Dependency injection is not a virtue

27 Feb 2019

In languages less open than Ruby, hard-coded class references can make testing tough. If your Java code has Date date = new Date();` buried in its guts, how do you set it to a known value you can then compare against in your tests? Well, you don’t. So what you do instead is pass in the date as part of the parameters to your method. You inject the dependency on Date. Yay, testable code!

As has unfortunately happened with a variety of patterns that originate from rigid languages like Java, Dependency Injection has spread and been advocated as a cross-language best practice on trumped up benefits of flexibility and malleability. If your code never knows exactly who it’s talking to, it can talk to anyone! Testing stubs, mocks, and future collaborators. Hogwash.

If you really want to get to grips with Ruby development and gain a solid understanding of Object Oriented Design, I thoroughly recommend Sandi Metz's Practical Object Oriented Design in Ruby. It's the perfect introduction to OOP and pragmatic Ruby. You can buy it here.

“Meticulously pragmatic and exquisitely articulate, Practical Object Oriented Design in Ruby makes otherwise elusive knowledge available to an audience which desperately needs it. The prescriptions are appropriate both as rules for novices and as guidelines for experienced professionals.”

– Katrina Owen, Creator, Exercism

Dependency injection is not a virtue

“Meticulously pragmatic and exquisitely articulate, Practical Object Oriented Design in Ruby makes otherwise elusive knowledge available to an audience which desperately needs it. The prescriptions are appropriate both as rules for novices and as guidelines for experienced professionals.”

Essential Reading: Learn Rails 6