Inheritance Mapping Selector

When to Use

Relational databases have no native concept of inheritance. Every time an OO hierarchy needs to be persisted, a structural decision must be made: how does the class tree become table(s)? This skill routes that decision.

Apply this skill when:

• You have an OO inheritance hierarchy and need to choose a database schema strategy
• You are configuring @Inheritance in Hibernate/JPA or equivalent ORM annotation
• Your current STI table has grown so wide that NULLs dominate most rows
• Your current CTI (joined) queries are slow because every polymorphic read needs a multi-table join
• You are refactoring a legacy schema where inheritance was never explicitly designed

Prerequisites: none. If the domain-logic pattern (Transaction Script / Domain Model) is not yet settled, clarify that first — it affects whether you need inheritance mapping at all.

Context & Input Gathering

Before scoring, collect:

Sufficiency check: You need at minimum the class hierarchy shape and an estimate of polymorphic read frequency. Everything else refines the recommendation.

Process

Step 1 — Map the hierarchy structure

List all classes: which are abstract, which are concrete, how deep the tree is, and where fields cluster.

Why: STI and CTI (joined) require a clear understanding of which classes share common fields. Concrete Table requires knowing all concrete classes, since each gets its own table. Deep hierarchies with many levels favor CTI; shallow hierarchies with low field divergence favor STI.

Step 2 — Score on six trade-off dimensions

For each of the three candidate strategies, score the trade-offs given this specific hierarchy:

Why: These six dimensions are the primary axes Fowler identifies for distinguishing the three patterns. Scoring them forces an explicit trade-off rather than defaulting to whichever the ORM does by default.

Step 3 — Apply the primary routing rule

Use the dominant signals to select a strategy:

→ Single Table Inheritance when:

• Few subclass-specific columns (most fields live on the superclass)
• Polymorphic reads are frequent and join cost is a concern
• Refactoring the hierarchy is likely (field promotions/demotions should not require schema migration)
• Constraints on subclass columns are not needed

→ Class Table Inheritance (Joined) when:

• Subclasses have substantially divergent fields (many NULL columns in STI would be a problem)
• FK integrity on subclass-specific columns is required
• Domain model clarity matters to DBAs or reporting tools
• Polymorphic read frequency is moderate (joins are acceptable)

→ Concrete Table Inheritance when:

• Subclasses are largely independent (rarely queried polymorphically)
• Other applications or reporting tools read the database directly and benefit from standalone tables
• Each concrete class stands alone with a full, self-contained schema

Why: Fowler offers no single universal recommendation — the decision is genuinely context-dependent. But each strategy has dominant use cases where it wins cleanly.

Step 4 — Check for mixing

Evaluate whether different branches of the hierarchy have significantly different trade-off profiles.

IF the hierarchy has a stable branch with few subclass fields (STI-favorable) AND a divergent branch with many subclass-specific fields and constraint needs (CTI-favorable): → Use Inheritance Mappers as the coordinator layer and apply different strategies per branch.

Fowler explicitly permits this: "The trio of inheritance patterns can coexist in a single hierarchy."

Why: Mixing avoids forcing the entire hierarchy into the strategy that fits only the worst-case branch. The Inheritance Mappers pattern provides the implementation scaffold — an abstract mapper per class plus a separate coordinator mapper for the supertype — that makes mixed strategies workable without duplicating load/save logic.

Step 5 — Map to ORM idiom

Translate the chosen strategy to the team's ORM:

Note: Hibernate's TABLE_PER_CLASS (Concrete) generates UNION queries automatically for polymorphic finds. This convenience hides the performance cost — monitor actual query plans.

Why: The choice must translate to idiomatic ORM configuration. Choosing CTI and then fighting the ORM defaults erodes the benefit.

Step 6 — Produce the decision record

Write the inheritance mapping decision record (see Outputs).

Inputs

• Required: OO inheritance hierarchy (class names, field locations, depth, abstract vs. concrete)
• Required: Polymorphic read frequency estimate (high / medium / low)
• Helpful: ORM/framework in use
• Helpful: FK constraint and data integrity requirements
• Helpful: Whether other systems read the database directly
• Helpful: Existing schema (if refactoring)

Outputs

Inheritance Mapping Decision Record containing:

## Inheritance Mapping Decision — [Hierarchy Name]

### Hierarchy Summary
[Superclass] → [Subclasses list]
Abstract classes: [list]
Concrete classes: [list]
Field distribution: [X fields on superclass, Y per subclass avg]

### Six-Dimension Score
| Dimension           | STI | CTI | Concrete |
|---------------------|-----|-----|----------|
| Joins               | ✅  | ⚠️  | ❌       |
| Wasted space        | ⚠️  | ✅  | ✅       |
| FK enforcement      | ❌  | ✅  | ⚠️       |
| Ad-hoc readability  | ⚠️  | ✅  | ✅       |
| Refactoring impact  | ✅  | ⚠️  | ⚠️       |
| Polymorphism cost   | ✅  | ⚠️  | ❌       |

### Recommendation
**Primary strategy:** [STI / CTI / Concrete / Mixed]
**Rationale:** [2-3 sentences]

### SQL Schema Sketch
[simplified CREATE TABLE statements]

### ORM Configuration
[annotated class snippet for the team's ORM]

### Mixing Note
[IF mixing: which branches use which strategy, and why]

### Migration Path
[steps to get from current state to chosen strategy]

Key Principles

1. Single Table is Fowler's default for simple hierarchies — with explicit caveats. STI wins on query simplicity and refactoring tolerance. Its costs (NULLs, no constraints) are real but manageable for hierarchies with few subclass-specific fields. Fowler's own Player/Footballer/Cricketer example uses it as the primary illustration. The caveat: once subclass-specific field count grows substantially, the NULL bloat and constraint inability become genuine liabilities.

2. Class Table (Joined) buys normalization at the cost of joins — and joins compound with depth. CTI is the most "object-aligned" strategy: one table per class mirrors the class hierarchy. Its cost is that loading any object requires touching multiple tables. For a two-level hierarchy (parent + one child table), one join is tolerable. For a four-level hierarchy, you may be joining four tables per load. Monitor query plans; the supertype table also becomes a bottleneck since every query touches it.

3. Concrete Table's UNION penalty is the silent killer. Concrete Table looks attractive in isolation: no joins, no NULLs, self-contained tables. The danger emerges when someone queries at the supertype level. The ORM (or developer) must UNION all concrete tables. As the hierarchy grows, this query becomes increasingly expensive. Concrete Table is appropriate only when polymorphic supertype queries are rare or absent.

4. FK constraints are incompatible with STI for subclass-specific columns. If a Footballer must have a club and that column must be NOT NULL and FK-constrained, STI cannot enforce this — the column exists on all rows, including Cricketers who have no club. CTI (or Concrete) is required when FK integrity on subclass fields is non-negotiable.

5. Mixing is the escape hatch — use it deliberately, not ad-hoc. Fowler explicitly endorses mixing strategies within one hierarchy. The right way to implement mixing is the Inheritance Mappers pattern: each class has its own mapper (abstract or concrete), and a separate coordinator mapper handles supertype-level operations. Do not mix strategies without this scaffolding — the result is tangled load/save logic that is difficult to maintain.

6. Keys must be unique across all tables in Concrete Table — the database won't enforce this for you. When using Concrete Table, the database's primary key constraint only guarantees uniqueness within one table. A Footballer with id=42 and a Cricketer with id=42 are valid in the database — but they will collide on any polymorphic Identity Field lookup. You need a cross-table key allocation strategy (e.g., a shared sequence, UUID keys, or application-level key tracking).

Examples

Example 1: Sports Player Hierarchy (STI recommended)

Scenario: SportsDB system models Player (name, dateOfBirth) with three subclasses: Footballer (club, position), Cricketer (battingAverage), Bowler (bowlingSpeed, bowlingStyle). The application frequently loads "all active Players" for reporting and roster views. Hierarchy is unlikely to deepen further.

Trigger: "Should we use STI or separate tables for Player/Footballer/Cricketer? We need to query all players at once frequently."

Process:

1. Hierarchy: 3 concrete classes, 1 abstract base. Subclass-specific fields: 1–2 per subclass. Field divergence is low.
2. Dimension score: STI wins on joins (none), refactoring tolerance (none), polymorphism (single query). Cost: wasted columns for ~2–3 nullable columns per row — tolerable. No FK constraint requirement on club or battingAverage.
3. Routing: STI. Fowler uses this exact example to demonstrate Single Table Inheritance.
4. Mixing: Not needed — all branches are similar in shape.
5. ORM config: Hibernate @Inheritance(SINGLE_TABLE) with @DiscriminatorColumn(name="type") on Player; @DiscriminatorValue("F") on Footballer.

Output (schema sketch):

CREATE TABLE players (
  id         BIGINT PRIMARY KEY,
  type       VARCHAR(1) NOT NULL,      -- discriminator: F, C, B
  name       VARCHAR(100) NOT NULL,
  club       VARCHAR(100),             -- Footballer only
  batting_avg DECIMAL(5,2),            -- Cricketer only
  bowling_speed INT,                   -- Bowler only
  bowling_style VARCHAR(50)            -- Bowler only
);

Example 2: Organization Hierarchy with Strict Constraints (CTI recommended)

Scenario: HR system with LegalEntity (taxId, registeredAddress) as abstract base, Corporation (stockExchange, tickerSymbol), Partnership (partnerCount), SoleTrader (tradingName) as concrete subclasses. Every Corporation must have a non-null stockExchange. The compliance team requires FK integrity on all subtype-specific columns. Polymorphic reads ("load any LegalEntity by id") occur at application startup only; most queries are type-specific.

Trigger: "We need strict FK constraints on Corporation fields. Can we still use inheritance?"

Process:

1. Hierarchy depth: 2 levels. Subclass fields: 3–5 per subclass, highly divergent.
2. Dimension score: FK enforcement → CTI required (STI cannot enforce NOT NULL on stockExchange for Corporations). Refactoring impact acceptable (hierarchy is stable by design). Polymorphic reads are infrequent → join cost acceptable.
3. Routing: Class Table Inheritance (Joined). Hibernate @Inheritance(JOINED).
4. Mixing: Not needed.

Output (schema sketch):

CREATE TABLE legal_entities (id BIGINT PRIMARY KEY, tax_id VARCHAR(20), address TEXT, type VARCHAR(20));
CREATE TABLE corporations (id BIGINT REFERENCES legal_entities(id), stock_exchange VARCHAR(10) NOT NULL, ticker VARCHAR(10) NOT NULL);
CREATE TABLE partnerships (id BIGINT REFERENCES legal_entities(id), partner_count INT NOT NULL);
CREATE TABLE sole_traders (id BIGINT REFERENCES legal_entities(id), trading_name VARCHAR(100) NOT NULL);

Example 3: Independent Product Types, Rarely Queried Polymorphically (Concrete recommended)

Scenario: E-commerce catalog with Product as abstract base and three highly divergent concrete types: PhysicalProduct (weight, dimensions, shippingClass), DigitalProduct (fileSize, downloadUrl, licenseType), SubscriptionProduct (billingCycle, trialDays, renewalPolicy). Each has 8–12 unique fields. The reporting team queries each type independently ("all digital downloads this month"). Polymorphic "all products" queries exist only in one admin view.

Trigger: "Our Product table is a mess — 40 columns, half NULL on any given row. What's the alternative?"

Process:

1. Subclass fields: 8–12 per subclass, highly divergent. Only 3 shared fields on Product (id, name, price).
2. Dimension score: Current STI is the problem (wasted space). Concrete Table wins on no NULLs, self-contained tables, per-class query performance. Cost: polymorphic "all products" query → UNION. But this query is rare (one admin view).
3. Routing: Concrete Table Inheritance. Cross-table key uniqueness → use UUID primary keys.
4. Key uniqueness: UUIDs eliminate the cross-table collision problem.
5. Polymorphic admin view: Accept a UNION query here — document the performance expectation; cache if needed.

Output (schema sketch):

CREATE TABLE physical_products (id UUID PRIMARY KEY, name VARCHAR, price DECIMAL, weight_kg DECIMAL, dim_cm VARCHAR, shipping_class VARCHAR);
CREATE TABLE digital_products (id UUID PRIMARY KEY, name VARCHAR, price DECIMAL, file_size_mb INT, download_url TEXT, license_type VARCHAR);
CREATE TABLE subscription_products (id UUID PRIMARY KEY, name VARCHAR, price DECIMAL, billing_cycle VARCHAR, trial_days INT, renewal_policy VARCHAR);
-- Polymorphic admin query:
SELECT id, name, price, 'physical' AS type FROM physical_products
UNION ALL SELECT id, name, price, 'digital' FROM digital_products
UNION ALL SELECT id, name, price, 'subscription' FROM subscription_products;

References

• references/six-dimension-trade-off-matrix.md — Full trade-off matrix with rationale per cell and worked examples
• references/inheritance-mappers-scaffold.md — Inheritance Mappers implementation pattern for mixed-strategy hierarchies
• references/orm-config-reference.md — ORM configuration snippets for Hibernate, Rails, Django, SQLAlchemy, Doctrine

License

This skill is licensed under CC-BY-SA-4.0. Source: BookForge — Patterns of Enterprise Application Architecture by Martin Fowler, David Rice, Matthew Foemmel, Edward Hieatt, Robert Mee, Randy Stafford.

Related BookForge Skills

Install related skills from ClawhHub:

• clawhub install bookforge-data-source-pattern-selector
• clawhub install bookforge-object-relational-structural-mapping-guide

Or install the full book set from GitHub: bookforge-skills