undefined method `-' for nil:NilClass for active record callback of existing field

Question

I have a form with a checkbox of different styles. The form works fine, but I am getting an empty string saved in the array of style options. I would like to remove it. So I created a before_save callback..

Parameters: {"utf8"=>"✓","car_form"=>{"styles"=>["","Black"]}}

VIEW

<%= f.input :styles, as: :check_boxes, collection: FormCollection::Styles.all %>

MODEL

class Car < ApplicationRecord
  before_save :remove_empty_from_styles

  def remove_empty_from_styles
     styles = (styles - ['']) if styles_changed?
  end
end

However, I get:

undefined method `-' for nil:NilClass

However, if I comment out the call back everything works fine, the values get saved, like normal.

NOTES: styles is not nil.

Answer 1

You need to access styles on self:

def remove_empty_from_styles
  self.styles = styles - [''] if styles_changed?
end

styles = styles - [''] assigns the result of styles - [''] to a local variable called styles.

self.styles = styles - [''] invokes a styles= method on self passing styles - [''] as an argument.