与：inverse_of的多态ActiveRecord关联的“未定义方法'scan'for nil：NilClass”

Question

I have an issue with a "has_many" relationship, where I am getting a random exception from within ActiveRecord:

product = Product.create!(valid_attributes)
product.prices 
# throws:
NoMethodError:
       undefined method `scan' for nil:NilClass

This appears to have to do with "inverse_of", but I've apparently done something that ActiveRecord didn't expect, but couldn't be bothered to have a good error for. Best guess is it has something to do with my column named "column" (although that isn't on the blacklist AFAIK). I'm using PostgreSQL. EDIT: tried renaming the column to "column_name" and "parent_column", and that didn't fix it. Will try some other things.

Here is the relevant model code and schema:

class Price < ApplicationRecord
  belongs_to :parent, polymorphic: true
end

class Product < ApplicationRecord
  has_many :prices, as: :parent, inverse_of: :parent
end

class CreatePrices < ActiveRecord::Migration[5.2]
  def change
    create_table :prices do |t|
      t.string :parent_type, null: false
      t.bigint :parent_id, null: false
      t.string :column, null: false
      t.decimal :price, null: false, precision: 15, scale: 2
      t.timestamp :effective_date, null: false
    end

    add_index :prices, [:parent_type, :parent_id, :column]
  end
end

And the full stacktrace:

NoMethodError:
       undefined method `scan' for nil:NilClass
     # /Users/william/.rbenv/versions/2.5.5/lib/ruby/gems/2.5.0/gems/activerecord-5.2.2.1/lib/active_record/inheritance.rb:185:in `compute_type'
     # /Users/william/.rbenv/versions/2.5.5/lib/ruby/gems/2.5.0/gems/activerecord-5.2.2.1/lib/active_record/reflection.rb:422:in `compute_class'
     # /Users/william/.rbenv/versions/2.5.5/lib/ruby/gems/2.5.0/gems/activerecord-5.2.2.1/lib/active_record/reflection.rb:379:in `klass'
     # /Users/william/.rbenv/versions/2.5.5/lib/ruby/gems/2.5.0/gems/activerecord-5.2.2.1/lib/active_record/reflection.rb:234:in `inverse_of'
     # /Users/william/.rbenv/versions/2.5.5/lib/ruby/gems/2.5.0/gems/activerecord-5.2.2.1/lib/active_record/reflection.rb:239:in `check_validity_of_inverse!'
     # /Users/william/.rbenv/versions/2.5.5/lib/ruby/gems/2.5.0/gems/activerecord-5.2.2.1/lib/active_record/reflection.rb:474:in `check_validity!'
     # /Users/william/.rbenv/versions/2.5.5/lib/ruby/gems/2.5.0/gems/activerecord-5.2.2.1/lib/active_record/associations/association.rb:26:in `initialize'
     # /Users/william/.rbenv/versions/2.5.5/lib/ruby/gems/2.5.0/gems/activerecord-5.2.2.1/lib/active_record/associations.rb:237:in `new'
     # /Users/william/.rbenv/versions/2.5.5/lib/ruby/gems/2.5.0/gems/activerecord-5.2.2.1/lib/active_record/associations.rb:237:in `association'
     # /Users/william/.rbenv/versions/2.5.5/lib/ruby/gems/2.5.0/gems/activerecord-5.2.2.1/lib/active_record/associations/builder/association.rb:108:in `prices'

Answer 1

The answer was that I was violating ActiveRecord's expectations in a way that I didn't realize, and should have included in the original question, but did not due to hubris. I was using an anonymous model in testing. For example:

module Priced
  extend ActiveSupport::Concern

  included do
    has_many :prices, as: :parent, inverse_of: :parent
  end
end

RSpec.describe Priced do
  let(:model_class) {
    Class.new(ActiveRecord::Base).tap do |klass|
      klass.table_name = "products"
      klass.send(:include, Priced)
    end
  end

  it "defines a 'has_many :prices' association" do
    model = model_class.create!
    price = Price.create!(parent: model, price: 100)
    expect(model.prices).to eq([price])
  end
end

This lets me use an ActiveRecord model with an existing table, but de-couple it from all the logic in the actual Product model.

I had never encountered any issues with this setup before, so I've been using it happily for testing modules for years. So mea culpa; if I had put this code in the original question, I probably would have realized the answer before posting, but I'm answering my own question as a warning to other "clever" Ruby coders.