Add Jackson Converter for type field of Entities

This decouples type identifiers used in CouchDB from Java type names.

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

@@ -25,7 +25,7 @@ public interface Entity {
 	void setId(String id);
 
 	@JsonView(View.Persistence.class)
-	default String getType() {
-		return getClass().getSimpleName();
+	default Class<? extends Entity> getType() {
+		return getClass();
 	}
 }

src/main/java/de/thm/arsnova/entities/serialization/CouchDbDocumentMixIn.java

@@ -20,6 +20,8 @@ package de.thm.arsnova.entities.serialization;
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.annotation.JsonSerialize;
+import de.thm.arsnova.entities.Entity;
 
 @JsonIgnoreProperties(value = {"type"}, allowGetters = true)
 public abstract class CouchDbDocumentMixIn {
@@ -34,4 +36,7 @@ public abstract class CouchDbDocumentMixIn {
 	abstract String getRevision();
 
 	@JsonProperty("_rev") abstract String setRevision(String rev);
+
+	@JsonSerialize(converter = CouchDbTypeFieldConverter.class)
+	abstract Class<? extends Entity> getType();
 }

src/main/java/de/thm/arsnova/entities/serialization/CouchDbTypeFieldConverter.java

+/*
+ * This file is part of ARSnova Backend.
+ * Copyright (C) 2012-2017 The ARSnova Team
+ *
+ * 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.entities.serialization;
+
+import com.fasterxml.jackson.databind.JavaType;
+import com.fasterxml.jackson.databind.type.TypeFactory;
+import com.fasterxml.jackson.databind.util.Converter;
+import de.thm.arsnova.entities.Entity;
+import java.util.HashMap;
+import java.util.Map;
+
+public class CouchDbTypeFieldConverter implements Converter<Class<? extends Entity>, String> {
+	private static final Map<Class<? extends Entity>, String> typeMapping = new HashMap<>();
+
+	{
+	}
+
+	@Override
+	public String convert(Class<? extends Entity> aClass) {
+		return typeMapping.get(aClass);
+	}
+
+	@Override
+	public JavaType getInputType(TypeFactory typeFactory) {
+		return typeFactory.constructGeneralizedType(typeFactory.constructType(Class.class), Entity.class);
+	}
+
+	@Override
+	public JavaType getOutputType(TypeFactory typeFactory) {
+		return typeFactory.constructType(String.class);
+	}
+}