Fixed detection of UnscopedFind if optional: true is defined in the model and is not self-reference association

[NerdyBoyCool]

ref #1139
ref #1153

The change in #1153 avoids checking UnscopedFind for model classes with optional: true settings, but I think UnscopedFind warning should be issued for the following cases.

The current code only checks if optional: true is defined in the model class, so the check for the Group model is skipped.

class Group < ApplicationRecord
  belongs_to :user, optional: true
end

class GroupsController < ApplicationController
  def show
    @group = Group.find(params[:id])
  end
end

This PR solves this problem by also checking self-reference associations.
We have also added implementation and testing for the issues mentioned in #1139 so that they are not affected.

[NerdyBoyCool]

@presidentbeef
Can you give me your opinion on this change?
I would appreciate any advice and opinions.

[presidentbeef]

I'm not sure I understand the intent here. If the relationship is optional, wouldn't you have legitimate cases where the application can't look up a model via the relationship..? In other words, is this change just as likely to increase false positives as it is to decrease false negatives?

[NerdyBoyCool]

The first example I gave was a little confusing, so I will give an example that is similar to the problem we encountered.

class User < ApplicationRecord
end

class Organization < ApplicationRecord
  has_many :members, dependent: :destroy
  has_many :users, dependent: :destroy
end

class Member < ApplicationRecord
  belongs_to :user
  belongs_to :organization

  has_many :articles, dependent: :destroy
end

class Article < ApplicationRecord
  belongs_to :author, class_name: "User", foreign_key: "author_id", optional: true
  belongs_to :organization
end

# Bad
artciles = Article.find(params[:id])

# Good
organization = Organization.first
articles = organization.articles.find(params[:id])

I expected brakeman to detect that the Article was not being retrieved via an Organization instance, but it did not because the Article model already defines optional: true for the User.
As you say, this type of problem could increase the number of false positives, but I thought the problem would be greater if the code that calls find directly against Organization was skipped.

[presidentbeef]

I think what we actually want is to check if there is any belongs_to relationship that is not optional?

Is the self-referential piece actually that important?

[presidentbeef]

Is class_name going to show up more than once as a key? Couldn't this also use hash_access(:class_name) instead of iterating?