• Don't confuse attr_accessor and attr_accessible -- they have very similar names
  • attr_accessor is a pure Ruby function to create reader and writer methods for an instance variable
  • attr_accessible is a pre-Rails 4 function to list the model attributes (e.g., database fields) that can be set via mass assignment, providing access control

You can use attr_accessor on a Rails model to create non-persisted instance variables. For example:

class Stock
  attr_accessor :current_price
end

This allows you to get the @current_price of a stock -- something that would make no sense to persist.