EF

public void UpdateEntity(ParentItem parent)
{
// Load original parent including the child item collection
var originalParent = _dbContext.ParentItems
.Where(p => p.ID == parent.ID)
.Include(p => p.ChildItems)
.SingleOrDefault();
// We assume that the parent is still in the DB and don't check for null

// Update scalar properties of parent,
// can be omitted if we don't expect changes of the scalar properties
var parentEntry = _dbContext.Entry(originalParent);
parentEntry.CurrentValues.SetValues(parent);

foreach (var childItem in parent.ChildItems)
{
var originalChildItem = originalParent.ChildItems
.Where(c => c.ID == childItem.ID && c.ID != 0)
.SingleOrDefault();
// Is original child item with same ID in DB?
if (originalChildItem != null)
{
// Yes -> Update scalar properties of child item
var childEntry = _dbContext.Entry(originalChildItem);
childEntry.CurrentValues.SetValues(childItem);
}
else
{
// No -> It's a new child item -> Insert
childItem.ID = 0;
originalParent.ChildItems.Add(childItem);
}
}

// Don't consider the child items we have just added above.
// (We need to make a copy of the list by using .ToList() because
// _dbContext.ChildItems.Remove in this loop does not only delete
// from the context but also from the child collection. Without making
// the copy we would modify the collection we are just interating
// through - which is forbidden and would lead to an exception.)
foreach (var originalChildItem in
originalParent.ChildItems.Where(c => c.ID != 0).ToList())
{
// Are there child items in the DB which are NOT in the
// new child item collection anymore?
if (!parent.ChildItems.Any(c => c.ID == originalChildItem.ID))
// Yes -> It's a deleted child item -> Delete
_dbContext.ChildItems.Remove(originalChildItem);
}

_dbContext.SaveChanges();
}

//---------------------------------------------------
//---------------------------------------------------

var parent = context.Parents.GetById(1); // Make sure that childs are loaded as well
foreach(var child in modifiedParent.ChildItems)
{
var attachedChild = FindChild(parent, child.Id);
if (attachedChild != null)
{
// Existing child - apply new values
context.Entry(attachedChild).CurrentValues.SetValues(child);
}
else
{
// New child
// Don't insert original object. It will attach whole detached graph
parent.ChildItems.Add(child.Clone());
}
}

// Now you must delete all entities present in parent.ChildItems but missing
// in modifiedParent.ChildItems
// ToList should make copy of the collection because we can't modify collection
// iterated by foreach
foreach(var child in parent.ChildItems.ToList())
{
var detachedChild = FindChild(modifiedParent, child.Id);
if (detachedChild == null)
{
parent.ChildItems.Remove(child);
context.Childs.Remove(child);
}
}

context.SaveChanges();