Nested attributes are not updated whenhas_many is present in both parent and child STI models

[sergey-alekseev]

Steps to reproduce

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "rails", github: "rails/rails"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :companies do |t|
    t.string :type
  end

  create_table :employees do |t|
    t.references :company, foreign_key: true
    t.string :name
  end
end

class Company < ActiveRecord::Base
  has_many :employees
end

class PublicCompany < Company
  has_many :employees, foreign_key: :company_id

  accepts_nested_attributes_for :employees
end

class Employee < ActiveRecord::Base
  belongs_to :company
end

class BugTest < Minitest::Test
  def test_nested_attributes_with_sti_and_has_many
    public_company = PublicCompany.create!
    public_company_employee = Employee.create!(name: 'Initial', company: public_company)
    public_company.update!({
      'employees_attributes' => {
        '0' => {
          'name' => 'Updated',
          'id' => public_company_employee.id
        }
      }
    })
    public_company_employee.reload

    assert_equal 'Updated', public_company_employee.name
  end
end

Expected behavior

1 runs, 1 assertions, 0 failures, 0 errors, 0 skips

Actual behavior

Failure:
BugTest#test_nested_attributes_with_sti_and_has_many:
Expected: "Updated"
  Actual: "Initial"

1 runs, 1 assertions, 1 failures, 0 errors, 0 skips

System configuration

Rails version: 6.0.0.alpha
Ruby version: 2.5.1