diff --git a/navalplanner-business/src/main/java/org/navalplanner/business/materials/entities/Material.java b/navalplanner-business/src/main/java/org/navalplanner/business/materials/entities/Material.java
index f5c9c5886..cb5245cf4 100644
--- a/navalplanner-business/src/main/java/org/navalplanner/business/materials/entities/Material.java
+++ b/navalplanner-business/src/main/java/org/navalplanner/business/materials/entities/Material.java
@@ -34,6 +34,7 @@ import org.navalplanner.business.common.BaseEntity;
  */
 public class Material extends BaseEntity {
 
+    @NotNull
     @NotEmpty
     private String code;
 
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 43de29182..40ecf17d9 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
@@ -24,9 +24,10 @@ import java.util.Collections;
 import java.util.HashSet;
 import java.util.Set;
 
-import org.apache.commons.collections.Unmodifiable;
 import org.apache.commons.lang.Validate;
+import org.hibernate.validator.AssertTrue;
 import org.hibernate.validator.NotEmpty;
+import org.hibernate.validator.Valid;
 import org.navalplanner.business.common.BaseEntity;
 
 /**
@@ -42,8 +43,10 @@ public class MaterialCategory extends BaseEntity {
 
     private MaterialCategory parent = null;
 
+    @Valid
     private Set<MaterialCategory> subcategories = new HashSet<MaterialCategory>();
 
+    @Valid
     private Set<Material> materials = new HashSet<Material>();
 
     // Default constructor, needed by Hibernate
@@ -97,7 +100,21 @@ public class MaterialCategory extends BaseEntity {
         materials.add(material);
         material.setCategory(this);
     }
+
     public void removeMaterial(Material material) {
         materials.remove(material);
     }
+
+    @AssertTrue(message="material code must be unique within a category")
+    public boolean checkConstraintNonRepeatedMaterialCodes() {
+        Set<String> materialCodes = new HashSet<String>();
+        for (Material each: materials) {
+            final String code = each.getCode();
+            if (materialCodes.contains(code)) {
+                return false;
+            }
+            materialCodes.add(code);
+        }
+        return true;
+    }
 }
diff --git a/navalplanner-webapp/src/main/java/org/navalplanner/web/materials/MaterialsController.java b/navalplanner-webapp/src/main/java/org/navalplanner/web/materials/MaterialsController.java
index ce25f4e48..1193f45e9 100644
--- a/navalplanner-webapp/src/main/java/org/navalplanner/web/materials/MaterialsController.java
+++ b/navalplanner-webapp/src/main/java/org/navalplanner/web/materials/MaterialsController.java
@@ -24,6 +24,7 @@ import static org.navalplanner.web.I18nHelper._;
 
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Iterator;
 import java.util.List;
 
 import org.apache.commons.logging.LogFactory;
@@ -31,6 +32,7 @@ import org.hibernate.validator.InvalidValue;
 import org.navalplanner.business.common.exceptions.ValidationException;
 import org.navalplanner.business.materials.entities.Material;
 import org.navalplanner.business.materials.entities.MaterialCategory;
+import org.navalplanner.web.common.ConstraintChecker;
 import org.navalplanner.web.common.IMessagesForUser;
 import org.navalplanner.web.common.Level;
 import org.navalplanner.web.common.MessagesForUser;
@@ -211,6 +213,13 @@ public class MaterialsController extends
         }
     }
 
+    /**
+     * Finds which element in categoryTree has the same name as {@link MaterialCategory},
+     * and returns name {@link Textbox} component
+     *
+     * @param materialCategory
+     * @return
+     */
     private Component findInMaterialCategoryTree(MaterialCategory materialCategory) {
         final Treechildren children = categoriesTree.getTreechildren();
         for(Treeitem each: (Collection<Treeitem>) children.getItems()) {
@@ -241,21 +250,64 @@ public class MaterialsController extends
     }
 
     public void saveAndExit() {
-        save();
-        messagesForUser.showMessage(Level.INFO, _("Materials saved"));
-    }
-
-    private void save() {
-        validate();
-        try {
-            materialsModel.confirmSave();
-        } catch (ValidationException e) {
-            messagesForUser.showInvalidValues(e);
+        if (save()) {
+            messagesForUser.showMessage(Level.INFO, _("Materials saved"));
         }
     }
 
-    private void validate() {
-        ConstraintChecker.
+    private boolean save() {
+        try {
+            materialsModel.confirmSave();
+            return true;
+        } catch (ValidationException e) {
+            showInvalidValues(e);
+        }
+        return false;
+    }
+
+    private void showInvalidValues(ValidationException validationException) {
+        final InvalidValue[] invalidValues = validationException.getInvalidValues();
+        for (InvalidValue each: invalidValues) {
+            if (each.getBean() instanceof Material) {
+                final Material material = (Material) each.getBean();
+                showConstraintErrorsFor(material.getCategory());
+            }
+        }
+    }
+
+    private void showConstraintErrorsFor(MaterialCategory materialCategory) {
+        Treeitem treeitem = findTreeItemByMaterialCategory(categoriesTree.getRoot(), materialCategory);
+        if (treeitem != null) {
+            treeitem.setSelected(true);
+
+            // Load materials for category
+            final List<Material> materials = getMaterials(materialCategory);
+            gridMaterials.setModel(new SimpleListModel(materials));
+            gridMaterials.renderAll();
+
+            // Show errors
+            ConstraintChecker.isValid(gridMaterials);
+        }
+    }
+
+    private Treeitem findTreeItemByMaterialCategory(Component node, MaterialCategory materialCategory) {
+        if (node instanceof Treeitem) {
+            final Treeitem treeitem = (Treeitem) node;
+            final MaterialCategory _materialCategory = (MaterialCategory) treeitem.getValue();
+            if (_materialCategory.equals(materialCategory)) {
+                return treeitem;
+            }
+        }
+        for (Iterator i = node.getChildren().iterator(); i.hasNext(); ) {
+            Object obj = i.next();
+            if (obj instanceof Component) {
+                Treeitem treeitem =  findTreeItemByMaterialCategory((Component) obj, materialCategory);
+                if (treeitem != null) {
+                    return treeitem;
+                }
+            }
+        }
+        return null;
     }
 
     public void saveAndContinue() {
@@ -279,10 +331,13 @@ public class MaterialsController extends
     private List<Material> getMaterials(Treeitem treeitem) {
         final List<Material> result = new ArrayList<Material>();
         if (treeitem != null) {
-            final MaterialCategory materialCategory = (MaterialCategory) treeitem.getValue();
-            result.addAll(materialsModel.getMaterials(materialCategory));
+            result.addAll(getMaterials((MaterialCategory) treeitem.getValue()));
         }
         return result;
     }
 
+    private List<Material> getMaterials(MaterialCategory materialCategory) {
+        return materialsModel.getMaterials(materialCategory);
+    }
+
 }
diff --git a/navalplanner-webapp/src/main/webapp/materials/materials.zul b/navalplanner-webapp/src/main/webapp/materials/materials.zul
index 81db15d58..f7a8f2c44 100644
--- a/navalplanner-webapp/src/main/webapp/materials/materials.zul
+++ b/navalplanner-webapp/src/main/webapp/materials/materials.zul
@@ -48,7 +48,7 @@
                                     <button label="${i18n:_('Add')}"
                                         onClick="materialsController.addMaterialCategory()" />
                                     <tree id="categoriesTree"
-                                        width="280px" rows="10" vflex="true" multiple="true"
+                                        width="280px" rows="10" vflex="true" multiple="false"
                                         model="@{materialsController.materialCategories}"
                                         treeitemRenderer="@{materialsController.materialCategoryRenderer}"
                                         onSelect="materialsController.refreshMaterials()">
@@ -72,30 +72,20 @@
                                         fixedLayout="true"
                                         model="@{materialsController.materials}">
                                         <columns>
-                                            <column
-                                                label="${i18n:_('Code')}" />
-                                            <column
-                                                label="${i18n:_('Description')}" />
-                                            <column
-                                                label="${i18n:_('Unit price')}" />
-                                            <column
-                                                label="${i18n:_('Unit type')}" />
-                                            <column
-                                                label="${i18n:_('Disabled')}" />
+                                            <column label="${i18n:_('Code')}" />
+                                            <column label="${i18n:_('Description')}" />
+                                            <column label="${i18n:_('Unit price')}" />
+                                            <column label="${i18n:_('Unit type')}" />
+                                            <column label="${i18n:_('Disabled')}" />
                                         </columns>
                                         <rows>
-                                            <row
-                                                self="@{each='material'}" value="@{material}">
-                                                <textbox
-                                                    value="@{material.code}" constraint="no empty:${i18n:_('cannot be null or empty')}" />
-                                                <textbox
-                                                    value="@{material.description}" />
-                                                <intbox
-                                                    value="@{material.defaultUnitPrice}" constraint="no empty:${i18n:_('cannot be null or empty')}" />
-                                                <textbox
-                                                    value="@{material.unitType}" constraint="no empty:${i18n:_('cannot be null or empty')}" />
-                                                <checkbox
-                                                    checked="@{material.disabled}" />
+                                            <row self="@{each='material'}" value="@{material}">
+                                                <textbox id="code" value="@{material.code}"
+                                                    constraint="no empty:${i18n:_('cannot be null or empty')}" />
+                                                <textbox value="@{material.description}" />
+                                                <intbox value="@{material.defaultUnitPrice}" />
+                                                <textbox value="@{material.unitType}" />
+                                                <checkbox checked="@{material.disabled}" />
                                             </row>
                                         </rows>
                                     </grid>