Ensure association target classes exist

Fixes https://github.com/rails/rails/issues/42122

Currently if you call a reader on a singular association, no error is raised if the target class doesn't exist (it just returns `nil`). If you call a reader on a collection association an error is raised, but the error appears to not be intended for this case and doesn't have a very good message.

`ActiveRecord::Reflection::AssociationReflection#compute_class` appears to be set up to handle this, but it is not being called correctly. So this PR just makes a call to that (via `reflection.klass`) from `SingularAssociation#reader` and `CollectionAssociation#reader`.

activerecord/lib/active_record/associations/association.rb

@@ -215,6 +215,12 @@ module ActiveRecord
       end
 
       private
+        # Reader and writer methods call this so that consistent errors are presented
+        # when the association target class does not exist.
+        def ensure_klass_exists!
+          klass
+        end
+
         def find_target
           if violates_strict_loading? && owner.validation_context.nil?
             Base.strict_loading_violation!(owner: owner.class, reflection: reflection)

activerecord/lib/active_record/associations/collection_association.rb

@@ -30,6 +30,8 @@ module ActiveRecord
     class CollectionAssociation < Association #:nodoc:
       # Implements the reader method, e.g. foo.items for Foo.has_many :items
       def reader
+        ensure_klass_exists!
+
         if stale_target?
           reload
         end

activerecord/lib/active_record/associations/singular_association.rb

@@ -5,6 +5,8 @@ module ActiveRecord
     class SingularAssociation < Association #:nodoc:
       # Implements the reader method, e.g. foo.bar for Foo.has_one :bar
       def reader
+        ensure_klass_exists!
+
         if !loaded? || stale_target?
           reload
         end

activerecord/lib/active_record/reflection.rb

@@ -413,14 +413,22 @@ module ActiveRecord
           raise ArgumentError, "Polymorphic associations do not support computing the class."
         end
 
-        active_record.send(:compute_type, name).tap do |klass|
+        msg = <<-MSG.squish
+          Rails couldn't find a valid model for #{name} association.
+          Please provide the :class_name option on the association declaration.
+          If :class_name is already provided make sure is an ActiveRecord::Base subclass.
+        MSG
+
+        begin
+          klass = active_record.send(:compute_type, name)
+
           unless klass < ActiveRecord::Base
-            raise ArgumentError, <<-MSG.squish
+            raise ArgumentError, msg
-              Rails couldn't find a valid model for #{name} association.
-              Please provide the :class_name option on the association declaration.
-              If :class_name is already provided make sure is an ActiveRecord::Base subclass.
-            MSG
           end
+
+          klass
+        rescue NameError
+          raise NameError, msg
         end
       end
 

activerecord/test/cases/associations_test.rb

@@ -359,6 +359,28 @@ class OverridingAssociationsTest < ActiveRecord::TestCase
       end
     end
   end
+
+  class ModelAssociatedToClassesThatDoNotExist < ActiveRecord::Base
+    self.table_name = "accounts" # this is just to avoid adding a new model just for this test
+
+    has_one :non_existent_has_one_class
+    belongs_to :non_existent_belongs_to_class
+    has_many :non_existent_has_many_classes
+  end
+
+  def test_associations_raise_with_name_error_if_associated_to_classes_that_do_not_exist
+    assert_raises NameError do
+      ModelAssociatedToClassesThatDoNotExist.new.non_existent_has_one_class
+    end
+
+    assert_raises NameError do
+      ModelAssociatedToClassesThatDoNotExist.new.non_existent_belongs_to_class
+    end
+
+    assert_raises NameError do
+      ModelAssociatedToClassesThatDoNotExist.new.non_existent_has_many_classes
+    end
+  end
 end
 
 class PreloaderTest < ActiveRecord::TestCase