使用扩展方法从Linq到列表

Question

I have this code that runs fine:

// my controller

public HttpResponseMessage GetUserFavorites(string id)
{

    var response = Request.CreateResponse();

    response.Content = new StringContent(JsonConvert.SerializeObject(jobRepository.GetUserFavorites(id)));
    response.StatusCode = HttpStatusCode.OK;

    return response;

}

// my jobRepository

     public IQueryable<ProfileInfo> GetUserFavorites(string iUserId)
        {


            var profiles = from favoriate in db.TB_Res_Favorites
                           join profile in db.TB_Res_Profile on favoriate.ProfileID equals profile.ProfileID
                           where favoriate.UserId == iUserId

                           select new ProfileInfo()
                           {
                               ProfileID = profile.ProfileID,
                               FullName = profile.FullName,
                               Headline = profile.Headline,
                               Location = profile.Location,
                               Industry = profile.Industry,
                               ImageUrl = profile.ImageUrl,
                               EducationInstitute = profile.EducationInstitute,
                               Degree = profile.Degree
                           };


            return profiles;


        }

The issue is, that the mapping to ProfileInfo will return in many places in my code. I obviously want to avoid that. I thought of using extension method, that will map from EF to my class like this:

// my extension:

 public static class ModelExtensions
    {

        public static ProfileInfo ToProfileInfo(this TB_Res_Profile dbProfile)
        {
            return new ProfileInfo
            {

                 ProfileID= dbProfile.ProfileID,
                 FullName = dbProfile.FullName, 
                 Headline = dbProfile.Headline,
                 Location = dbProfile.Location,
                 Industry = dbProfile.Industry,
                 ImageUrl = dbProfile.ImageUrl,
                 EducationInstitute = dbProfile.EducationInstitute,
                 Degree = dbProfile.Degree
            };
        }




    }

and use it like this (controler is not changed):

public IQueryable<ProfileInfo> GetUserFavorites(string iUserId)
{


    var profiles = from favoriate in db.TB_Res_Favorites
                   join profile in db.TB_Res_Profile on   favoriate.ProfileID equals profile.ProfileID
                   where favoriate.UserId == iUserId
                   select profile.ToProfileInfo();


    return profiles;


}

It is compiled OK, but I got this error in run time:

An exception of type 'System.NotSupportedException' occurred in Newtonsoft.Json.dll but was not handled in user code

Additional information: LINQ to Entities does not recognize the method 'myApp.Models.InfoClass.ProfileInfo ToProfileInfo(myApp.Models.TB_Res_Profile)' method, and this method cannot be translated into a store expression.

I know there are auto mappers that I can use. but it really interest me to make this work, and understand what is wrong here.

Answer 1

You can make it work using extension method if you first force a call to a database using for example ToList() and then use a mapper:

var profiles = (from favoriate in db.TB_Res_Favorites
               join profile in db.TB_Res_Profile on   favoriate.ProfileID equals profile.ProfileID
               where favoriate.UserId == iUserId
               select profile).ToList()
               .Select(p => p.ToProfileInfo());
return profiles;

The reason why you need to make a call to a database first, is because Entity Framework is making a translation of your LINQ code to SQL code, and when it finds custom Extension method it obviously cannot make a translation.

When you force a request to database using ToList() you make a request to extension method in C# code, which is obviously valid thing to do.

Answer 2

You should pass the whole IQueryable and return another IQueryable . Something like this:

public static IQueryable<ProfileInfo> ToProfileInfo(this IQueryable<TB_Res_Profile> query)
{
  return query.Select(p => new ProfileInfo()
  {
    ProfileID= p.ProfileID,
    FullName = p.FullName, 
    Headline = p.Headline,
    Location = p.Location,
    Industry = p.Industry,
    ImageUrl = p.ImageUrl,
    EducationInstitute = p.EducationInstitute,
    Degree = p.Degree
  });
}

Now you only have Expression trees, which EF will be pleased to work with:

var profiles = (from favoriate in db.TB_Res_Favorites
                join profile in db.TB_Res_Profile
                on favoriate.ProfileID equals profile.ProfileID
                where favoriate.UserId == iUserId
                select profile)
                .ToProfileInfo();

Answer 3

You could use the navigation properties , if they're properly set in place, and do something like this:

from p in db.TB_Res_Profile 
where p.TB_Res_Favorite.UserId == iUserId select p;

This way you'll also avoid all that mapping. Mind using the correct name for the navigation property.