///--------------------------------------------------------------------------------
        /// <summary>This method adds a relationship to the view model.</summary>
        ///
        /// <param name="sourceEntity">The source entity of the relationship to add.</param>
        /// <param name="sinkEntity">The sink entity of the relationship to add.</param>
        /// <param name="entityRelationship">The associated entity relationship.</param>
        ///--------------------------------------------------------------------------------
        public void AddRelationship(DiagramEntityViewModel sourceEntity, DiagramEntityViewModel sinkEntity, RelationshipViewModel entityRelationship)
        {
            // add diagram relationship to solution diagram
            DiagramRelationshipViewModel relationship = new DiagramRelationshipViewModel(sourceEntity, sinkEntity, sourceEntity.Diagram, entityRelationship);

            DiagramRelationships.Add(relationship);
            Items.Add(relationship);
        }
        ///--------------------------------------------------------------------------------
        /// <summary>This method adds a relationship to the view model.</summary>
        ///
        /// <param name="relationship">The relationship to add.</param>
        ///--------------------------------------------------------------------------------
        public void DeleteRelationship(DiagramRelationshipViewModel relationship)
        {
            // remove diagram relationship from solution diagram
            RemoveRelationship(relationship);

            // add to deleted relationship list
            ItemsToDelete.Add(relationship);
            Refresh(false);
        }
        ///--------------------------------------------------------------------------------
        /// <summary>This method adds a relationship to the view model.</summary>
        ///
        /// <param name="relationship">The relationship to add.</param>
        ///--------------------------------------------------------------------------------
        public void CreateRelationship(DiagramRelationshipViewModel relationship)
        {
            // add diagram relationship to solution diagram
            DiagramRelationships.Add(relationship);
            Items.Add(relationship);

            // add to diagram relationships to add list
            ItemsToAdd.Add(relationship);
            Refresh(false);
        }
 ///--------------------------------------------------------------------------------
 /// <summary>This method adds a relationship to the view model.</summary>
 ///
 /// <param name="relationship">The relationship to add.</param>
 ///--------------------------------------------------------------------------------
 public void RemoveRelationship(DiagramRelationshipViewModel relationship)
 {
     // remove diagram relationship from solution diagram
     DiagramRelationships.Remove(relationship);
     Items.Remove(relationship);
 }