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Find definitions of Rails methods using source_location and bundle open

Given it’s a dynamic language, it’s important that Ruby comes with several excellent debugging and introspection features out of the box.

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Finding the exact place a method or block of code is defined, and being able to read the related source code, is essential for effective debugging and code comprehension. In Ruby, the source_location method provides a powerful tool for retrieving the file and line number for where a particular method or block is defined.

Explore Rails using…

…the #source_location method.

#=> ["/.../activesupport-7.0.8/lib/active_support/core_ext/string/inflections.rb", 60]

Then open from the command line:

bundle open activesupport

…which opens the source code of the gem in your editor of choice.


Use of source_location is invaluable when you’re new to an application, re-exploring unfamiliar code, or trying to understand which gem is providing the functionality you’re using.

Reading source code is a great way to learn. Reading battle-tested code like that of Rails itself, or other gems, even more so.

Thanks to the authors of the framework, source_location also works for “magic” meta-programmed methods in Rails. Methods on Active Record associations that are generated using class_eval pass special syntax to enable the lookup to still work. If it weren’t for this, when you called source_location on these methods, you’d always just see (eval) as the first result.

class Car < ApplicationRecord
  has_many :seats

  # ...

#=> [".../activerecord-7.0.8/lib/active_record/associations/builder/association.rb", 103]

Then open from the command line:

bundle open activerecord

Why not?

While you’re experimenting, you might find source_location doesn’t always provide a helpful result. Methods included as part of “core” Ruby are often implemented in C, and their definitions are not directly accessible from Ruby code. Therefore, calling source_location on a core method will typically return nil.

#=> nil

It also won’t work for methods that use a C extension (where Ruby code calls out to C). source_location only works for methods defined in gems where the source code is in Ruby.

And while source_location can be invaluable during development and debugging, don’t accidentally include it in production code!

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Last updated on December 4th, 2023 by @andycroll

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