Inheritance & Polymorphism In Relational Databases solving some common problems in web development

The Problem Books & Televisions Both SKU Title Description Manufacturer Books ISBN Author Televisions: Screen Size Resolution

Books & Televisions

Both

SKU

Title

Description

Manufacturer

Books

ISBN

Author

Televisions:

Screen Size

Resolution

Own Tables Duplication of shared fields Gets worse with more products Lose indexing of shared fields i.e. SKU unique across products SELECT * FROM books WHERE id = 7 SELECT id, title, ‘book’ AS type FROM books UNION SELECT id, title, ‘television’ AS type FROM televisions

Duplication of shared fields

Gets worse with more products

Lose indexing of shared fields

i.e. SKU unique across products

Single table inheritance (STI) No duplication Wasted fields Gets worse with more products Rails built in inheritance SELECT * FROM products WHERE id = 7 SELECT id, title, type FROM products

No duplication

Wasted fields

Gets worse with more products

Rails built in inheritance

Class Table Inheritance (CTI) No duplication No wasted fields Index shared fields Referential integrity Difficult to enforce books or televisions Creating and reading becomes more complex SELECT books.*, products.* FROM books JOIN products ON(products.id = books.product_id) WHERE books.id = 7 SELECT products.id, products.title, books.id, televisions.id FROM products LEFT JOIN books ON(books.product_id = products.id) LEFT JOIN televisions ON(televisions.product_id = products.id)

No duplication

No wasted fields

Index shared fields

Referential integrity

Difficult to enforce books or televisions

Creating and reading becomes more complex

Another Problem Tags for Books and Televisions Reference Products from Tags What about tagging other things besides Products as well?...

Tags for Books and Televisions

Reference Products from Tags

What about tagging other things besides Products as well?...

Multiple Tables Duplication of Tag structure Need new table for each thing Can’t index across all tags

Duplication of Tag structure

Need new table for each thing

Can’t index across all tags

Multiple Relationships Single Tag structure Can index all Tags Need similar tables for anything we want to tag Need some app logic Deleting Product deletes relationship, not Tags themselves

Single Tag structure

Can index all Tags

Need similar tables for anything we want to tag

Need some app logic

Deleting Product deletes relationship, not Tags themselves

Class Table Inheritance Generalize Products and Images Avoids duplication Alter existing structures Increasing complexity Books span 3 tables Tightly coupled Adding Comments: “TaggableCommentables”?

Generalize Products and Images

Avoids duplication

Alter existing structures

Increasing complexity

Books span 3 tables

Tightly coupled

Adding Comments: “TaggableCommentables”?

Polymorphism Simple Just one Tags table to Tag anything No changes to existing entities Can handle tagging of any future entities Loses referential integrity Need to enforce in application logic Comments could be added similarly Rails has it built in

Simple

Just one Tags table to Tag anything

No changes to existing entities

Can handle tagging of any future entities

Loses referential integrity

Need to enforce in application logic

Comments could be added similarly

Rails has it built in