diff --git a/application/Api/v1/PreorderApicontroller.php b/application/Api/v1/PreorderApicontroller.php
index 628886196..390b15758 100644
--- a/application/Api/v1/PreorderApicontroller.php
+++ b/application/Api/v1/PreorderApicontroller.php
@@ -241,12 +241,13 @@ class PreorderApicontroller extends mfBaseApicontroller {
      * search wohneinheit
      */
     $update_unit = false;
+    $assign_unit = false;
     $where = "1=1 ";
     $unit = false;
     if(count($unit_search)) {
       //var_dump($unit_search);exit;
       foreach($unit_search as $field => $value) {
-        if($field == "stock" || $field == "stiege") continue; // only check for block and tuer
+        if($field == "stock") continue; // only check for block, stiege and tuer
         $where .= " AND `$field` = '$value'";
       }
       
@@ -260,9 +261,9 @@ class PreorderApicontroller extends mfBaseApicontroller {
       $res = $this->db()->query($sql);
       if($this->db()->num_rows($res)) {
         $unit = $this->db()->fetch_object($res);
-        //return mfResponse::NotFound(['message' => "Wohneinheit nicht gefunden"]);
+      } else {
+        $update_unit = true;
       }
-      //var_dump($this->db()->num_rows($res), $this->db()->fetch_object($res));
     } else {
       // if all unit values are empty try to find the unit with all empty values
       // failure is not an error, but must be checked by a human at some point
@@ -282,7 +283,9 @@ class PreorderApicontroller extends mfBaseApicontroller {
       $res = $this->db()->query($sql);
       if($this->db()->num_rows($res)) {
         $unit = $this->db()->fetch_object($res);
-        $update_unit = true;
+      } else {
+        // XXX
+        //$assign_unit = true;
       }
     }
     
@@ -434,6 +437,100 @@ class PreorderApicontroller extends mfBaseApicontroller {
       }
     }
     
+    
+    
+    
+    //var_dump($this->allow_unit_update, $update_unit);exit;
+    
+    // update wohneinheit if allowed
+    // and unit search data was submitted
+    
+    $unit_changed = false;
+    
+    if($this->allow_unit_update && $update_unit) {
+      //var_dump($unit_search);
+      //var_dump($unit);
+      //exit;
+      
+      // get available units
+      $exclude_wohneinheit_ids = [];
+      $preorders_in_hausnummer = PreorderModel::search(['adb_hausnummer_id' => $address->hausnummer_id]);
+      if(count($preorders_in_hausnummer)) {
+        
+        foreach($preorders_in_hausnummer as $pih) {
+          if($pih->adb_wohneinheit_id) {
+            $exclude_wohneinheit_ids[] = $pih->adb_wohneinheit_id;
+          }
+        }
+      }
+      // get units excluding unavailable units
+      $where = "hausnummer_id=".$address->hausnummer_id;
+      $where .= " AND block IS NULL";
+      $where .= " AND stiege IS NULL";
+      $where .= " AND stock IS NULL";
+      $where .= " AND tuer IS NULL";
+      
+      if(count($exclude_wohneinheit_ids)) {
+        $where .= " AND wohneinheit_id NOT IN (". implode(",",$exclude_wohneinheit_ids).")";
+      }
+      $sql = "SELECT * FROM view_wohneinheit WHERE $where";
+      $this->log->debug($sql);
+      $res = $this->db()->query($sql);
+      if(!$this->db()->num_rows($res)) {
+        return mfResponse::Forbidden(['message' => "Keine Wohneinheiten verfügbar"]);
+      }
+      
+      $unit_row = $this->db()->fetch_object($res);
+      $unit = new ADBWohneinheit($unit_row->wohneinheit_id);
+      if(!$unit->id) {
+        return mfResponse::InternalServerError(['message' => "unit not found"]);
+      }
+      //echo "blah";exit;
+      
+      $change_note_data = [];
+      foreach(['block','stiege','stock','tuer'] as $unit_address_part) {
+        if($unit_search[$unit_address_part]) {
+          $unit->$unit_address_part = $unit_search[$unit_address_part];
+          $unit_changed = true;
+          $change_note_data[] = [
+            'field' => $unit_address_part,
+            'from' => $unit->_old_data->$unit_address_part,
+            'to' => $unit_search[$unit_address_part]
+          ];
+        }
+      }
+      
+      if($unit_changed) {
+        $change_note_text = ($unit->note) ? $unit->note : "";
+        $change_note_text .= date("Y-m-d H:i:s").": ";
+        foreach($change_note_data as $cn) {
+          $field = $cn['field'];
+          $from = ($cn['from'] === null) ? "NULL" : "'".$cn['from']."'";
+          $to = ($cn['to'] === null) ? "NULL" : "'".$cn['to']."'";
+          $change_note_text .= "$field from $from to $to; ";
+        }
+        $change_note_text .= "\n";
+        $unit->note = $change_note_text;
+        
+        $unit->startTransaction();
+        $unit->save();
+      }
+      //var_dump($unit);exit;
+      
+      $preorder_data['adb_wohneinheit_id'] = $unit->id;
+      $preorder_data['oaid'] = $unit->oaid;
+    }
+    
+    
+    // assign next wohneinheit if no unit search data was submitted
+    /*
+    if($this->allow_unit_update && $assign_unit) {
+      var_dump($unit_search);
+      var_dump($unit);
+      exit;
+    }*/
+    
+    
     /*
      * create preorder record
      */
@@ -443,18 +540,16 @@ class PreorderApicontroller extends mfBaseApicontroller {
     $preorder_id = $preorder->save();
     
     if(!$preorder_id || !$preorder->ucode) {
+      if($unit_changed) $unit->rollbackTransaction();
       return mfResponse::InternalServerError();
     }
     
-    // update wohneinheit if allowed
-    /*if($this->allow_unit_update && $update_unit) {
-      var_dump($unit_search);
-      var_dump($unit);
-      exit;
-    }*/
-    
+    if($unit_changed) $unit->commitTransaction();
     
     $return = ["code" => $preorder->ucode];
+    if($preorder->oaid) {
+      $return ['oaid'] = $preorder->oaid;
+    }
     if($addon_data) {
       $return["additionalData"] = $addon_data;
     }
diff --git a/lib/mvcfronk/mfBase/mfBaseModel.php b/lib/mvcfronk/mfBase/mfBaseModel.php
index 951e85d4f..51cb71dc3 100644
--- a/lib/mvcfronk/mfBase/mfBaseModel.php
+++ b/lib/mvcfronk/mfBase/mfBaseModel.php
@@ -223,6 +223,18 @@ class mfBaseModel {
       return "update";
     }
   }
+  
+  public function startTransaction() {
+    $this->db->query("START TRANSACTION");
+  }
+  
+  public function commitTransaction() {
+    $this->db->query("COMMIT");
+  }
+  
+  public function rollbackTransaction() {
+    $this->db->query("ROLLBACK");
+  }
 
   // generic functions for entity-classes
   public function toArray() {