refactor a little so we can remove some methods

activerecord/lib/active_record/associations/join_dependency.rb

@@ -80,7 +80,7 @@ def merge_outer_joins!(other)
       end
 
       def join_constraints
-        join_root.flat_map(&:join_constraints)
+        join_root.children.flat_map { |c| c.flat_map(&:join_constraints) }
       end
 
       def columns

activerecord/lib/active_record/associations/join_dependency/join_part.rb

@@ -29,9 +29,6 @@ def initialize(base_klass, parent)
           @children = []
         end
 
-        def join_constraints; []; end
-        def join_relation(rel); rel; end
-
         def name
           reflection.name
         end