enable AR caching, refactor tree hash loop

lib/gitlab/import_export/project_tree_restorer.rb

@@ -23,9 +23,11 @@ module Gitlab
 
         @project_members = @tree_hash.delete('project_members')
 
+          ActiveRecord::Base.uncached do
             ActiveRecord::Base.no_touching do
             create_relations
           end
+        end
       rescue => e
         @shared.error(e)
         false
@@ -52,6 +54,7 @@ module Gitlab
         @saved = []
         default_relation_list.each do |relation|
           next unless relation.is_a?(Hash) || @tree_hash[relation.to_s].present?
+
           if relation.is_a?(Hash)
             create_sub_relations(relation, @tree_hash)
           else
@@ -82,6 +85,10 @@ module Gitlab
         relation_hash = create_relation(relation_key, relation_hash_batch)
 
         @saved << restored_project.append_or_update_attribute(relation_key, relation_hash)
+        @restored_project = nil
+        @project = nil
+        relation_hash = nil
+        relation_hash_batch = nil
         @restored_project = Project.find_by_id(@project_id)
       end
 
@@ -117,7 +124,9 @@ module Gitlab
         relation_key = relation.keys.first.to_s
         return if tree_hash[relation_key].blank?
 
-        [tree_hash[relation_key]].flatten.each do |relation_item|
+        tree_array = [tree_hash[relation_key]].flatten
+
+        while relation_item = tree_array.shift
             relation.values.flatten.each do |sub_relation|
               # We just use author to get the user ID, do not attempt to create an instance.
               next if sub_relation == :author
@@ -128,9 +137,15 @@ module Gitlab
               relation_item[sub_relation.to_s] = create_relation(sub_relation, relation_hash) unless relation_hash.blank?
             end
 
-          save_relation_hash([relation_item], relation_key) if save
-          tree_hash.delete(relation_key) if save
+            if save
+              save_relation_hash([relation_item], relation_key)
             end
+
+          tree_hash[relation_key].delete(relation_item) if save
+          relation_item = nil
+        end
+
+        tree_hash.delete(relation_key) if save
       end
 
       def assign_relation_hash(relation_item, sub_relation)