diff --git a/src/main/java/org/gcube/informationsystem/base/reference/SchemaMixedElement.java b/src/main/java/org/gcube/informationsystem/base/reference/SchemaMixedElement.java
new file mode 100644
index 0000000..f916445
--- /dev/null
+++ b/src/main/java/org/gcube/informationsystem/base/reference/SchemaMixedElement.java
@@ -0,0 +1,20 @@
+package org.gcube.informationsystem.base.reference;
+
+import java.util.Map;
+
+/**
+ * This interfaces is an helper to identify elements supporting Schema Mixed.
+ * i.e. elements which instances could have additional properties in respect to the ones defined in the schema  
+ * @author Luca Frosini (ISTI - CNR)
+ */
+public interface SchemaMixedElement extends Element {
+	
+	public Map<String,Object> getAdditionalProperties();
+	
+	public void setAdditionalProperties(Map<String,Object> additionalProperties);
+	
+	public Object getAdditionalProperty(String key);
+	
+	public void setAdditionalProperty(String key, Object value);
+	
+}
diff --git a/src/main/java/org/gcube/informationsystem/model/reference/entities/Facet.java b/src/main/java/org/gcube/informationsystem/model/reference/entities/Facet.java
index 46634e6..fb5ce13 100644
--- a/src/main/java/org/gcube/informationsystem/model/reference/entities/Facet.java
+++ b/src/main/java/org/gcube/informationsystem/model/reference/entities/Facet.java
@@ -2,6 +2,7 @@ package org.gcube.informationsystem.model.reference.entities;
 
 import java.util.Map;
 
+import org.gcube.informationsystem.base.reference.SchemaMixedElement;
 import org.gcube.informationsystem.types.annotations.Abstract;
 import org.gcube.informationsystem.utils.AdditionalPropertiesSerializer;
 
@@ -15,40 +16,25 @@ import com.fasterxml.jackson.databind.annotation.JsonSerialize;
  */
 @Abstract
 // @JsonDeserialize(as=FacetImpl.class) Do not uncomment to manage subclasses
-public interface Facet extends Entity {
+public interface Facet extends Entity, SchemaMixedElement {
 	
 	public static final String NAME = "Facet"; //Facet.class.getSimpleName();
 	public static final String DESCRIPTION = "This is the base class for Facets";
 	public static final String VERSION = "1.0.0";
 	
-	/**
-	 * Return all properties. The returned Map is a copy of
-	 * the internal representation. Any modification to the returned Map MUST
-	 * not affect the object
-	 * @return a Map containing the properties
-	 */
 	@JsonAnyGetter
 	@JsonSerialize(using = AdditionalPropertiesSerializer.class)
+	@Override
 	public Map<String,Object> getAdditionalProperties();
 	
-	/**
-	 * Set all properties, replacing existing ones
-	 */
+	@Override
 	public void setAdditionalProperties(Map<String,Object> additionalProperties);
 	
-	/**
-	 * Return the value of the given property.
-	 * @param key the key of the requested property 
-	 * @return the value of the given property
-	 */
+	@Override
 	public Object getAdditionalProperty(String key);
 	
-	/**
-	 * Set the value of the given property.
-	 * @param key the key of the requested property 
-	 * @param value the value of the given resource property
-	 */
 	@JsonAnySetter
+	@Override
 	public void setAdditionalProperty(String key, Object value);
 	
 }
diff --git a/src/main/java/org/gcube/informationsystem/model/reference/relations/Relation.java b/src/main/java/org/gcube/informationsystem/model/reference/relations/Relation.java
index 1cdf89b..66db264 100644
--- a/src/main/java/org/gcube/informationsystem/model/reference/relations/Relation.java
+++ b/src/main/java/org/gcube/informationsystem/model/reference/relations/Relation.java
@@ -5,6 +5,7 @@ package org.gcube.informationsystem.model.reference.relations;
 
 import java.util.Map;
 
+import org.gcube.informationsystem.base.reference.SchemaMixedElement;
 import org.gcube.informationsystem.base.reference.relations.RelationElement;
 import org.gcube.informationsystem.model.reference.entities.Entity;
 import org.gcube.informationsystem.model.reference.entities.Resource;
@@ -26,7 +27,7 @@ import com.fasterxml.jackson.databind.annotation.JsonSerialize;
  */
 @Abstract
 // @JsonDeserialize(as=RelationImpl.class) Do not uncomment to manage subclasses
-public interface Relation<S extends Resource, T extends Entity> extends RelationElement<S,T> {
+public interface Relation<S extends Resource, T extends Entity> extends RelationElement<S,T>, SchemaMixedElement {
 	
 	public static final String NAME = "Relation"; //Relation.class.getSimpleName();
 	
@@ -34,48 +35,37 @@ public interface Relation<S extends Resource, T extends Entity> extends Relation
 	
 	@JsonIgnoreProperties({Resource.CONSISTS_OF_PROPERTY, Resource.IS_RELATED_TO_PROPERTY})
 	@JsonGetter(value = SOURCE_PROPERTY)
+	@Override
 	public S getSource();
 	
 	@JsonIgnore
+	@Override
 	public void setSource(S source);
 	
 	@JsonGetter(value = TARGET_PROPERTY)
+	@Override
 	public T getTarget();
 	
 	@JsonIgnore
+	@Override
 	public void setTarget(T target);
 	
 	@ISProperty(name = PROPAGATION_CONSTRAINT)
 	public PropagationConstraint getPropagationConstraint();
 	
-	/**
-	 * Return all properties. The returned Map is a copy of
-	 * the internal representation. Any modification to the returned Map MUST
-	 * not affect the object
-	 * @return a Map containing the properties
-	 */
 	@JsonAnyGetter
 	@JsonSerialize(using = AdditionalPropertiesSerializer.class)
+	@Override
 	public Map<String,Object> getAdditionalProperties();
 	
-	/**
-	 * Set all properties, replacing existing ones
-	 */
+	@Override
 	public void setAdditionalProperties(Map<String,Object> additionalProperties);
 	
-	/**
-	 * Return the value of the given property.
-	 * @param key the key of the requested property 
-	 * @return the value of the given property
-	 */
+	@Override
 	public Object getAdditionalProperty(String key);
 	
-	/**
-	 * Set the value of the given property.
-	 * @param key the key of the requested property 
-	 * @param value the value of the given resource property
-	 */
 	@JsonAnySetter
+	@Override
 	public void setAdditionalProperty(String key, Object value);
 	
 }