Linq-to-Entities Left JOIN

Question

This is my query:

from forum in Forums
    join post in Posts on forum equals post.Forum into postGroup    

    from p in postGroup     
    where p.ParentP         


        

Answer 1

Here is some code to help you to work out Left Join with Link

    private class EntityRole
    {
        public int EntityId { get; set; }
        public int RoleId { get; set; }
    }

    private IList GetSourceEntityRole()
    {
        var list = new List() {new EntityRole(){EntityId = 123, RoleId = 1},
                                           new EntityRole(){EntityId = 123, RoleId = 2},
                                           new EntityRole(){EntityId = 123, RoleId = 3},
                                           new EntityRole(){EntityId = 123, RoleId = 4}};

        list.Reverse();

        return list;
    }

    private IList GetEmptyEntityRole()
    {
        var list = new List();

        return list;
    }

    public void TestToDelete()
    {
        var source = this.GetSourceEntityRole();
        var destination = this.GetEmptyEntityRole();

        this.TestLeftJoin(source, destination);
    }

    private void TestLeftJoin(IList source, IList destination)
    {
        var inserting = this.GetMissing(source, destination);
        var deleting = this.GetMissing(destination, source);

        this.Enumerate("Source", source);
        this.Enumerate("Destination", destination);

        this.Enumerate("Deleting", deleting);
        this.Enumerate("Inserting", inserting);
    }

    private IEnumerable GetMissing(IList sourceEntities, IList destinationEntities)
    {
        return from source in sourceEntities
               join dest in destinationEntities on source.RoleId equals dest.RoleId into joined
               from source2 in joined.DefaultIfEmpty()
               where source2 == null
               select source;
    }

    private void Enumerate(string source, IEnumerable roles)
    {
        foreach (var item in roles)
        {
            Console.WriteLine("{0}:{1}", source, item.RoleId);
        }
    }