Design Principles & SOLID

Design Principles & SOLID covers: Head First OOA&D, The Object-Oriented Thought Process, The Design of Design. Master the fundamental principles that govern all software design: SOLID, coupling and cohesion, separation of concerns, composition over inheritance. Move from "I know these acronyms" to "I use these to make daily decisions." Rails violates SOLID in intentional, documented ways. ActiveRecord violates Single Responsibility (model + persistence). Concerns violate Interface Segregation (wide interfaces mixed in). Convention over configuration violates Open/Closed in places. These are design decisions by DHH, not mistakes. Understanding SOLID means understanding why Rails chose to violate specific principles and when those violations cost you. Any mature Rails codebase has both — places where Rails conventions are perfect and places where they have created 500-line models. Module 1 gives you the language to tell the difference.