What's the deal with super?

Context

• was explaining this user model
• one very clever student noticed super in password_digest getter (line 23)
• his question was 'how' is super working here, but I answered 'why'
• he clarified that he understood what it was doing, but now how
• so I answered why again
• he clarified once more and I realized I had always just accepted it
• yet another time for me to look at the code and say "I don't know how that works", in front of 50 people
• we tolerate a lot of magic in Rails
• so I decided to try and make a model

The Model

Defining Accessors Lazily Based on Column Names

• needs to define attribute methods
• should do it lazily, upon method missing is a good enough time
• need to keep track of whether we have built our accessors or not, so we don't do it infinity times, use a class instance variable type boolean
• if we haven't built yet, call self.define_accessors
• return early if already defined
• also make an attributes method that returns a hash
• iterate over column names, write getters and setters for each using define_method

super

• accessors need to be overridable
• can't do it in parent class, other models share it
• can't just do it earlier in the class, redefined methods can't super
• module is the way to get this behavior
• when the class is inherited we get a callback, inherited, can insert a module then, use an instance variable to keep a pointer to this module
• then we write methods inside of the module using module_eval
• now we can override these methods and call the original using super

Key Points

• ActiveRecord stores data in a central attributes hash
• attribute methods are defined lazily
• previous rails versions defined attribute methods using method_missing, the first time they are used (4.0.2)
• now they are defined when the first instance is created (initialize in the base classe) (4.2.0)
• a module is created and set as an instance variable when ActiveRecord::Base is inherited
• when the methods are written they go into this module

Try it Yourself!

• here is the template I used to get started
• here is the finished version for your reference

References

• attribute methods (4.0.2)
• attribute methods (4.2.0)
• active model core (4.2.0)