Fix not equal operator (#935)

* Use nested query in getNotEqual Predicate
* Refactor RSQL Utility + use identifierField for field enums
* Don't join in case of not equal rsql operator
* Bugfix for automatic cross join + small refactoring
* Fix rsql out operator
* Fix tests for out-operator + extend TargetFieldTest
* Use inner join for subquery
* Don't use subquery for simple rsql queries
* Refactor RSQLUtility
* Change some methods to static
* Only use outer joins when they are needed
* Add tests for empty tag names
* Minor changes and refactoring for RSQLUtility
* Refactor methods to remove duplications

Signed-off-by: Sebastian Firsching <sebastian.firsching@bosch-si.com>

hawkbit-core/src/main/java/org/eclipse/hawkbit/repository/DistributionSetMetadataFields.java

@@ -36,4 +36,9 @@ public enum DistributionSetMetadataFields implements FieldNameProvider {
     public String getFieldName() {
         return fieldName;
     }
+
+    @Override
+    public String identifierFieldName() {
+        return KEY.getFieldName();
+    }
 }

hawkbit-core/src/main/java/org/eclipse/hawkbit/repository/FieldNameProvider.java

@@ -81,4 +81,13 @@ public interface FieldNameProvider {
     default boolean isMap() {
         return false;
     }
+
+    /**
+     * Returns the name of the field, that identifies the entity.
+     * 
+     * @return the name of the identifier, by default 'id'
+     */
+    default String identifierFieldName() {
+        return "id";
+    }
 }

hawkbit-core/src/main/java/org/eclipse/hawkbit/repository/SoftwareModuleMetadataFields.java

@@ -41,4 +41,9 @@ public enum SoftwareModuleMetadataFields implements FieldNameProvider {
     public String getFieldName() {
         return fieldName;
     }
+
+    @Override
+    public String identifierFieldName() {
+        return KEY.getFieldName();
+    }
 }

hawkbit-core/src/main/java/org/eclipse/hawkbit/repository/TargetMetadataFields.java

@@ -33,4 +33,9 @@ public enum TargetMetadataFields implements FieldNameProvider {
     public String getFieldName() {
         return fieldName;
     }
+
+    @Override
+    public String identifierFieldName() {
+        return KEY.getFieldName();
+    }
 }