Single Table Inheritance — One Table for a Class Hierarchy

In this tutorial, you'll learn how the Single Table Inheritance pattern maps an entire class hierarchy into a single database table.

What You'll Learn

how the Single Table Inheritance pattern maps an entire class hierarchy into a single database table.

Why It Matters

Joining across hierarchy tables is slow. Single Table Inheritance avoids joins at the cost of nullable columns.

Real-World Use

Hibernate SINGLE_TABLE strategy, Entity Framework TPH, and Rails STI use this pattern.

The Single Table Inheritance Pattern

The Single Table Inheritance pattern addresses a specific recurring design problem by providing a reusable solution structure. Understanding when and how to apply it is essential for writing maintainable, scalable code.

Key Concepts

• Registry/Tracking: Single Table Inheritance maintains a registry of objects or operations.
• Atomicity: Changes are grouped into units that succeed or fail together.
• Isolation: Each unit operates independently.
• Consistency: The pattern ensures data integrity across operations.

Structure

The following diagram shows the structure of this pattern:

classDiagram
    class SingleTableInheritance {
        -new: List
        -dirty: List
        -removed: List
        +registerNew()
        +registerDirty()
        +registerRemoved()
        +commit()
    }
    class Entity { id data }
    class DataMapper { +insert() +update() +delete() }
    SingleTableInheritance --> Entity : tracks
    SingleTableInheritance --> DataMapper : persists

Implementation

from abc import ABC, abstractmethod
from dataclasses import dataclass, field
from typing import List, Dict

@dataclass
class Entity:
    id: int
    data: str = ""

class SingleTableInheritanceRegistry:
    def __init__(self):
        self._new: List[Entity] = []
        self._dirty: List[Entity] = []
        self._removed: List[Entity] = []

    def register_new(self, e: Entity):
        self._new.append(e)

    def register_dirty(self, e: Entity):
        if e not in self._dirty:
            self._dirty.append(e)

    def register_removed(self, e: Entity):
        self._removed.append(e)

    def commit(self):
        print(f"Inserting {len(self._new)} new entities")
        print(f"Updating {len(self._dirty)} dirty entities")
        print(f"Deleting {len(self._removed)} removed entities")
        self._new.clear()
        self._dirty.clear()
        self._removed.clear()

# Usage
reg = SingleTableInheritanceRegistry()
e1 = Entity(1, "Alice")
e2 = Entity(2, "Bob")
reg.register_new(e1)
reg.register_new(e2)
e1.data = "Alice Updated"
reg.register_dirty(e1)
reg.register_removed(e2)
reg.commit()

Expected output:

Inserting 2 new entities
Updating 1 dirty entities
Deleting 1 removed entities

Key Participants

• Single Table Inheritance: Coordinates tracking and persistence of changes.
• Entity: The domain object being tracked.
• Client: Code that uses the Single Table Inheritance.
• Data Mapper: Handles actual database operations.

Real-World Examples

• DodaTech uses this pattern internally for consistent cross-cutting concerns.
• Major frameworks and libraries implement this pattern as a core architectural element.
• Production systems at scale depend on this pattern for reliability.

Related Patterns

• Class Table Inheritance

• Concrete Table Inheritance

• Data Mapper

• Design Patterns — the complete patterns catalog.

Pros and Cons

Common Mistakes

1. **Over-engineering: Applying Single Table Inheritance where a simpler solution suffices, adding unnecessary complexity.

2. **Wrong granularity: Implementing Single Table Inheritance at the wrong level of abstraction.

3. **Thread Safety ignored: Using Single Table Inheritance in concurrent context without proper synchronization.

4. **Tight coupling: Violating the pattern intent by creating hidden dependencies.

5. **Premature optimization: Introducing Single Table Inheritance before there is evidence it is needed.

Practice Questions

1. What problem does the Single Table Inheritance pattern solve? Describe a real-world scenario where using it improves code quality.

2. How does Single Table Inheritance differ from alternative approaches? What are the trade-offs?

3. What testing strategy would you use for code that implements Single Table Inheritance?

4. How would you refactor legacy code to introduce Single Table Inheritance?

5. When should you NOT use Single Table Inheritance? Describe scenarios where it adds unnecessary complexity.

Challenge

Implement a complete Single Table Inheritance example in Python with unit tests. Include error handling, edge cases (empty data, null values, concurrent access), and a performance comparison against a simpler alternative. Document your design decisions.

Real-World Task

Find a section of code in your current project that could benefit from the Single Table Inheritance pattern. Refactor it, write tests, and measure the improvement in testability, coupling, and cohesion.

Security Tip: When implementing Single Table Inheritance, ensure proper input validation, avoid exposing internal state, and follow Least Privilege. At DodaTech, all implementations undergo security review.

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