[Fix #51720] Infer association klass as top level if model has same demodularized name

activerecord/CHANGELOG.md

@@ -1,3 +1,17 @@
+*   Fix inference of association model on nested models with the same demodularized name.
+
+    E.g. with the following setup:
+
+    ```ruby
+    class Nested::Post < ApplicationRecord
+      has_one :post, through: :other
+    end
+    ```
+
+    Before, `#post` would infer the model as `Nested::Post`, but now it correctly infers `Post`.
+
+    *Joshua Young*
+
 *   Add public method for checking if a table is ignored by the schema cache.
 
     Previously, an application would need to reimplement `ignored_table?` from the schema cache class to check if a table was set to be ignored. This adds a public method to support this and updates the schema cache to use that directly.

activerecord/lib/active_record/reflection.rb

@@ -428,13 +428,17 @@ module ActiveRecord
       # a new association object. Use +build_association+ or +create_association+
       # instead. This allows plugins to hook into association object creation.
       def klass
-        @klass ||= compute_class(class_name)
+        @klass ||= compute_class(compute_name(class_name))
       end
 
       def compute_class(name)
         name.constantize
       end
 
+      def compute_name(name) # :nodoc:
+        active_record.name.demodulize == name ? "::#{name}" : name
+      end
+
       # Returns +true+ if +self+ and +other_aggregation+ have the same +name+ attribute, +active_record+ attribute,
       # and +other_aggregation+ has an options hash assigned to it.
       def ==(other_aggregation)
@@ -994,7 +998,7 @@ module ActiveRecord
       end
 
       def klass
-        @klass ||= delegate_reflection.compute_class(class_name)
+        @klass ||= delegate_reflection.compute_class(compute_name(class_name))
       end
 
       # Returns the source of the through reflection. It checks both a singularized

activerecord/test/cases/reflection_test.rb

@@ -30,6 +30,9 @@ require "models/recipe"
 require "models/user_with_invalid_relation"
 require "models/hardback"
 require "models/sharded/comment"
+require "models/admin"
+require "models/admin/user"
+require "models/user"
 
 class ReflectionTest < ActiveRecord::TestCase
   include ActiveRecord::Reflection
@@ -181,6 +184,30 @@ class ReflectionTest < ActiveRecord::TestCase
     end
   end
 
+  def test_reflection_klass_with_same_demodularized_name
+    reflection = ActiveRecord::Reflection.create(
+      :has_one,
+      :user,
+      nil,
+      {},
+      Admin::User
+    )
+
+    assert_equal User, reflection.klass
+  end
+
+  def test_reflection_klass_with_same_demodularized_different_modularized_name
+    reflection = ActiveRecord::Reflection.create(
+      :has_one,
+      :user,
+      nil,
+      { class_name: "Nested::User" },
+      Admin::User
+    )
+
+    assert_equal Nested::User, reflection.klass
+  end
+
   def test_aggregation_reflection
     reflection_for_address = AggregateReflection.new(
       :address, nil, { mapping: [ %w(address_street street), %w(address_city city), %w(address_country country) ] }, Customer

activerecord/test/models/user.rb

@@ -23,3 +23,9 @@ end
 class UserWithNotification < User
   after_create -> { Notification.create! message: "A new user has been created." }
 end
+
+module Nested
+  class User < ActiveRecord::Base
+    self.table_name = "users"
+  end
+end