1. Introduction

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ContentType solves the problem where two fields in one table are associated with two (or more) other tables.

# Course Table - Free Courses
# id         name     
   1         Linux Basics                   
   2         Python Basics                       


# DegreeCourse Table - Degree Courses
# id         name        
   1         Python Full Stack               


# PricePolicy Table - Two fields correspond to the id and table name of the above tables
# id        day        price        course_id        course_type
   1         7          0            1                Free Course
   2         7          0            2                Free Course
   3         14         9.9          2                Free Course
   4         180        18888        1                Degree Course   

Modify the PricePolicy table to associate with Django's content_type, which stores all table names in Django.

# PricePolicy Table
# id         day         price        course_id    course_type(foreign key)
   1         7           0            1            1
   2         14          9.9          1            1
   3         180         18888        1            2  


# ContentType Table
id         model            app_label
1          course            app01
2          degreecourse      app01  

2. Custom Methods for Operations

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Model Definitions

from django.db import models

class Course(models.Model):
    name = models.CharField(max_length=32)
    def __str__(self):
        return self.name

class DegreeCourse(models.Model):
    name = models.CharField(max_length=32)
    def __str__(self):
        return self.name

# Using ContentType but not its methods
from django.contrib.contenttypes.models import ContentType
class PricePolicy(models.Model):
    day = models.IntegerField()
    price = models.CharField(max_length=8)
    course_id = models.IntegerField()  
    content_type = models.ForeignKey(to=ContentType, null=True)  # Will be changed to foreign key

   # Custom operations
   # 1. Insert a strategy into course data; use _meta.model_name to get table name string
   # Method: First query course; get table name through course; insert data into policy table
   course = models.DegreeCourse.objects.first()
   contenttype = models.ContentType.objects.filter(model=course._meta.model_name).first()
   models.PricePolicy.objects.create(day=7, price=0, course_id=course.id, content_type=contenttype)


   # 2. Query policies by course
   # Method: Same as above
   course = models.DegreeCourse.objects.first()
   contenttype = models.ContentType.objects.filter(model=course._meta.model_name).first()
   price_policy = models.PricePolicy.objects.filter(course_id=course.id, content_type=contenttype).values('day', 'price')
   print(price_policy)


   # Query course via policy using apps.get_model to retrieve table class
   # Method: First query policy table; get table class via get_model; then query data in course table
   from django.apps import apps
   price_policy_obj = models.PricePolicy.objects.filter(day=7, price=0).first()
   model = apps.get_model('API', price_policy_obj.content_type.model)   # First parameter is app name, second is table name, returns table class
   course = model.objects.filter(pk=price_policy_obj.course_id)
   print(course)

3. Using ContentType Methods for Operations

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Model Definiitons

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

class Course(models.Model):
    name = models.CharField(max_length=32)
    price_policy = GenericRelation(to='PricePolicy')
    def __str__(self):
        return self.name

class DegreeCourse(models.Model):
    name = models.CharField(max_length=32)
    price_policy = GenericRelation(to='PricePolicy')
    def __str__(self):
        return self.name

class PricePolicy(models.Model):
    day = models.IntegerField()
    price = models.CharField(max_length=8)
    object_id = models.IntegerField()
    content_type = models.ForeignKey(to=ContentType, null=True)
    model_obj = GenericForeignKey()

# Note:
#     1. model_obj refers to DegreeCourse or Course table objects
#     2. object_id refers to course ID, corresponding to parameters in GenericForeignKey
#     3. If customizing object_id and content_type, parameters in GenericForeignKey and GenericRelation must also be modified, e.g.:
#         course_id = models.IntegerField()  
#         course_type = models.ForeignKey(to=ContentType, null=True)
#         model_obj = GenericForeignKey(ct_field='course_type', fk_field='course_id')
#
#         price_policy = GenericRelation(to='PricePolicy', object_id_field='course_id', content_type_field='course_type')

# 1. Insert a strategy for course data
degree_course = models.DegreeCourse.objects.first()
models.PricePolicy.objects.create(day=180, price=18880, model_obj=degree_course)


# 2. Find all price policies for all courses
price_policy_list = models.PricePolicy.objects.all()
for price_policy in price_policy_list:
    print(price_policy.model_obj.name, price_policy.day, price_policy.price)


# 3. Query policies by course
course = models.Course.objects.first()
price_policys = course.price_policy.all()
for price_policy in price_policys:
    print(course.name, price_policy.day, price_policy.price)