A.2 ActiveRecord (A single record)

A.2.1 All column names

You get a method for every column in the table; for example, if you have retrieved an individual record from the Contact table, you can call .first_name, .last_name, etc, on it.

This means that every ActiveRecord object will have methods .id, .created_at, and .updated_at, since every table has those columns.

Full explanation

A.2.2 Any other instance methods you define in the class

It’s often helpful to define your own instance methods in the model file; for example, you might want to define a method .full_name:

class Contact < ApplicationRecord
  def full_name
    return self.first_name + " " + self.last_name
  end
end

You would then be able to call .full_name anywhere in the application that you wind up with an individual Contact object.

Full explanation