fix some edge cases

lib/gitlab/import_export/project_tree_restorer.rb

 module Gitlab
   module ImportExport
     class ProjectTreeRestorer
-      # Relations which cannot have both group_id and project_id at the same time
-      RESTRICT_PROJECT_AND_GROUP = %i(milestone milestones).freeze
+      # Relations which cannot be saved at project level
+      GROUP_MODELS = [GroupLabel, Milestone].freeze
 
       def initialize(user:, shared:, project:)
         @path = File.join(shared.export_path, 'project.json')
@@ -70,12 +70,20 @@ module Gitlab
       def save_relation_hash(relation_hash_batch, relation_key)
         relation_hash = create_relation(relation_key, relation_hash_batch)
 
+        remove_group_models(relation_hash) if relation_hash.is_a?(Array)
+
         @saved = false unless restored_project.append_or_update_attribute(relation_key, relation_hash)
 
         # Restore the project again, extra query that skips holding the AR objects in memory
         @restored_project = Project.find(@project_id)
       end
 
+      def remove_group_models(relation_hash)
+        relation_hash.reject! do |value|
+          value.respond_to?(:group_id) && value.group_id && GROUP_MODELS.include?(value.class)
+        end
+      end
+
       def default_relation_list
         reader.tree.reject do |model|
           model.is_a?(Hash) && model[:project_members]
@@ -181,13 +189,7 @@ module Gitlab
       end
 
       def parsed_relation_hash(relation_hash, relation_type)
-        if RESTRICT_PROJECT_AND_GROUP.include?(relation_type)
-          params = {}
-          params['group_id'] = restored_project.group.try(:id) if relation_hash['group_id']
-          params['project_id'] = restored_project.id if relation_hash['project_id']
-        else
         params = { 'group_id' => restored_project.group.try(:id), 'project_id' => restored_project.id }
-        end
 
         relation_hash.merge(params)
       end

lib/gitlab/import_export/relation_factory.rb

@@ -285,9 +285,14 @@ module Gitlab
           label.save!
           label
         else
-          parsed_relation_hash.delete('type') if milestone?
+          object = GroupProjectFinder.find_or_create(relation_class, finder_hash)
 
-          GroupProjectFinder.find_or_create(relation_class, finder_hash)
+          if milestone?
+            parsed_relation_hash.delete('group_id') if object.project_id
+            parsed_relation_hash.delete('project_id') if object.group_id
+          end
+
+          object
         end
       end
 

spec/lib/gitlab/import_export/project_tree_restorer_spec.rb

@@ -400,7 +400,8 @@ describe Gitlab::ImportExport::ProjectTreeRestorer do
 
         restored_project_json
 
-        expect(project.milestones.count).to eq(1)
+        expect(project.group.milestones.count).to eq(1)
+        expect(project.milestones.count).to eq(0)
       end
     end
   end