Entity Framework Core 在查询时抛出异常

Question

我在我的项目中使用了一个通用的存储库类，所以一个实际的具体存储库是通过依赖注入来实例化的：

services.AddTransient<IRepository<Passenger>, EntityFrameworkRepository<Passenger>>();

下面是通用存储库本身，请注意我正在使用规范模式进行查询过滤。规范本身只返回一个Expression<TEntity, bool> 表达式对象。

public class EntityFrameworkRepository<TEntity> : IRepository<TEntity>
    where TEntity : class
{
    public async Task<IEnumerable<TEntity>> Find(Specification<TEntity> specification)
    {
        return await _context.Set<TEntity>()
            .Where(specification.ToExpression())
            .AsNoTracking()
            .ToListAsync();;
    }

    public async Task<TEntity> FindOne(Specification<TEntity> specification)
    {
        return await _context.Set<TEntity>()
            .AsNoTracking()
            .FirstOrDefaultAsync(specification.ToExpression());
    }

    public async Task<TEntity> GetById(object id)
    {
        return await _context.Set<TEntity>().FindAsync(id);
    }
}

仅返回确认电子邮件地址的乘客的规范的实现：

public class PermanentPassengerSpecification : Specification<Passenger>
{
    public override Expression<Func<Passenger, bool>> ToExpression()
    {
        return passenger => passenger.EmailConfirmed == true;
    }
}

另一个通过电话号码匹配乘客

public class PassengerByPhoneSpecification : Specification<Passenger>
{
    private readonly PhoneNumber _phoneNumber;

    public PassengerByPhoneSpecification(PhoneNumber phoneNumber)
    {
        if (phoneNumber == null)
            throw new ArgumentNullException();
        _phoneNumber = phoneNumber;
    }

    public override Expression<Func<Passenger, bool>> ToExpression()
    {
        return passenger => passenger.PhoneNumber == _phoneNumber;
    }
}

所以，基本上当我使用 FindOne 方法从存储库中查询数据时，EF Core 会抛出这个：

InvalidOperationException: The EF.Property<T> method may only be used within LINQ queries.
Microsoft.EntityFrameworkCore.EF.Property<TProperty>(object entity, string propertyName)
lambda_method(Closure , TransparentIdentifier<Passenger, PhoneNumber> )
System.Linq.AsyncEnumerable+WhereSelectEnumerableAsyncIterator+<MoveNextCore>d__8.MoveNext()
System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
System.Runtime.CompilerServices.ConfiguredTaskAwaitable+ConfiguredTaskAwaiter.GetResult()
System.Linq.AsyncEnumerable+AsyncIterator+<MoveNext>d__10.MoveNext()
System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
System.Runtime.CompilerServices.ConfiguredTaskAwaitable+ConfiguredTaskAwaiter.GetResult()
System.Linq.AsyncEnumerable+<FirstOrDefault_>d__165.MoveNext()
System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
System.Runtime.CompilerServices.TaskAwaiter.GetResult()
Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider+TaskResultAsyncEnumerable+Enumerator+<MoveNext>d__3.MoveNext()
System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
System.Runtime.CompilerServices.TaskAwaiter.GetResult()
Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider+ExceptionInterceptor+EnumeratorExceptionInterceptor+<MoveNext>d__5.MoveNext()
System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
System.Runtime.CompilerServices.TaskAwaiter.GetResult()
Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler+<ExecuteSingletonAsyncQuery>d__23.MoveNext()
System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
System.Runtime.CompilerServices.TaskAwaiter.GetResult()
Taksapp.Infrastructure.Repositories.EntityFrameworkRepository+<FindOne>d__3.MoveNext() in EntityFrameworkRepository.cs

有什么方法可以解决这个问题吗？

Answer 1

这是因为表达式将实体成员 Passenger.EmailConfirmed 与常量值 true 进行比较。

public class PermanentPassengerSpecification : Specification<Passenger> {
    public override Expression<Func<Passenger, bool>> ToExpression() {
        return passenger => passenger.EmailConfirmed == true;
    }
}

但是在这种情况下

public class PassengerByPhoneSpecification : Specification<Passenger> {
    private readonly PhoneNumber _phoneNumber;

    public PassengerByPhoneSpecification(PhoneNumber phoneNumber) {
        if (phoneNumber == null)
            throw new ArgumentNullException();
        _phoneNumber = phoneNumber;
    }

    public override Expression<Func<Passenger, bool>> ToExpression() {
        return passenger => passenger.PhoneNumber == _phoneNumber; //<--THIS WONT WORK
    }
}

表达式将实体成员Passenger.PhoneNumber（我假设它是一个string）与局部变量_phoneNumber 进行比较，后者是一个ValueOject 派生的PhoneNumber。

Entity Framework 无法将表达式中的值对象转换为有效的 SQL。

PhoneNumber 值对象没有隐式或显式转换，因此在生成查询时很可能只调用与任何电话号码都不匹配的 ToString。

我的建议是允许值对象能够转换为最有可能与之比较的对象，或者至少修改规范以进行有效比较。

让我们说类似

public override Expression<Func<Passenger, bool>> ToExpression() {
    string formattedNumber = 
        string.Format("{0}{1}",_phoneNumber.RegionCode, _phoneNumber.Number);
    return passenger => passenger.PhoneNumber == formattedNumber;
}