Building a Mini ORM with Ruby Metaprogramming
By Albin Yesudasan
Object-Relational Mapping frameworks like ActiveRecord power modern development, but the mechanisms behind them remain mysterious to many developers. Let’s demystify them by building our own.
The MiniRecord Base Class
Our minimal ORM demonstrates building a complete data layer leveraging Ruby’s metaprogramming capabilities. The MiniRecord class serves as the base with several powerful macros:
column— defines model attributeshas_many— one-to-many associationsbelongs_to— inverse associationsbefore_save/after_save— lifecycle callbacks
Dynamic Associations
Association methods are dynamically created using define_method. The belongs_to method generates getter and setter methods automatically, while has_many generates collection methods that query related records.
The Power of column
The column method demonstrates powerful metaprogramming at work:
- Stores column metadata for the model
- Generates accessors via
attr_accessor - Creates dynamic finder methods like
find_by_email
Each column definition creates multiple methods automatically, reducing boilerplate code dramatically.
Callbacks
Callbacks store methods as class variables and invoke them at appropriate lifecycle points. The before_save and after_save callbacks wrap the persistence operation, allowing custom logic to run automatically.
Persistence Layer
For simplicity, our ORM uses CSV files for storage — but the principles apply directly to SQL databases. The key patterns — dynamic method generation, metadata tracking, and lifecycle hooks — are the same ones ActiveRecord uses under the hood.
Key Metaprogramming Concepts
send— invokes dynamically named methods- Ruby’s assignment syntax — assignments are actually method calls under the hood
define_singleton_methodvsdefine_method— class methods vs instance methods
Conclusion
Building a mini ORM reveals the elegant metaprogramming that powers frameworks like ActiveRecord. Understanding these fundamentals makes you a more effective Rails developer.