add some convenient methods for avoiding array allocations

activerecord/lib/active_record/associations/join_dependency.rb

@@ -76,17 +76,17 @@ def graft(*associations)
       end
 
       def reflections
-        join_root.drop(1).map(&:reflection)
+        join_root.drop(1).map!(&:reflection)
       end
 
       def join_relation(relation)
-        join_root.drop(1).inject(relation) do |rel,association|
+        join_root.inject(relation) do |rel,association|
           association.join_relation(rel)
         end
       end
 
       def join_constraints
-        join_root.drop(1).flat_map(&:join_constraints)
+        join_root.flat_map(&:join_constraints)
       end
 
       def columns

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

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