Django：如何根据查找表的值连接表

Question

I am trying to do what one might consider an advanced sql query and would like to know if its possible in Django without resorting to raw sql (I will if its necessary).

I want to join 1 or another table based on a value located in a table lookup table and would like to do this entirely in python/django.

The following are rough examples of the models I am using:

class SpecificProduct(models.Model):
    specific_product_id = models.AutoField(primary_key=True)
    a_field = models.TextField()
    something_specific_to_this_model = models.CharField()


class GeneralProduct(models.Model):
    other_product_id = models.AutoField(primary_key=True)
    text = models.TextField()

TABLE_CATEGORIES = {
    1 : SpecificProduct,
    2 : GeneralProduct,
}

class ProductCategory(models.Model):
    category_id = models.AutoField(primary_key=True)
    table_category = models.IntegerField()  # Technically represents a table.
    category_text = models.CharField(max_length=20)


class Inventory(models.Model):
    inventory_id = models.AutoField(primary_key=True)
    product_category = models.ForeignKey(ProductCategory, on_delete=models.CASCADE)
    product_pk = models.IntegerField()  # Technically foreign key to a product table.
    quantity = models.IntegerField()

What I want is a method like:

def get_product(category_id, product_pk):
    # SQL query  magic
    return one_object_of_a_specific_product_type

This method should be able to do things like...

• Give me the product (model) where the product_category = 1 and the product_pk = 1 . (returns a SpecificProduct model)
• Give me the product where product_category = 2 and the product_pk = 50 (returns a GeneralProduct model)

How do you do this query in Django and is this even possible?

Edit: Based on Kireeti K's response I have created models that look like the following:

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


class SpecificProduct(models.Model):
    specific_product_id = models.AutoField(primary_key=True)
    specific_text = models.TextField()

class GeneralProduct(models.Model):
    general_product_id = models.AutoField(primary_key=True)
    text = models.TextField()

class ProductCategoryLookup(models.Model):
    category_id = models.ForeignKey(ContentType, on_delete=models.CASCADE, primary_key=True)
    category_text = models.CharField(max_length=20)


class Inventory(models.Model):
    inventory_id = models.AutoField(primary_key=True)
    product_category = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    product_id = models.PositiveIntegerField()
    product = GenericForeignKey('product_category', 'product_id')
    quantity = models.IntegerField()


def get_product(category_id, product_pk):
    content_type = ContentType.objects.get(id=category_id)
    inventory = Inventory.objects.get(product_category=content_type, product_id=product_pk).first()
    return inventory.product

Answer 1

You can use a generic-foreign-key to get foreign-key relation with any model sort of dynamically, read about it here. https://docs.djangoproject.com/en/2.1/ref/contrib/contenttypes/#generic-relations

If you rewrite your models using generic-foreign-key then it looks something like this.

class ProductCategory(models.Model):
    category_id = models.AutoField(primary_key=True)
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    object_id = models.PositiveIntegerField()
    table_category = models. GenericForeignKey()  # Technically represents a table.
    category_text = models.CharField(max_length=20)


class Inventory(models.Model):
    inventory_id = models.AutoField(primary_key=True)
    product_category = models.ForeignKey(ProductCategory, 
                       on_delete=models.CASCADE)
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    object_id = models.PositiveIntegerField()
    product = models. GenericForeignKey()  # Technically foreign key to a product table.
    quantity = models.IntegerField()

Now to achieve what you want, you can implement your function like this.

def get_product(model=None, category_id, product_pk):
    model = "specificproduct" if model else "generalproduct"
    content_type = ContentType.objects.get(model=model)
    inventory = Inventory.objects.get(product_category_id=category_id, object_id=product_pk, content_type=content_type)

    return inventory.product