Fix for #15354: Count unique users who answered questions

src/main/java/de/thm/arsnova/dao/CouchDBDao.java

@@ -1080,11 +1080,14 @@ public class CouchDBDao implements IDatabaseDao, ApplicationEventPublisherAware
 		try {
 			final View statsView = new View("statistics/statistics");
 			final View creatorView = new View("statistics/unique_session_creators");
+			final View studentUserView = new View("statistics/active_student_users");
 			statsView.setGroup(true);
 			creatorView.setGroup(true);
+			studentUserView.setGroup(true);
 
 			final ViewResults statsResults = getDatabase().view(statsView);
 			final ViewResults creatorResults = getDatabase().view(creatorView);
+			final ViewResults studentUserResults = getDatabase().view(studentUserView);
 
 			if (!isEmptyResults(statsResults)) {
 				final JSONArray rows = statsResults.getJSONArray("rows");
@@ -1125,6 +1128,15 @@ public class CouchDBDao implements IDatabaseDao, ApplicationEventPublisherAware
 				}
 				stats.setCreators(creators.size());
 			}
+			if (!isEmptyResults(studentUserResults)) {
+				final JSONArray rows = studentUserResults.getJSONArray("rows");
+				Set<String> students = new HashSet<String>();
+				for (int i = 0; i < rows.size(); i++) {
+					final JSONObject row = rows.getJSONObject(i);
+					students.add(row.getString("key"));
+				}
+				stats.setActiveStudents(students.size());
+			}
 			return stats;
 		} catch (final Exception e) {
 			LOGGER.error("Error while retrieving session count", e);

src/main/java/de/thm/arsnova/entities/Statistics.java

@@ -26,6 +26,7 @@ public class Statistics {
 	private int closedSessions;
 	private int creators;
 	private int activeUsers;
+	private int activeStudents;
 	private int loggedinUsers;
 	private int interposedQuestions;
 	private int conceptQuestions;
@@ -118,6 +119,14 @@ public class Statistics {
 		this.conceptQuestions = conceptQuestions;
 	}
 
+	public int getActiveStudents() {
+		return activeStudents;
+	}
+
+	public void setActiveStudents(int activeStudents) {
+		this.activeStudents = activeStudents;
+	}
+
 	@Override
 	public int hashCode() {
 		return (this.getClass().getName()