refactor createGame-Method to return GameId instead of Game

fh.fd.ci.server/src/main/java/de/fd/fh/server/game/GameService.java

@@ -1,10 +1,14 @@
 package de.fd.fh.server.game;
 
+import lombok.RequiredArgsConstructor;
+
+@RequiredArgsConstructor
 public class GameService
 {
+    private final GameRepository repository;
 
-    public Game createGame()
+    public GameId createGame(final Game game)
     {
-        return null;
+        return (GameId) repository.save(game).getId();
     }
 }

fh.fd.ci.server/src/test/java/de/fd/fh/server/game/GameServiceTest.java

@@ -1,17 +1,25 @@
 package de.fd.fh.server.game;
 
+import dev.morphia.Key;
 import org.junit.jupiter.api.Test;
 
 import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
 
 class GameServiceTest
 {
     @Test
     void testCreateGame()
     {
+        final GameRepository repository = when(mock(GameRepository.class).save(any()))
+                .thenReturn(new Key<>(Game.class, "testCollection", GameId.random()))
+                .getMock();
+
         final Game game = new Game();
 
-        final Game result = new GameService().createGame();
+        final GameId result = new GameService(repository).createGame(game);
 
         assertNotNull(result);
     }