.net - How to select the next and previous entities via Entity Framework?

Question

I have a web app that displays the details of some entity, let's call it Log. The entity is loaded from SQL Server via Entity Framework 4.

I would like to provide 'next' and 'previous' links to walk through logs bidirectionally.

Logs are ordered by two properties/columns:

• Date
• Time

Both of these columns may contain null, and there is no guarantee of uniqueness. If both of these values are null, then in order to guarantee a stable sort I order by the database Id, which is guaranteed to be non-null and unique.

Furthermore there may not actually be an entity before or after a given Log.

There are some other questions that address doing this with SQL directly. I'd like to know how to do this with the Entity Framework, ideally making only a single trip to the DB to bring back a few fields for this pair of Logs (id, title, etc).

Answer 1

I'm not sure if this works but let get it a try:

var query =(
        from l in context.Logs
        where l.UserId == log.UserId &&
             (    l.Date < log.Date
              || (l.Date == log.Date && l.Time < log.Time)
              || (l.Date == log.Date && l.Time == log.Time && l.Id < log.Id)
             )
        orderby l.Date descending, l.Time descending
        select l
    ).Take(1)
    .Concat((
        from l in context.Logs
        where l.UserId == log.UserId &&
             (    l.Date > log.Date
              || (l.Date == log.Date && l.Time > log.Time)
              || (l.Date == log.Date && l.Time == log.Time && l.Id > log.Id)
             )
        orderby l.Date, l.Time
        select l
    ).Take(1));