public function Merge($OldUserID, $NewUserID) {
      $OldUser = $this->GetID($OldUserID, DATASET_TYPE_ARRAY);
      $NewUser = $this->GetID($NewUserID, DATASET_TYPE_ARRAY);
      
      if (!$OldUser || !$NewUser) {
         throw new Gdn_UserException("Could not find one or both users to merge.");
      }
      
      $Map = array('UserID', 'Name', 'Email', 'CountVisits', 'CountDiscussions', 'CountComments');
      
      $Result = array('MergeID' => NULL, 'Before' => array(
         'OldUser' => ArrayTranslate($OldUser, $Map),
         'NewUser' => ArrayTranslate($NewUser, $Map)));
      
      // Start the merge.
      $MergeID = $this->MergeStart($OldUserID, $NewUserID);
      
      // Copy all discussions from the old user to the new user.
      $this->MergeCopy($MergeID, 'Discussion', 'InsertUserID', $OldUserID, $NewUserID);
      
      // Copy all the comments from the old user to the new user.
      $this->MergeCopy($MergeID, 'Comment', 'InsertUserID', $OldUserID, $NewUserID);
      
      // Copy all of the activities.
      $this->MergeCopy($MergeID, 'Activity', 'NotifyUserID', $OldUserID, $NewUserID);
      $this->MergeCopy($MergeID, 'Activity', 'InsertUserID', $OldUserID, $NewUserID);
      $this->MergeCopy($MergeID, 'Activity', 'ActivityUserID', $OldUserID, $NewUserID);
      
      // Copy all of the activity comments.
      $this->MergeCopy($MergeID, 'ActivityComment', 'InsertUserID', $OldUserID, $NewUserID);
      
      // Copy all conversations.
      $this->MergeCopy($MergeID, 'Conversation', 'InsertUserID', $OldUserID, $NewUserID);
      $this->MergeCopy($MergeID, 'ConversationMessage', 'InsertUserID', $OldUserID, $NewUserID, 'MessageID');
      $this->MergeCopy($MergeID, 'UserConversation', 'UserID', $OldUserID, $NewUserID, 'ConversationID');
      
      $this->MergeFinish($MergeID);
      
      $OldUser = $this->GetID($OldUserID, DATASET_TYPE_ARRAY);
      $NewUser = $this->GetID($NewUserID, DATASET_TYPE_ARRAY);
      
      $Result['MergeID'] = $MergeID;
      $Result['After'] = array(
         'OldUser' => ArrayTranslate($OldUser, $Map),
         'NewUser' => ArrayTranslate($NewUser, $Map));
      
      return $Result;
   }