Fix workers query error

app/workers/delete_bitstream_worker.rb

@@ -24,7 +24,7 @@ class DeleteBitstreamWorker
   sidekiq_options queue: :dspace
 
   def perform(attachment_id = nil)
-    attachment = LearningObject::Attachment.where(id: attachment_id)
+    attachment = LearningObject::Attachment.where(id: attachment_id).first
     return false if attachment.blank? || !wait_bitstream(attachment)
 
     # Require dspace gem

app/workers/dspace_upload_worker.rb

@@ -27,7 +27,7 @@ class DspaceUploadWorker
 
   def perform(learning_object_id, media_path, attachment_id = nil, description = nil)
     # find learning object
-    learning_object = LearningObject.where(id: learning_object_id)
+    learning_object = LearningObject.where(id: learning_object_id).first
 
     return false if learning_object.blank?
 
@@ -48,7 +48,7 @@ class DspaceUploadWorker
 
   def create_attachment(learning_object, bitstream, attachment_id = nil)
     unless attachment_id.nil?
-      attachment = learning_object.attachments.where(id: attachment_id)
+      attachment = learning_object.attachments.where(id: attachment_id).first
       unless attachment.blank?
         attachment.update(map_bitstream2attachment(bitstream))
         return attachment

app/workers/thumbnail_generate_worker.rb

@@ -22,7 +22,7 @@ class ThumbnailGenerateWorker
   sidekiq_options queue: :thumbnail
 
   def perform(attachment_id, media_path = nil, force = false)
-    @attachment = ::LearningObject::Attachment.where(id: attachment_id)
+    @attachment = ::LearningObject::Attachment.where(id: attachment_id).first
     return false if @attachment.blank? || (!@attachment.thumbnail.blank? && !force)
 
     # object from dspace