LINQ to Entities不识别方法 [英] LINQ to Entities does not recognize the method

问题描述

我正在MSDN上关注这篇文章。我将其移植到EF Code First。

I am following this article on MSDN. I ported it to 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();

这个模型是伟大的，因为它给了我可测试性。但是，像上面那样运行的查询会抛出这个错误。

This model is great as it gives me testability. However, running queries like the one above throws this

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.

我明白了错误，但是有什么办法可以避免这种错误，但仍然保留这些存储库的可测试性？ / p>

I understand the error but is there any way to avoid it but still keep the repositories for testability?

推荐答案

由于 IQueryable< T> 和查询提供者工作：查看此主题了解更多信息什么是IQueryable< T>之间的区别。和IEnumerable< T>？

Your select statement cannot be translated due to the nature of how IQueryable<T> and the query providers work: see this thread for more info What is the difference between IQueryable<T> and IEnumerable<T>?

您可以通过将表达式分成单独的语句来帮助linq提供者：

You can 'help' the linq provider by dividing your expression into separate statements like this:

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< T> 而不是 IQueryable< T> 然后你的原始表达式应该工作。

Or you can let FindAll() return IEnumerable<T> instead of IQueryable<T> and then your original expression should work.

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