EntityFramework Upsert parent and children

SomeRepository.cs

public void Upsert(ParentEntity parentEntity) {
    ParentEntity existing = DB.Parents.Find(parentEntity.Id);

    if (existing == null) {
        DB.Parents.Add(parentEntity);
    } else {
        var newChildKeys = new HashSet<int>(parentEntity.Children.Select(m => m.Id));

        var existingChildren = DB.Children.Where(m => m.ParentId == parentEntity.Id).ToArray();
        var existingChildrenKeys = new HashSet<int>(existingChildren.Select(m => m.Id));

        foreach (var existingChild in existingChildren) {
            if (newChildKeys.Contains(existingChild.Id)) {
                DB.Entry(existingChild).State = EntityState.Detached;
            } else {
                DB.Children.Remove(existingChild);
            }
        }

        foreach (var child in parentEntity.Children) {
            child.ParentId = parentEntity.Id;
            if (existingChildrenKeys.Contains((child.Id))) {
                DB.Children.Attach(child);
                DB.Entry(child).State = EntityState.Modified;
            } else {
                DB.Children.Add(child);
            }
        }

        DB.Entry(existing).State = EntityState.Detached;
        DB.Parents.Attach(parentEntity);
        DB.Entry(parentEntity).State = EntityState.Modified;
    }

    SaveChanges();
}

The basic idea is

• Find the common keys across the entities (existing in database vs replacement entity)
• For common keys, detach the old entity, attach the new entity & mark as modified
• For keys that no-longer exist - remove the entity
• For new keys - add the entity