Include null values into JSON produced by the REST API.

src/main/java/cz/cvut/kbss/termit/config/WebAppConfig.java

@@ -20,12 +20,14 @@
 import com.fasterxml.jackson.databind.module.SimpleModule;
 import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;
 import cz.cvut.kbss.jopa.model.MultilingualString;
+import cz.cvut.kbss.jopa.sessions.UnitOfWorkImpl;
 import cz.cvut.kbss.jsonld.JsonLd;
 import cz.cvut.kbss.jsonld.jackson.JsonLdModule;
 import cz.cvut.kbss.termit.rest.servlet.DiagnosticsContextFilter;
 import cz.cvut.kbss.termit.util.AdjustedUriTemplateProxyServlet;
 import cz.cvut.kbss.termit.util.ConfigParam;
 import cz.cvut.kbss.termit.util.Constants;
+import cz.cvut.kbss.termit.util.json.ManageableIgnoreMixin;
 import cz.cvut.kbss.termit.util.json.MultilingualStringDeserializer;
 import cz.cvut.kbss.termit.util.json.MultilingualStringSerializer;
 import org.springframework.boot.web.servlet.FilterRegistrationBean;
@@ -79,12 +81,14 @@ public ObjectMapper objectMapper() {
      */
     public static ObjectMapper createJsonObjectMapper() {
         final ObjectMapper objectMapper = new ObjectMapper();
-        objectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
+        objectMapper.setSerializationInclusion(JsonInclude.Include.ALWAYS);
         objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
         final SimpleModule multilingualStringModule = new SimpleModule();
         multilingualStringModule.addSerializer(MultilingualString.class, new MultilingualStringSerializer());
         multilingualStringModule.addDeserializer(MultilingualString.class, new MultilingualStringDeserializer());
         objectMapper.registerModule(multilingualStringModule);
+        // Ignore UoW references injected into entities
+        objectMapper.addMixIn(UnitOfWorkImpl.class, ManageableIgnoreMixin.class);
         // JSR 310 (Java 8 DateTime API)
         objectMapper.registerModule(new JavaTimeModule());
         return objectMapper;

src/main/java/cz/cvut/kbss/termit/util/json/ManageableIgnoreMixin.java

@@ -0,0 +1,10 @@
+package cz.cvut.kbss.termit.util.json;
+
+import com.fasterxml.jackson.annotation.JsonIgnoreType;
+
+/**
+ * Mixin declared to allow ignoring JOPA's Manageable reference to the current UnitOfWork
+ */
+@JsonIgnoreType
+public class ManageableIgnoreMixin {
+}