

coontent_type indicates which table is associated and ʻobject_id` indicates which record is associated.

Implementation

Modeling

from django.db import models
from django.contrib.contenttypes.models import ContentType


class Picture(models.Model):
    object_id = models.IntegerField(db_index=True)
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    file_name = models.CharField()


class Employee(models.Model):
    name = models.CharField()
    email = models.EmailField()


class Product(models.Model):
    name = models.CharField()
    price = models.IntegerField()

Implementation of Imageable class

from django.db import models
from django.contrib.contenttypes.fields import GenericRelation, GenericForeignKey
from django.contrib.contenttypes.models import ContentType


class Picture(models.Model):
    object_id = models.IntegerField(db_index=True)
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    file_name = models.CharField(max_length=256)
    content_object = GenericForeignKey('content_type', 'object_id')


class Imageable(models.Model):
    class Meta:
        abstract = True

    pictures = GenericRelation(Picture)


class Employee(Imageable):
    name = models.CharField(max_length=256)
    email = models.EmailField()


class Product(Imageable):
    name = models.CharField(max_length=256)
    price = models.IntegerField()

Operation check

#migration
$ python manage.py makemigrations polymorphic_associations
$ python manage.py migrate polymorphic_associations
$ python manage.py shell


#Data creation
>>> from polymorphic_associations.models import Employee, Product
>>>
>>> employee = Employee(name='John', email='[email protected]')
>>> employee.save()
>>> employee.pictures.create(file_name='employee.jpg')
<Picture: Picture object (1)>
>>>
>>> product = Product(name='Desk', price=1000)
>>> product.save()
>>> product.pictures.create(file_name='product.jpg')
<Picture: Picture object (2)>


#Data acquisition
>>> employee.pictures.all()
<QuerySet [<Picture: Picture object (1)>]>
>>> employee.pictures.first().file_name
'employee.jpg'
>>>
>>> product.pictures.all()
<QuerySet [<Picture: Picture object (2)>]>
>>> product.pictures.first().file_name
'product.jpg'


#SQL confirmation
>>> str(employee.pictures.all().query)
'SELECT
    "polymorphic_associations_picture"."id",
    "polymorphic_associations_picture"."object_id",
    "polymorphic_associations_picture"."content_type_id",
    "polymorphic_associations_picture"."file_name"
FROM
    "polymorphic_associations_picture"
WHERE
    (
        "polymorphic_associations_picture"."content_type_id" = 2
    AND "polymorphic_associations_picture"."object_id" = 1
    )'
>>>
>>> str(product.pictures.all().query)
'SELECT
    "polymorphic_associations_picture"."id",
    "polymorphic_associations_picture"."object_id",
    "polymorphic_associations_picture"."content_type_id",
    "polymorphic_associations_picture"."file_name"
FROM
    "polymorphic_associations_picture"
WHERE
    (
        "polymorphic_associations_picture"."content_type_id" = 3
    AND "polymorphic_associations_picture"."object_id" = 1
    )'

You can see that the created data can identify the table and record by content_type_id and ʻobject_id. This allows all tables with images to be implemented quickly by inheriting ʻImageable. Also, by implementing image-related processing in Imageable, it is possible to prevent logic from being distributed to each model and service. This source code is posted on Git.

reference

• https://guides.rubyonrails.org/association_basics.html#polymorphic-associations
• https://stackoverflow.com/questions/51824934/how-to-make-a-django-model-commentable-likeable-and-rateable