LINQ to Entities 无法识别该方法

Question

我正在 MSDN 上关注这篇文章。我将其移植到 EF Code First。

public interface IUnitOfWork
{
    IRepository<Employee> Employees { get; }
    IRepository<TimeCard> TimeCards { get; }
    void Commit();
}

---

public class HrContext : DbContext
{
    public DbSet<Employee> Employees { get; set; }
    public DbSet<TimeCard> TimeCards { get; set; }
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Employee>()
                    .HasMany(e => e.TimeCards)
                    .WithOptional(tc => tc.Employee);
     }
}

---

public class SqlRepository<T> : IRepository<T>
    where T : class
{
    private readonly DbSet<T> entitySet;
    public SqlRepository(DbContext context)
    {
        this.entitySet = context.Set<T>();
    }
    public void Add(T newEntity)
    {
        this.entitySet.Add(newEntity);
    }
    public IQueryable<T> FindAll()
    {
        return this.entitySet;
    }
    public T FindById(params object[] keys)
    {
        return this.entitySet.Find(keys);
    }
    public IQueryable<T> FindWhere(Expression<Func<T, bool>> predicate)
    {
        return this.entitySet.Where(predicate);
    }
    public void Remove(T entity)
    {
        this.entitySet.Remove(entity);
    }
}

---

public class SqlUnitOfWork : IUnitOfWork, IDisposable
{
    private readonly HrContext context;
    private IRepository<Employee> employees;
    private IRepository<TimeCard> timeCards;
    public SqlUnitOfWork()
    {
        this.context = new HrContext();
    }
    public IRepository<Employee> Employees
    {
        get
        {
            return new SqlRepository<Employee>(context);
        }
    }
    public IRepository<TimeCard> TimeCards
    {
        get
        {
            return new SqlRepository<TimeCard>(context);
        }
    }
    public void Commit()
    {
        this.context.SaveChanges();
    }
    public void Dispose()
    {
        context.Dispose();
    }
}

---

var query = from e in unitOfWork.Employees.FindAll()
            from tc in unitOfWork.TimeCards.FindAll()
            where tc.Employee.Id == e.Id && e.Name.StartsWith("C")
            select tc;
var timeCards = query.ToList();

这个模型很棒，因为它给了我可测试性。然而，运行像上面这样的查询会抛出这个

LINQ to Entities does not recognize the method
 'System.Linq.IQueryable`1[DomainModel.Models.TimeCard] FindAll()' 
  method, and this method cannot be translated into a store expression.

错误，但有什么办法可以避免它，但仍然保留存储库以实现可测试性？

Answer 1

由于 IQueryable 和查询提供程序工作方式的性质，您的 select 语句无法翻译：请参阅此线程以获取更多信息 IQueryable和 IQueryable之间有什么区别和 IEnumerable？

您可以通过将表达式分成单独的语句来“帮助”linq 提供程序，如下所示：

var ems = unitOfWork.Employees.FindAll();
var tcs = unitOfWork.TimeCards.FindAll();

var query = from e in ems
            from tc in tcs
            where tc.Employee.Id == e.Id && e.Name.StartsWith("C")
            select tc;

或者您可以让 FindAll() 返回 IEnumerable 而不是 < code>IQueryable然后你的原始表达式应该可以工作。