Started implementation of fix

Signed-off-by: Christian Wolf <github@christianwolf.email>

lib/Migration/Version000000Date20210701093123.php

@@ -4,16 +4,57 @@ declare(strict_types=1);
 
 namespace OCA\Cookbook\Migration;
 
-use Closure;
+use OCP\IDBConnection;
 use OCP\DB\ISchemaWrapper;
 use OCP\Migration\IOutput;
 use OCP\Migration\SimpleMigrationStep;
+use Closure;
 
 /**
  * Auto-generated migration step: Please modify to your needs!
  */
 class Version000000Date20210701093123 extends SimpleMigrationStep {
 
+    /**
+     * @var IDBConnection
+     */
+    private $db;
+    
+    public function __construct(IDBConnection $db){
+        $this->db = $db;
+    }
+    
+    function preSchemaChange(IOutput $output, \Closure $schemaClosure, array $options){
+        $this->db->beginTransaction();
+        try {
+            
+            $qb = $this->db->getQueryBuilder();
+            
+            // Fetch all rows that are non-unique
+            $qb->selectAlias('n.user_id', 'user')
+                ->selectAlias($qb->createFunction('COUNT(n.user_id)'), 'cnt')
+                ->from('cookbook_names', 'n')
+                ->groupBy('n.user_id', 'n.recipe_id')
+                ->having('cnt > 1');
+            //echo $qb->getSQL() . "\n";
+            
+            $cursor = $qb->execute();
+            $result = $cursor->fetchAll();
+            
+            if(sizeof($result) > 0){
+                // We have to fix the database
+                throw new \Exception('This is not yet implemented.');
+            }
+            
+            // Finish the transaction
+            $this->db->commit();
+        } catch (\Exception $e) {
+            // Abort the transaction
+            $this->db->rollBack();
+            throw $e;
+        }
+    }
+    
 	/**
 	 * @param IOutput $output
 	 * @param Closure $schemaClosure The `\Closure` returns a `ISchemaWrapper`