Fix: bi-direction accepts_nested_attributes_for validation.

[akhilgkrishnan]

Motivation / Background

Fixes #41811

Addresses the issue with bi-directional accepts_nested_attributes where validation of a collection fails due to only the last record being considered. This occurs because each child item triggers a valid? call on the parent record, which overwrites previous validation errors.

class Post < ActiveRecord::Base
  has_many :comments, inverse_of: :post
  accepts_nested_attributes_for :comments

  validates :title, presence: true
end

class Comment < ActiveRecord::Base
  belongs_to :post, inverse_of: :comments
  accepts_nested_attributes_for :post

  validates :body, presence: true
end

class BugTest < Minitest::Test
  # currently fails
  def test_invalid_attributes
    post = Post.new(title: "foo", comments_attributes: [{ body: "body 1" }, { body: nil }, { body: "body 3" }])

    refute post.valid?
    refute post.errors.empty?
  end

  # currently passes
  def test_invalid_attributes_end_of_list
    post = Post.new(title: "foo", comments_attributes: [{ body: "bar" }, { body: "baz" }, { body: nil }])

    refute post.valid?
    refute post.errors.empty?
  end

  # currently passes
  def test_invalid_attributes_one_attribute
    post = Post.new(title: "foo", comments_attributes: [{ body: nil }])

    refute post.valid?
    refute post.errors.empty?
  end
end

Detail

This PR is followup of #41878 which done by @intrip in 2 years back, additionally I've a added minor change for initialising the variable. This fix involves tracking the current record under validation and avoiding cyclic valid? calls.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

cc/ @byroot