来自类中Private方法的NoMethodError（未定义方法）

Question

Why can't I use a private method from within my class? How do I fix my code to prevent the error?

module CarRegistration
  class Basics < Base

    fields_of_model(:car).each do |attr|
      delegate attr.to_sym, "#{attr}=".to_sym, to: :car
    end

    private

    car_structure = #array of hashes

    def fields_of_model(model)
      car_structure.select {|record| record[:model] == model}.map{|record| record[:name]}
    end
end

error

NoMethodError (undefined method `fields_of_model' for CarRegistration::Basics:Class):

Answer 1

I think you have a number of problems going on here.

First, you've defined fields_of_model as an instance method, here:

def fields_of_model(model)
  car_structure.select {|record| record[:model] == model}.map{|record| record[:name]}
end

but you're trying to call it from the class, here:

fields_of_model(:car).each do |attr|
  delegate attr.to_sym, "#{attr}=".to_sym, to: :car
end

So, you'll want to make fields_of_model a class method, and define it before you call it. Something like:

module CarRegistration
  class Basics < Base

    private

    car_structure = #array of hashes

    class << self

      def fields_of_model(model)
        car_structure.select {|record| record[:model] == model}.map{|record| record[:name]}
      end

    end

    fields_of_model(:car).each do |attr|
      delegate attr.to_sym, "#{attr}=".to_sym, to: :car
    end

end

You'll also have problems with that car_structure variable, I think, because it'll be out of scope for the class method. So, I think you need to make a class-level instance variable. So, give this a try:

module CarRegistration
  class Basics < Base

    @car_structure = #array of hashes

    class << self

      def fields_of_model(model)
        @car_structure.select {|record| record[:model] == model}.map{|record| record[:name]}
      end

      private :fields_of_model

    end

    fields_of_model(:car).each do |attr|
      delegate attr.to_sym, "#{attr}=".to_sym, to: :car
    end

end

Note that I made the class method, :fields_of_models private using private :fields_of_model .

To demonstrate the whole thing, I ginned up this RSpec test:

require 'rails_helper'

class Car

  attr_accessor *%w(
    color
    make
    year
  ).freeze

end

module CarRegistration
  class Basic

    @car_structure = [
      {model: :car, name: :color},
      {model: :car, name: :make},
      {model: :car, name: :year}
    ]

    class << self

      def fields_of_model(model)
        @car_structure.select {|record| record[:model] == model}.map{|record| record[:name]}
      end

      private :fields_of_model

    end

      fields_of_model(:car).each do |attr|
        delegate attr.to_sym, "#{attr}=".to_sym, to: :car
      end

      def car
        @car ||= Car.new 
      end

  end
end

RSpec.describe CarRegistration::Basic do
    it "has :fields_of_model as a private class method" do 
      expect(CarRegistration::Basic.public_methods).not_to include(:fields_of_model)
      expect(CarRegistration::Basic.private_methods).to include(:fields_of_model)
    end
    it "responds to :color and :color=" do
      expect(car_registration).to respond_to(:color)
      expect(car_registration).to respond_to(:color=)
    end
    it "sets and gets attributes on car" do
      expect(car_registration.color).to be_nil
      expect(car_registration.car.color).to be_nil
      car_registration.color = :red
      expect(car_registration.car.color).to eq(:red)
      expect(car_registration.color).to eq(:red)
      expect(car_registration.instance_variable_get(:@color)).to be_nil 
    end
end

def car_registration
  @car_registration ||= described_class.new
end

Which, when run, yields:

CarRegistration::Basic
  has :fields_of_model as a private class method
  responds to :color and :color=
  sets and gets attributes on car

Finished in 0.733 seconds (files took 27.84 seconds to load)
3 examples, 0 failures

BTW, having this code in your class outside of a def - end is just fine and not the root of your problem. In fact, it's quite normal.

Also, I will note that Jörg W Mittag wishes to say:

I am one of those Ruby Purists who likes to point out that there is no such thing as a class method in Ruby. I am perfectly fine, though, with using the term class method colloquially, as long as it is fully understood by all parties that it is a colloquial usage . In other words, if you know that there is no such thing as a class method and that the term "class method" is just short for "instance method of the singleton class of an object that is an instance of Class ", then there is no problem. But otherwise, I have only seen it obstruct understanding.

Let it be fully understood by all parties that the term class method is used above in its colloquial sense.

Answer 2

Because you wrote the method not in def - end clause; you should write it like

def my_method
  fields_of_model(:car).each do |attr|
    delegate attr.to_sym, "#{attr}=".to_sym, to: :car
  end
end

That is why the error message says CarRegistration::Basics:Class as opposed to CarRegistration::Basics

Here is a sample code that works. Usually there is no need to put a class inside Module , but if you must for some reason, this is a way.

module CarRegistration
  class Basics < Object
    def run(model)
      fields_of_model(model)
    end

    private

    def fields_of_model(model)
      puts model
    end
  end
end

a = CarRegistration::Basics.new
a.run('xyz')  # => 'xyz' is printed.