Convert empty strings to nil in ImportExport::BaseObjectBuilder

- BaseObjectBuilder is used to look up existing objects
  like labels during import, instead of creating one
- For example, if a label is used on an epic and a board,
  we do not want to create 2 labels and instead reuse an
  one
- If a label is exported with description: '', and then imported
  back, rails will convert description to be nil
- Later on, when we try to identify a label BaseObjectBuilder
  uses description: '' as one of the where causes and
  nil != '', hence label lookup fails and the relation creation
  fails as well
- This MR converts any empty strings to nil inside BaseObjectBuilder
  so it does not happen anymore

changelogs/unreleased/georgekoltsov-object_builder_convert_empty_strings.yml

+---
+title: Fix Group Import existing objects lookup when description attribute is an empty
+  string
+merge_request: 25187
+author:
+type: fixed

lib/gitlab/import_export/group_object_builder.rb

@@ -18,12 +18,21 @@ module Gitlab
         super
 
         @group = @attributes['group']
+
+        update_description
       end
 
       private
 
       attr_reader :group
 
+      # Convert description empty string to nil
+      # due to existing object being saved with description: nil
+      # Which makes object lookup to fail since nil != ''
+      def update_description
+        attributes['description'] = nil if attributes['description'] == ''
+      end
+
       def where_clauses
         [
           where_clause_base,

spec/lib/gitlab/import_export/group_object_builder_spec.rb

@@ -26,6 +26,16 @@ describe Gitlab::ImportExport::GroupObjectBuilder do
 
       expect(label.persisted?).to be true
     end
+
+    context 'when description is an empty string' do
+      let(:label_attributes) { base_attributes.merge('type' => 'GroupLabel', 'description' => '') }
+
+      it 'finds the existing group label' do
+        group_label = create(:group_label, label_attributes)
+
+        expect(described_class.build(Label, label_attributes)).to eq(group_label)
+      end
+    end
   end
 
   context 'milestones' do
@@ -41,4 +51,16 @@ describe Gitlab::ImportExport::GroupObjectBuilder do
       expect(milestone.persisted?).to be true
     end
   end
+
+  describe '#initialize' do
+    context 'when attributes contain description as empty string' do
+      let(:attributes) { base_attributes.merge('description' => '') }
+
+      it 'converts empty string to nil' do
+        builder = described_class.new(Label, attributes)
+
+        expect(builder.send(:attributes)).to include({ 'description' => nil })
+      end
+    end
+  end
 end