diff --git a/navalplanner-business/src/main/java/org/navalplanner/business/materials/entities/MaterialCategory.java b/navalplanner-business/src/main/java/org/navalplanner/business/materials/entities/MaterialCategory.java
index 9e2bfcfed..3f301cfbf 100644
--- a/navalplanner-business/src/main/java/org/navalplanner/business/materials/entities/MaterialCategory.java
+++ b/navalplanner-business/src/main/java/org/navalplanner/business/materials/entities/MaterialCategory.java
@@ -146,6 +146,7 @@ public class MaterialCategory extends IntegrationEntity {
     }
 
     public void removeMaterial(Material material) {
+        material.setCategory(null);
         materials.remove(material);
     }
 
diff --git a/navalplanner-business/src/main/resources/org/navalplanner/business/materials/entities/Materials.hbm.xml b/navalplanner-business/src/main/resources/org/navalplanner/business/materials/entities/Materials.hbm.xml
index 8c5e58008..500cdcb8b 100644
--- a/navalplanner-business/src/main/resources/org/navalplanner/business/materials/entities/Materials.hbm.xml
+++ b/navalplanner-business/src/main/resources/org/navalplanner/business/materials/entities/Materials.hbm.xml
@@ -80,7 +80,7 @@
         index="idx_material_category_on_parent"/>
 
     <!-- Indexed the other side -->
-    <set name="materials" inverse="true" cascade="all">
+    <set name="materials" inverse="true" cascade="all-delete-orphan">
         <key column="category_id"/>
         <one-to-many class="Material"/>
     </set>
@@ -140,4 +140,4 @@
 
 </class>
 
-</hibernate-mapping>
\ No newline at end of file
+</hibernate-mapping>