Add FindQueryService for Comment

Currently supports finding comments by `roomId`.

See !83.

src/main/java/de/thm/arsnova/services/CommentFindQueryService.java

+/*
+ * This file is part of ARSnova Backend.
+ * Copyright (C) 2012-2018 The ARSnova Team and Contributors
+ *
+ * ARSnova Backend is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * ARSnova Backend is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package de.thm.arsnova.services;
+
+import de.thm.arsnova.entities.FindQuery;
+import de.thm.arsnova.entities.Comment;
+import org.springframework.stereotype.Service;
+
+import java.util.HashSet;
+import java.util.Set;
+
+@Service
+public class CommentFindQueryService implements FindQueryService<Comment> {
+	private CommentService commentService;
+	private UserService userService;
+
+	public CommentFindQueryService(final CommentService commentService, final UserService userService) {
+		this.commentService = commentService;
+		this.userService = userService;
+	}
+
+	@Override
+	public Set<String> resolveQuery(final FindQuery<Comment> findQuery) {
+		Set<String> ids = new HashSet<>();
+		if (findQuery.getProperties().getRoomId() != null) {
+			for (Comment c : commentService.getByRoomId(findQuery.getProperties().getRoomId(), 0, 0)) {
+				ids.add(c.getId());
+			}
+		}
+
+		return ids;
+	}
+}