Add @group_where condition

lib/tallty_import_export/export.rb

@@ -44,7 +44,7 @@ module TalltyImportExport
           else
             records.group(@group_by).count.keys.each do |key|
               @group_key = key
-              export_workbook workbook, records.ransack("#{@group_by}_eq" => key).result
+              export_workbook workbook, records.ransack("#{@group_where}" => key).result
             end
           end
         else
@@ -95,6 +95,7 @@ module TalltyImportExport
       @width ||= options.delete(:width) || 30
       @filename ||= options.delete(:filename)
       @group_by ||= options.delete(:group_by)
+      @group_where = "#{@group_by}_eq" if @group_by.present?
     end
 
     def with_scope records