NoMethodError (undefined method) from Private method in class - ruby-on-rails

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.