Entity framework Generic query in Nongeneric Property - entity-framework

Question

In Entity framework I have objectsets like
public partial class Building
{
public int BuildingID { get; set; }
public string BuildingName { get; set; }
}
public partial class Town
{
public int TownID { get; set; }
public string TownName { get; set; }
}
I want to create a generic query like
T.OrderBy(o=>o.Id).Skip(maxDispItem * (page - 1)).Take(maxDispItem).ToList();
T is generic class can be Building or Town but problem is BuildingId and TownId has different name.I don't want to change their name as Id and create interface IIdentity.

Answer 1

Maybe you could try something like this:
var query = (typeof(T) == typeof(Building) ?
context.Buildings.Select(b => new { Id = b.BuildingId, Name = b.BuildingName }) :
context.Towns.Select(t => new { Id = t.TownId, Name = b.TownName }))
.OrderBy(o => o.Id)...
Not tested but that's worth a test...

Answer 2

You can create generic method which find a field decorated with KeyAttribute, and then performs sorting by found key field. I have tested your model, works perfectly. Look at code snippet.
DbContext:
using System.Collections.Generic;
using System.Data.Entity;
namespace ConsoleApplication28.Entities
{
public class AppDbContext : DbContext
{
public AppDbContext()
{
Database.Connection.ConnectionString = #"Data Source=NOTEBOOK-PC;Initial Catalog=StackOverflowTest;Integrated Security=True";
Database.SetInitializer(new AppDbInitializer());
}
public DbSet<Town> Towns { get; set; }
public DbSet<Building> Buildings { get; set; }
}
public class AppDbInitializer : DropCreateDatabaseIfModelChanges<AppDbContext>
{
protected override void Seed(AppDbContext context)
{
context.Buildings.AddRange(new List<Building>
{
new Building {BuildingName = "Building1"},
new Building {BuildingName = "Building2"},
});
context.Towns.AddRange(new List<Town>
{
new Town {TownName = "Town1"},
new Town {TownName = "Town2"},
});
context.SaveChanges();
base.Seed(context);
}
}
}
Building
using System.ComponentModel.DataAnnotations;
namespace ConsoleApplication28.Entities
{
public class Building
{
[Key]
public int BuildingID { get; set; }
public string BuildingName { get; set; }
}
}
Town
using System.ComponentModel.DataAnnotations;
namespace ConsoleApplication28.Entities
{
public class Town
{
[Key]
public int TownID { get; set; }
public string TownName { get; set; }
}
}
Program
using System;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
using ConsoleApplication28.Entities;
using System.ComponentModel.DataAnnotations;
namespace ConsoleApplication28
{
class Program
{
static void Main(string[] args)
{
const int maxDispItem = 10;
const int page = 1;
var db = new AppDbContext();
var towns = db.Towns.OrderByKey().Skip(maxDispItem * (page - 1)).Take(maxDispItem).ToList();
var buildings = db.Buildings.OrderByKey().Skip(maxDispItem * (page - 1)).Take(maxDispItem).ToList();
}
}
public static class Extensions
{
/// <summary>
/// Sorts the elements of a sequence in ascending order according to a key specified using KeyAttribute
/// </summary>
public static IOrderedQueryable<T> OrderByKey<T>(this IQueryable<T> source, bool isAsc = true)
{
var type = typeof(T);
var keyProperty = type.GetProperties().Single(x => x.GetCustomAttributes(typeof(KeyAttribute)).Any());
return source.OrderBy(keyProperty.Name, isAsc);
}
#region COPIED FROM THERE http://stackoverflow.com/questions/41244/dynamic-linq-orderby-on-ienumerablet
public static IOrderedQueryable<T> OrderBy<T>(this IQueryable<T> source, string property, bool isAsc)
{
return isAsc ? source.OrderBy(property) : source.OrderByDescending(property);
}
public static IOrderedQueryable<T> OrderBy<T>(this IQueryable<T> source, string property)
{
return ApplyOrder<T>(source, property, "OrderBy");
}
public static IOrderedQueryable<T> OrderByDescending<T>(this IQueryable<T> source, string property)
{
return ApplyOrder<T>(source, property, "OrderByDescending");
}
public static IOrderedQueryable<T> ThenBy<T>(this IOrderedQueryable<T> source, string property)
{
return ApplyOrder<T>(source, property, "ThenBy");
}
public static IOrderedQueryable<T> ThenByDescending<T>(this IOrderedQueryable<T> source, string property)
{
return ApplyOrder<T>(source, property, "ThenByDescending");
}
static IOrderedQueryable<T> ApplyOrder<T>(IQueryable<T> source, string property, string methodName)
{
string[] props = property.Split('.');
Type type = typeof(T);
ParameterExpression arg = Expression.Parameter(type, "x");
Expression expr = arg;
foreach (string prop in props)
{
PropertyInfo pi = type.GetProperty(prop);
expr = Expression.Property(expr, pi);
type = pi.PropertyType;
}
Type delegateType = typeof(Func<,>).MakeGenericType(typeof(T), type);
LambdaExpression lambda = Expression.Lambda(delegateType, expr, arg);
object result = typeof(Queryable).GetMethods().Single(
method => method.Name == methodName
&& method.IsGenericMethodDefinition
&& method.GetGenericArguments().Length == 2
&& method.GetParameters().Length == 2)
.MakeGenericMethod(typeof(T), type)
.Invoke(null, new object[] { source, lambda });
return (IOrderedQueryable<T>)result;
}
#endregion
}
}