update attribute

guitar.rb

# update_attribute

guitar.update_attribute(:state, 'used')

# method on model

class Guitar
  def mark_as_used
    self.state = 'used'
    self.save
  end
end

guitar.mark_as_used

# skipping validations the short way :)

class Guitar
  def mark_as_used
    update_attribute(:state, 'used')
  end
end

guitar.mark_as_used