The following collection select helper for an element of ‘class ComponentOrder’ is failing with the error shown

<%= f.collection_select(:supplier_req_id, component.suppliers, :id, :price_per_um, prompt: t('component.assign_supplier') ) %>

undefined method `price_per_um' for an instance of Supplier

it runs when referencing its own variable f.collection_select(:supplier_req_id, component.suppliers, :id, :name_supplier, using the collection of child records, as

class Component < ApplicationRecord
  has_many   :component_suppliers
[...]
class Supplier < ApplicationRecord
  has_many   :component_suppliers
[...]
class ComponentSupplier < ApplicationRecord
  belongs_to :component
  belongs_to :supplier
[...]
class ComponentOrder < ApplicationRecord
  belongs_to :supplier_req, class_name: 'Supplier', optional: true

the join class defines a method to determine a value. The goal is to use it both as a method for storing data, as well as a view helper for the collection_select.

class ComponentSupplier < ApplicationRecord
  def price_per_um
    if self.last_quote_per_package
      min_units = self.units_per_package * self.unit_quantity 
      value_per_um = self.last_quote_per_package / min_units
      value_per_um.to_s + ' ' + unit_um.name
    else
      '?? ' + unit_um.name
    end
  end

self.units_per_package and self.unit_quantity have default values of 1

Rails raises the error because it is relying on the model definition of ComponentOrder.

How can the collection select therefore invoke the desired price_per_um values?