diff --git a/ECS/DataStructures/TypeSafeFasterListForECS.cs b/ECS/DataStructures/TypeSafeFasterListForECS.cs
index 4bca085..4f15c15 100644
--- a/ECS/DataStructures/TypeSafeFasterListForECS.cs
+++ b/ECS/DataStructures/TypeSafeFasterListForECS.cs
@@ -16,6 +16,7 @@ namespace Svelto.ECS.Internal
         ITypeSafeDictionary CreateIndexedDictionary();
         IEntityView[] ToArrayFast(out int count);
         void ReserveCapacity(int capacity);
+        int GetIndexFromID(int entityID);
     }
 
     class TypeSafeFasterListForECS<T>: FasterList<T> where T:IEntityView
@@ -69,6 +70,11 @@ namespace Svelto.ECS.Internal
                 Resize(capacity);
         }
 
+        public int GetIndexFromID(int entityID)
+        {
+            return _mappedIndices[entityID];
+        }
+
         readonly Dictionary<int, int> _mappedIndices;
     }
 
diff --git a/ECS/EnginesRootEntities.cs b/ECS/EnginesRootEntities.cs
index f9c3896..6b58966 100644
--- a/ECS/EnginesRootEntities.cs
+++ b/ECS/EnginesRootEntities.cs
@@ -171,22 +171,22 @@ namespace Svelto.ECS
             var entityViewBuilders = EntityDescriptorTemplate<T>.Default.entityViewsToBuild;
             int entityViewBuildersCount = entityViewBuilders.Length;
 
+            Dictionary<Type, ITypeSafeList> dictionary = _groupEntityViewsDB[fromGroupID];
+
+            Dictionary<Type, ITypeSafeList> groupedEntityViewsTyped;
+            if (_groupEntityViewsDB.TryGetValue(toGroupID, out groupedEntityViewsTyped) == false)
+            {
+                groupedEntityViewsTyped = new Dictionary<Type, ITypeSafeList>();
+
+                _groupEntityViewsDB.Add(toGroupID, groupedEntityViewsTyped);
+            }
+
             for (int i = 0; i < entityViewBuildersCount; i++)
             {
                 IEntityViewBuilder entityViewBuilder = entityViewBuilders[i];
                 Type entityViewType = entityViewBuilder.GetEntityViewType();
-                Dictionary<Type, ITypeSafeList> dictionary = _groupEntityViewsDB[fromGroupID];
 
                 ITypeSafeList fromSafeList = dictionary[entityViewType];
-
-                Dictionary<Type, ITypeSafeList> groupedEntityViewsTyped;
-                if (_groupEntityViewsDB.TryGetValue(toGroupID, out groupedEntityViewsTyped) == false)
-                {
-                    groupedEntityViewsTyped = new Dictionary<Type, ITypeSafeList>();
-
-                    _groupEntityViewsDB.Add(toGroupID, groupedEntityViewsTyped);
-                }
-
                 ITypeSafeList toSafeList;
 
                 if (groupedEntityViewsTyped.TryGetValue(entityViewType, out toSafeList) == false)
@@ -198,9 +198,9 @@ namespace Svelto.ECS
 
                 if (fromSafeList.UnorderedRemove(entityID) == false)
                     dictionary.Remove(entityViewType);
-
-                if (dictionary.Count == 0) _groupEntityViewsDB.Remove(fromGroupID);
             }
+
+            if (dictionary.Count == 0) _groupEntityViewsDB.Remove(fromGroupID);
         }
 
         void InternalRemove(IEntityViewBuilder[] entityViewBuilders, int entityID,
@@ -241,16 +241,17 @@ namespace Svelto.ECS
         {
             int entityViewBuildersCount = entityViewBuilders.Length;
 
+            Dictionary<Type, ITypeSafeList> dictionary = _groupEntityViewsDB[groupID];
+
             for (int i = 0; i < entityViewBuildersCount; i++)
             {
                 Type entityViewType = entityViewBuilders[i].GetEntityViewType();
-                Dictionary<Type, ITypeSafeList> dictionary = _groupEntityViewsDB[groupID];
 
                 if (dictionary[entityViewType].UnorderedRemove(entityID) == false)
                     dictionary.Remove(entityViewType);
-
-                if (dictionary.Count == 0) _groupEntityViewsDB.Remove(groupID);
             }
+
+            if (dictionary.Count == 0) _groupEntityViewsDB.Remove(groupID);
         }
 
         static void RemoveEntityViewFromEngines(Dictionary<Type, FasterList<IHandleEntityViewEngine>> entityViewEngines,
diff --git a/ECS/EntityDescriptor.cs b/ECS/EntityDescriptor.cs
index 91e09aa..9f3a46f 100644
--- a/ECS/EntityDescriptor.cs
+++ b/ECS/EntityDescriptor.cs
@@ -39,51 +39,37 @@ namespace Svelto.ECS.Internal
                                                      EntityDescriptorInfo entityViewsToBuildDescriptor,
                                                      object[] implementors)
         {
-            var entityViewsToBuild = entityViewsToBuildDescriptor.entityViewsToBuild;
-            int count = entityViewsToBuild.Length;
+            Dictionary<Type, ITypeSafeList> groupedEntityViewsTyped;
 
-            RemoveEntityImplementor removeEntityImplementor = null;
-
-            for (int index = 0; index < count; index++)
+            if (groupEntityViewsByType.TryGetValue(groupID, out groupedEntityViewsTyped) == false)
             {
-                var entityViewBuilder = entityViewsToBuild[index];
-                var entityViewType = entityViewBuilder.GetEntityViewType();
-
-                Dictionary<Type, ITypeSafeList> groupedEntityViewsTyped;
-
-                if (groupEntityViewsByType.TryGetValue(groupID, out groupedEntityViewsTyped) == false)
-                {
-                    groupedEntityViewsTyped = new Dictionary<Type, ITypeSafeList>();
-                    groupEntityViewsByType.Add(groupID, groupedEntityViewsTyped);
-                }
-
-                //only class EntityView will be returned
-                //struct EntityView cannot be filled so it will be null.
-                var entityViewObjectToFill =
-                    BuildEntityView(entityID, groupedEntityViewsTyped, entityViewType, entityViewBuilder);
+                groupedEntityViewsTyped = new Dictionary<Type, ITypeSafeList>();
+                groupEntityViewsByType.Add(groupID, groupedEntityViewsTyped);
+            }
 
-                //the semantic of this code must still be improved
-                //but only classes can be filled, so I am aware
-                //it's a EntityViewWithID
-                if (entityViewObjectToFill != null)
-                {
-                    if (removeEntityImplementor == null)
-                        removeEntityImplementor = new RemoveEntityImplementor(entityViewsToBuildDescriptor.entityViewsToBuild, groupID);
+            var removeEntityImplementor = new RemoveEntityImplementor(entityViewsToBuildDescriptor.entityViewsToBuild, groupID);
 
-                    FillEntityView(entityViewObjectToFill as EntityView, implementors, removeEntityImplementor, 
-                                   entityViewsToBuildDescriptor.name);
-                }
-            }
+            InternalBuildEntityViews(entityID, groupedEntityViewsTyped, entityViewsToBuildDescriptor, implementors, removeEntityImplementor);
         }
 
         internal static void BuildEntityViews(int entityID, 
                                               Dictionary<Type, ITypeSafeList> entityViewsByType,
                                               EntityDescriptorInfo entityViewsToBuildDescriptor,
                                               object[] implementors)
+        {
+            var removeEntityImplementor = entityViewsToBuildDescriptor.removeEntityImplementor;
+
+            InternalBuildEntityViews(entityID, entityViewsByType, entityViewsToBuildDescriptor, implementors, removeEntityImplementor);
+        }
+
+        private static void InternalBuildEntityViews(int entityID, 
+            Dictionary<Type, ITypeSafeList> entityViewsByType, 
+            EntityDescriptorInfo entityViewsToBuildDescriptor, 
+            object[] implementors, RemoveEntityImplementor removeEntityImplementor)
         {
             var entityViewsToBuild = entityViewsToBuildDescriptor.entityViewsToBuild;
             int count = entityViewsToBuild.Length;
-            
+
             for (int index = 0; index < count; index++)
             {
                 var entityViewBuilder = entityViewsToBuild[index];
@@ -99,7 +85,7 @@ namespace Svelto.ECS.Internal
                 //it's a EntityView
                 if (entityViewObjectToFill != null)
                 {
-                    FillEntityView(entityViewObjectToFill as EntityView, implementors, entityViewsToBuildDescriptor.removeEntityImplementor,
+                    FillEntityView(entityViewObjectToFill as EntityView, implementors, removeEntityImplementor,
                                    entityViewsToBuildDescriptor.name);
                 }
             }
diff --git a/ECS/EntityViewBuilder.cs b/ECS/EntityViewBuilder.cs
index b512cf9..73606af 100644
--- a/ECS/EntityViewBuilder.cs
+++ b/ECS/EntityViewBuilder.cs
@@ -48,7 +48,7 @@ namespace Svelto.ECS
             var fromCastedList = fromSafeList as TypeSafeFasterListForECSForClasses<EntityViewType>;
             var toCastedList = toSafeList as TypeSafeFasterListForECSForClasses<EntityViewType>;
 
-            toCastedList.Add(fromCastedList[entityID]);
+            toCastedList.Add(fromCastedList[fromCastedList.GetIndexFromID(entityID)]);
         }
 
         readonly Type _entityViewType = typeof(EntityViewType);
@@ -91,7 +91,7 @@ namespace Svelto.ECS
             var fromCastedList = fromSafeList as TypeSafeFasterListForECSForStructs<EntityViewType>;
             var toCastedList = toSafeList as TypeSafeFasterListForECSForStructs<EntityViewType>;
 
-            toCastedList.Add(fromCastedList[entityID]);
+            toCastedList.Add(fromCastedList[fromCastedList.GetIndexFromID(entityID)]);
         }
 
         readonly Type _entityViewType = typeof(EntityViewType);