Fix integrity constraints for database session driver.

src/Illuminate/Session/DatabaseSessionHandler.php

@@ -6,6 +6,7 @@
 use Illuminate\Support\Arr;
 use SessionHandlerInterface;
 use Illuminate\Contracts\Auth\Guard;
+use Illuminate\Database\QueryException;
 use Illuminate\Database\ConnectionInterface;
 use Illuminate\Contracts\Container\Container;
 
@@ -123,14 +124,42 @@ public function write($sessionId, $data)
         }
 
         if ($this->exists) {
-            $this->getQuery()->where('id', $sessionId)->update($payload);
+            $this->performUpdate($sessionId, $payload);
         } else {
-            $this->getQuery()->insert(Arr::set($payload, 'id', $sessionId));
+            $this->performInsert($sessionId, $payload);
         }
 
         return $this->exists = true;
     }
 
+    /**
+     * Perform an insert operation on the session ID.
+     *
+     * @param  string  $sessionId
+     * @param  string  $payload
+     * @return void
+     */
+    protected function performInsert($sessionId, $payload)
+    {
+        try {
+            return $this->getQuery()->insert(Arr::set($payload, 'id', $sessionId));
+        } catch (QueryException $e) {
+            $this->performUpdate($sessionId, $payload);
+        }
+    }
+
+    /**
+     * Perform an update operation on the session ID.
+     *
+     * @param  string  $sessionId
+     * @param  string  $payload
+     * @return int
+     */
+    protected function performUpdate($sessionId, $payload)
+    {
+        return $this->getQuery()->where('id', $sessionId)->update($payload);
+    }
+
     /**
      * Get the default payload for the session.
      *