EF6：映射到ViewModel可重用性

Question

Using Entity Framework 6, I have mapping functions between the Model objects and viewmodel objects contained in a class like so:

public class DataMappers
{
    public static Expression<Func<Data.Models.Employer, EmployerViewModel>> EmployerMapper = (e => new EmployerViewModel()
    {
        Name = e.Name,
        Location = e.Location
        ....
    });
}

Which I can then call in multiple places like so:

            results = db.Employers.OrderBy(e => e.Name)
                                  .Select(DataMappers.EmployerMapper)
                                  .ToList();

Which will generate a SQL statement with only the columns I need. My question is, is there anyway to reuse this if other tables reference my 'Employer' table? ie

    public static Expression<Func<Data.Models.Person, PersonViewModel>> Person = (p => new PersonViewModel()
    {
        FirstName = p.FirstName,
        LastName = p.LastName,
        Employer = *use the 'EmployerMapper' expression above on p.Employer*
        ....
    });

Can that be done, or will I need to duplicate that mapping code in every situation like this?

I tried using it as just a Func instead of Expression<Func> in that second example, which would compile ( Employer = EmployerMapper(p.Employer) ), however, you receive The LINQ expression node type 'Invoke' is not supported in LINQ to Entities exception at run-time.

Answer 1

You'll have to install LinqKit , and use its AsExpandable() :

using LinqKit;

results = db.Employers.AsExpandable()
                      .OrderBy(e => e.Name)
                      .Select(DataMappers.EmployerMapper)
                      .ToList();

And your projection function:

using LinqKit;

public static Expression<Func<Data.Models.Person, PersonViewModel>> Person = (p => new PersonViewModel()
    {
    FirstName = p.FirstName,
    LastName = p.LastName,
    Employer = DataMappers.EmployerMapper.Invoke(p.Employer)
    });

More information on LinqKit and how it works here .