Order column by a separate `List` variable

Question

I need to sort the results of an entity framework query by the index of elements in another list.

I tried the suggestion found elsewhere, like

.Then         


        

Answer 1

Here is a mad way to do it but it doesn't fall back to IEnumerable

var db = new BloggingContext();

var list = (new List<string> { "FR", "EN" }).
    Select((s,i)=> $"select '{s}' as {nameof(OrderIndex.Name)},{i} as {nameof(OrderIndex.Id)}");

var order = db.Set<OrderIndex>().FromSql(String.Join(" union ",list));

var orderedItems = from post in db.Posts
                   join ln in order on post.Lang equals ln.Name into lnPost
                   from od in lnPost.DefaultIfEmpty()
                   orderby od.Id
                   select post;
var data = orderedItems.ToList();

You can find the definition of the BloggingContext here I just added a Lang field for a language code. This will work in SQLite, SqlServer and MySQL for oracle you need to add from dual Due to how terrible EF core is you will need to go true the same procedure for OrderIndex as you would with an SQL view. In EF 6 there is a much nicer way of doing this with SqlQuery and you don't need to do the registration like with EF core. The created query is

SELECT "post"."PostId", "post"."BlogId", "post"."Content", "post"."Date", "post"."Lang", "post"."Title", "ln"."Id", "ln"."Name"
    FROM "Posts" AS "post"
    LEFT JOIN (
        select 'FR' as Name,0 as Id union select 'EN' as Name,1 as Id
    ) AS "ln" ON "post"."Lang" = "ln"."Name"
    ORDER BY "ln"."Id", "post"."Lang"

EDIT: Just remembered a different way to do it it's not as mad but might be better.

var lang = new List<string> { "FR", "EN" };    
var orderedItems = from post in db.Posts
                   orderby (lang[0] == post.Lang) ? 
                   0 :((lang[1] == post.Lang) ? 1 : 2)
                   select post;

var param = Expression.Parameter(typeof(Post));

var order = lang.Select((s, i) => new { s, i })
    .Aggregate((Expression)Expression.Constant(lang.Count), (agg, i) =>
        Expression.Condition(
            Expression.Equal(Expression.Property(param,nameof(Post.Lang)),
            Expression.Constant(i.s)),
            Expression.Constant(i.i),
            agg));
var exp = Expression.Lambda<Func<Post, int>>(order, param);
var data = db.Posts.OrderBy(exp).ToList();

And the SQL

SELECT "p"."PostId", "p"."BlogId", "p"."Content", "p"."Date", "p"."Lang", "p"."Title"
    FROM "Posts" AS "p"
    ORDER BY CASE
        WHEN "p"."Lang" = 'EN'
        THEN 1 ELSE CASE
            WHEN "p"."Lang" = 'FR'
            THEN 0 ELSE 2
        END
    END

I still think the mad way is useful for something I just don't know what.

Answer 2

this should do the trick:

private IEnumerable<MiaLog1A> PopulateQuery(string selectedCampus)
{
    var list = new List<string> {"Fall","Mid","Spring"};
    return _db.MiaLog1A.Where(m => m.Campus == selectedCampus)
        .AsEnumerable()
        .OrderBy(m => m.StudentName)
        .ThenBy(m=> list.IndexOf(m.Term));
}