Class vs Instance Methods

Learning Goals

Identify use cases for class methods and instance methods in the context of a Rails app
Identify available AR methods depending on current context of self
Differentiate when to use a class method or an instance method

Set Up

This lesson plan starts with the class-vs-instance-methods-setup branch of our good old Set List Tutorial Repo. Get it here. Then, follow the normal setup tasks:

Run bundle install
Run rails db:{drop,create,migrate,seed}

Exploration

Open up artist.rb and artist_spec.rb side-by-side in your code editor. You’ll notice a pry in some of the methods defined in the Artist class. You can run the spec with bundle exec rspec spec/models/artist_spec.rb to hit the first pry. You can enter exit into pry to continue on to the next pry. Use the code, test, and each of the prys to answer the following questions with your group in a break out room:

For each of the methods defined in the Artist class:

Is the method defined as a class or instance method?
What is the value of self inside the method?
How do you call the method? In other words, what can you call the method on?
How can you update the method body to use self?

Discussion Questions

What is a class method?
What is an instance method?
What is self in each context?
What does an instance of a model represent in our DB?
What does the class of a model represent in our DB?

Takeaways

Review these after your discussion

In a Rails app, a class method allows you to perform actions on or query data from an entire database table rather than one specific row (or instance) of the data. self refers to the entire class or table.
In an instance method, you can perform actions or retrieve information about one instance of the class, which represents one row of data in a database table. self in these methods refers to the single model object.

Practice Problems

Using model tests and the corresponding models only, write methods that will:

Return all songs sorted by title alphabetically
Return all of an artist’s songs sorted by title alphabetically
Return the x shortest songs, where x is an argument for the method
Return the x shortest songs for an artist, where x is an argument for the method

Spicy

Return a song’s artist’s name
Return the number of songs for an artist that have at least 1 play and a length greater than 0
Return a list of songs that have a title that contains the word “love”
Return the 3 songs that have the most plays, a length greater than x where x can be any integer value, and were updated within the last three days

Answers to these practice problems can be found on the class-vs-instance-complete branch here.

Checks for Understanding

Answer the following questions either in your notebook or by taking this review quiz.

How do you know whether a task requires a class or an instance method?
What are some common error messages we might see if we confuse a class method with an instance method and vice versa?
How can we list the methods available for self in pry?
What does an instance of a model represent in our DB?
What does the class of a model represent in our DB?