form-v1.rb

# Imagine we have a form that only sets user name for verified users
class Form
  include ActiveModel::Model

  attr_accessor :name

  def save(user)
    user.name = name if user.verified?
  end
end

# Now we want to make a new form that always sets user name, we have to reach into the superclass and copy its logic and introduce brittleness whenever the superclass logic changes.
class ApiForm < Form
  def save(user)
    # yuck, what if we add a new line of code to Form#save ?
    user.name = name
  end
end

form-v2.rb

# Instead, implement your base class based on an abstraction:
class Form
  include ActiveModel::Model

  attr_accessor :name

  def save(user)
    if update_user_name?
      user.name = name
    end
  end

  private

  def update_user_name?
    user.verified?
  end
end

# Simply override the template method from the base class
class ApiForm < Form
  private

  def update_user_name?
    # awesome, no longer have to change every time Form#save changes
    true
  end
end