/// <summary>
 /// Adds another collection to this collection and updates counts.
 /// </summary>
 /// <param name="collection">Collection to add.</param>
 public void Add(BaseCollectionModel <T> collection)
 {
     if (collection == null)
     {
         return;
     }
     this.Add(collection.Items);
 }
 /// <summary>
 /// Adds another collection to this collection and updates counts.
 /// </summary>
 /// <param name="collection">Collection to add.</param>
 public void AddRange(BaseCollectionModel <T> collection)
 {
     this.Add(collection);
 }