1 files changed, 27 insertions, 12 deletions
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 5bfc6727d8..2c8c4b6297 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -364,6 +364,17 @@ module ActiveRecord
         @constructable = calculate_constructable(macro, options)
         @association_scope_cache = {}
         @scope_lock = Mutex.new
+
+        if options[:class_name] && options[:class_name].class == Class
+          ActiveSupport::Deprecation.warn(<<-MSG.squish)
+            Passing a class to the `class_name` is deprecated and will raise
+            an ArgumentError in Rails 5.2. It eagerloads more classes than
+            necessary and potentially creates circular dependencies.
+
+            Please pass the class name as a string:
+            `#{macro} :#{name}, class_name: '#{options[:class_name]}'`
+          MSG
+        end
       end
 
       def association_scope_cache(conn, owner)
@@ -878,15 +889,13 @@ module ActiveRecord
         }
 
         if names.length > 1
-          example_options = options.dup
-          example_options[:source] = source_reflection_names.first
-          ActiveSupport::Deprecation.warn \
-            "Ambiguous source reflection for through association. Please " \
-            "specify a :source directive on your declaration like:\n" \
-            "\n" \
-            "  class #{active_record.name} < ActiveRecord::Base\n" \
-            "    #{macro} :#{name}, #{example_options}\n" \
-            "  end"
+          raise AmbiguousSourceReflectionForThroughAssociation.new(
+            active_record.name,
+            macro,
+            name,
+            options,
+            source_reflection_names
+          )
         end
 
         @source_reflection_name = names.first
@@ -933,6 +942,14 @@ module ActiveRecord
           raise HasOneThroughCantAssociateThroughCollection.new(active_record.name, self, through_reflection)
         end
 
+        if parent_reflection.nil?
+          reflections = active_record.reflections.keys.map(&:to_sym)
+
+          if reflections.index(through_reflection.name) > reflections.index(name)
+            raise HasManyThroughOrderError.new(active_record.name, self, through_reflection)
+          end
+        end
+
         check_validity_of_inverse!
       end
 
@@ -963,8 +980,7 @@ module ActiveRecord
         end
       end
 
-      protected
-
+      private
         def actual_source_reflection # FIXME: this is a horrible name
           source_reflection.send(:actual_source_reflection)
         end
@@ -975,7 +991,6 @@ module ActiveRecord
 
         def inverse_name; delegate_reflection.send(:inverse_name); end
 
-      private
         def derive_class_name
           # get the class_name of the belongs_to association of the through reflection
           options[:source_type] || source_reflection.class_name