Interview question

What is the difference between Django's abstract base classes and multi-table inheritance for models? Django models में abstract base classes और multi-table inheritance में क्या अंतर है?

Answer

Type	Database tables created	Use case
Abstract base class	Only for child models, parent has NO table	Share common fields/methods without a shared table
Multi-table inheritance	Separate table for BOTH parent and child, linked by implicit OneToOne	Real 'is-a' relationships needing independent parent queries
Proxy models	No new table - changes only behavior (Meta options, methods)	Different Python behavior for the same underlying data

# Abstract base class - no table for CommonInfo itself
class CommonInfo(models.Model):
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    class Meta:
        abstract = True  # KEY - prevents Django from creating a table for this

class Article(CommonInfo):
    title = models.CharField(max_length=200)
    # Article table includes: id, title, created_at, updated_at

# Multi-table inheritance - separate tables, linked automatically
class Place(models.Model):
    name = models.CharField(max_length=100)
    address = models.CharField(max_length=200)

class Restaurant(Place):  # NOT abstract - creates its own table too
    serves_pizza = models.BooleanField(default=False)

# Restaurant table has: place_ptr_id (OneToOne to Place), serves_pizza
# Place table has: id, name, address

restaurant = Restaurant.objects.create(name='Pizzeria', address='123 St', serves_pizza=True)
place = Place.objects.get(id=restaurant.id)  # accessible as a Place too
print(place.name)  # Pizzeria - works because Restaurant IS-A Place in the DB

# Proxy model - same table, different Python-level behavior
class OrderedArticle(Article):
    class Meta:
        proxy = True
        ordering = ['title']  # only affects default query ordering, no new table

    def summary(self):
        return self.title[:50]

Type	Database tables	Use case
Abstract base class	सिर्फ child models के लिए, parent का table नहीं	Common fields share करना बिना shared table के
Multi-table inheritance	Parent और child दोनों के अलग tables	Real 'is-a' relationships
Proxy models	नया table नहीं	Behavior बदलना, data नहीं

# Abstract base class
class CommonInfo(models.Model):
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    class Meta:
        abstract = True

class Article(CommonInfo):
    title = models.CharField(max_length=200)

# Multi-table inheritance
class Place(models.Model):
    name = models.CharField(max_length=100)
    address = models.CharField(max_length=200)

class Restaurant(Place):
    serves_pizza = models.BooleanField(default=False)

restaurant = Restaurant.objects.create(name='Pizzeria', address='123 St', serves_pizza=True)
place = Place.objects.get(id=restaurant.id)
print(place.name)  # Pizzeria

# Proxy model
class OrderedArticle(Article):
    class Meta:
        proxy = True
        ordering = ['title']

    def summary(self):
        return self.title[:50]

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