What is the most elegant way to update a child collection when using nhibernate (without creating unnecessary adds and deletes)?

Question

I have an domain object called Project which maps to a table in my SQL server database. It has a property that is a List called Dependencies.

   public class Project
   {
         public int Id;
         public List<ProjectDependency> Dependencies;   
   }

   public class ProjectDependency
   {
          public Project Project;
          public Project Dependency;
   }

and and I trying to figure out the most efficient way to update the list of dependencies given a new list of dependencyIds.

So here is a naive implementation:

 public void UpdateDependencies(Project p, List<int> newDependencyIds)
 {
       p.Dependencies.Clear();
       foreach (var dependencyId in newDependencyIds)
       {
             Project d = GetDependency(dependencyId)
             p.Dependencies.Add(new ProjectDependency{Project = p, Dependency = d});
       }
 }

but the issue here is that even if nothing changes, I am clearing out all items and doing inserts on the same items that were there before.

I am looking for an elegant way to determine the diff (whats been added, whats been removed) and just make those changes so if a dependency was there before and after then it doesn't get touched.

Answer 1

    public void UpdateDependencies(Project p, List<int> newDependencyIds)
    {
        p.Dependencies.RemoveAll(d => !newDependencyIds.Contains(d.Dependency.Id));
        var toAdd = newDependencyIds.Select(d => p.Dependencies.Any(pd => pd.Dependency.Id != d)).ToList();
        toAdd.ForEach(dependencyId => p.Dependencies.Add(new ProjectDependency{Project = p, Dependency = GetDependency(dependencyId)}));
    }