validate learning_object's dspace_id only in creation

app/models/learning_object.rb

@@ -59,7 +59,7 @@ class LearningObject < ApplicationRecord
   belongs_to :attachment, class_name: 'LearningObject::Attachment'
 
   validates_presence_of :name, :publisher, :object_type, :language, :author, unless: :draft?
-  validates :id_dspace, presence: true, uniqueness: true
+  validates :id_dspace, presence: true, uniqueness: true, unless: :published?
 
   default_scope { includes(:object_type, :attachment, :attachments) }
   scope :missing_thumbnail, ->() { where(thumbnail_file_name: nil) }