Ruby Mixin module not saving Active Record property - ruby-on-rails

Question

I have a eBook resource with a value property:
class EBook < ApplicationRecord
include Mixin
end
and a module:
module Mixin
extend ActiveSupport::Concern
included do
# validations
belongs_to :user
end
def change_value
#value = 200
end
end
I would like to be able to call EBook.change_value and have that instance's value set to 200. How can I do this? Is this an antipattern? I can't seem to find anything that will allow me to change instance state through a module.
Using the rails console I get this output:
EBook Load (0.3ms) SELECT `e_books`.* FROM `e_books` ORDER BY `e_books`.`id` ASC LIMIT 1 OFFSET 1
=> 200
but it doesn't update or save the model.

Answer 1

ActiveRecord does not use separate instance variables for attributes represented in database.
Change your method to
def change_value
self.value = 200
end
in order to use setter method generated by ActiveRecord for your model.
In order to clear it up a bit more, this is what your code did:
class Ebook < ApplicationRecord
attr_reader :value
def change_value
#value = 200
end
end
2.5.1 :001 > e = Ebook.new
=> #<Ebook id: nil, value: nil>
2.5.1 :002 > e.change_value # this sets your instance_variable
=> 200
2.5.1 :003 > e
=> #<Ebook id: nil, value: nil> # ActiveRecord's value remain nil
2.5.1 :004 > e.value # reads from instance variable as we've overwritten the method with attr_reader
=> 200
2.5.1 :005 > e.read_attribute(:value) # reads from ActiveRecord's attributes
=> nil
2.5.1 :006 > e.tap(&:save)
=> #<Ebook id: 3, value: nil> # as expected, nothing is saved