[Fix rails#51720] Infer association klass as top level if model has s…

…ame demodularized name

activerecord/lib/active_record/reflection.rb

@@ -422,13 +422,21 @@ def autosave=(autosave)
       # 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:
+        if active_record.name['::'] && active_record.name.demodulize == name
+          "::#{name}"
+        else
+          name
+        end
+      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)
@@ -979,7 +987,7 @@ def through_reflection?
       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/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 +184,18 @@ def test_reflection_klass_requires_ar_subclass
     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_aggregation_reflection
     reflection_for_address = AggregateReflection.new(
       :address, nil, { mapping: [ %w(address_street street), %w(address_city city), %w(address_country country) ] }, Customer