/**
  * Add diff between stage post and production post.
  *
  * @param Post_Env_Diff $diff
  */
 private function add_post_diff(Post_Env_Diff $diff)
 {
     // Store diff if it does not already exist.
     if (!isset($this->post_diffs[$diff->get_stage_id()])) {
         // Store diff in database.
         add_post_meta($this->batch->get_id(), 'sme_post_diff', $diff->to_array());
         // Store diff in property.
         $this->post_diffs[$diff->get_stage_id()] = $diff;
     }
 }