Apply OrderBy with DbSet

Question

I am trying to implement pagination and sorting with generic repository. How to take primary key column as default order by column in DbSet ?

DbSet = Context.Set<T>();

public IQueryable<T> GetAll(int pageNumber = 0, int pageSize = 10, string sortColumn = "")
{
    return DbSet.OrderBy("how to use primary key column here").Skip(pageNumber * pageSize)Take(pageSize);
}

side note, I'm pretty sure you'll want `Skip` then `Take` – Jonesopolis Oct 24 '14 at 15:20 — Jonesopolis, Oct 24 '14 at 15:20

Lee D · Accepted Answer · 2014-10-24T15:32:32.000

I have used these extension methods to achieve something similar:

public static string GetKeyField(Type type)
{
    var allProperties = type.GetProperties();

    var keyProperty = allProperties.SingleOrDefault(p => p.IsDefined(typeof(KeyAttribute)));

    return keyProperty != null ? keyProperty.Name : null;
}

public static IQueryable<T> OrderBy<T>(this IQueryable<T> source, string orderBy)
{
    return source.GetOrderByQuery(orderBy, "OrderBy");
}

public static IQueryable<T> OrderByDescending<T>(this IQueryable<T> source, string orderBy)
{
    return source.GetOrderByQuery(orderBy, "OrderByDescending");
}

private static IQueryable<T> GetOrderByQuery<T>(this IQueryable<T> source, string orderBy, string methodName)
    {
        var sourceType = typeof(T);
        var property = sourceType.GetProperty(orderBy);
        var parameterExpression = Expression.Parameter(sourceType, "x");
        var getPropertyExpression = Expression.MakeMemberAccess(parameterExpression, property);
        var orderByExpression = Expression.Lambda(getPropertyExpression, parameterExpression);
        var resultExpression = Expression.Call(typeof(Queryable), methodName,
                                               new[] { sourceType, property.PropertyType }, source.Expression,
                                               orderByExpression);

        return source.Provider.CreateQuery<T>(resultExpression);
    }

This allows you to pass the property name as a string and build up an expression which it passes to the regular LINQ OrderBy() function. So in your case, the usage would be:

DbSet = Context.Set<T>();

public IQueryable<T> GetAll(int pageNumber = 0, int pageSize = 10, string sortColumn = "")
{
    return DbSet.OrderBy(GetKeyField(typeof(T))).Skip(pageNumber * pageSize)Take(pageSize);
}

This assumes your key field in your entity class is properly decorated with the Key attribute.

Great and flexible solution, saved me lots of time. Thank you! — Oleg, Jan 23 '16 at 21:24
How does this even work? Shouldn't Order By return an IOrderedQueryable, This solution doesn't look like it handles compound Keys — johnny 5, Dec 06 '17 at 05:55

score 0 · Answer 2 · answered Oct 24 '14 at 15:27

One way would be to have all your entities inherit from some interface that allows you to retrieve their primary key value :

public interface IIdentifiableEntity 
{
    public int Id {get; set;}
}

Then implementations would be like :

public class User : IIdentifiableEntity
{
   public int UserId {get; set;}

   //other properties...

   public int Id { get { return UserId; } set { UserId = value; } } 
}

Then it would be as easy as ordering by the Id. This pattern can help you in other areas as well.

Apply OrderBy with DbSet

2 Answers2

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