Association with the same name as the demodularized model name is incorrectly inferred

[joshuay03]

Steps to reproduce

I think it's easiest to explain this with a script.

# frozen_string_literal: true

require "bundler/inline"

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

  gem "rails", github: "rails/rails", branch: "main"

  gem "sqlite3", "~> 1.4"
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 :posts, force: true do |t|
  end

  create_table :namespaced_posts, force: true do |t|
  end

  create_table :posts_namespaced_posts_joins, force: true do |t|
    t.references :post
    t.references :namespaced_post
  end
end

class Post < ActiveRecord::Base
  has_one :post_namespaced_post_join
  has_one :namespaced_post, through: :post_namespaced_post_join
end

module Namespaced
  class Post < ActiveRecord::Base
    self.table_name_prefix = "namespaced_"

    has_one :post_namespaced_post_join
    has_one :post, through: :post_namespaced_post_join
  end
end

class PostNamespacedPostJoin < ActiveRecord::Base
  self.table_name = "posts_namespaced_posts_joins"

  belongs_to :post
  belongs_to :namespaced_post, class_name: "Namespaced::Post"
end

class BugTest < Minitest::Test
  def test_association_with_same_demodularized_name
    post = Post.create!
    namespaced_post = Namespaced::Post.create!
    post_namespaced_post_join = PostNamespacedPostJoin.create!(post: post, namespaced_post: namespaced_post)

    assert_equal post_namespaced_post_join, post.post_namespaced_post_join # ✅
    assert_equal post, namespaced_post.post                                # ❌
  end
end

Results in:

F

Failure:
BugTest#test_ [test.rb:62]:
Expected: #<Post id: 1>
  Actual: #<Namespaced::Post id: 1>

The only workaround is to update the association on Namespaced::Post to be:

has_one :post, through: :post_namespaced_post_join, class_name: "::Post"

The ::ensures that it's resolved as an absolute constant.

See #51721 for a proposed fix.

Expected behavior

Association model is inferred to be a top level model that matches its name.

Actual behavior

Association model is inferred to be the model it's defined on.

System configuration

Rails version: main

Ruby version: 3.2.2