带EF 4.3的LINQ内部规范

我在LINQ查询中使用规范时遇到了麻烦。这里的问题是我的params规范。

让我们伪造一个简单的场景：

public class Car {
    public Guid Id { get; set; }
    public string Color { get; set; }
    public int UsedPieces { get; set; }
    // whatever properties
}
public class Piece {
    public Guid Id { get; set; }
    public string Color { get; set; }
    // whatever properties
}
public static class PieceSpecifications : ISpecification<Piece> {
    public static ISpecification<Piece> WithColor(string color) {
        return new Specification<Piece>(p => p.Color == color);
    }
}

// Get accepts ISpecification and returns IQueryable<Car> to force just one call to database
var carWithPieces = _carRepository.Get(CarSpecifications.UsedPiecesGreaterThan(10));
var piecesWithColor = from p in _pieceRepository.Get()
                      let car = carWithPieces.FirstOrDefault() // entire query will does one call to database
                      where PieceSpecifications.WithColor(car.Color).IsSatisfiedBy(p) // unfortunately it isn't possible
                   // where p.Color == car.Color -> it works, but it's not what I want
                      select p;

我知道这有点令人困惑，但我试图在我的真实（大）场景中避免很多往返，我知道实际上不可能使用带有实体框架的原始LINQ。我厌倦了尝试这么多博客和失败的（我的）方法。有人知道一些真正好的方法。还有别的办法吗？

错误

System.NotSupportedException:LINQ to Entities不识别method"Boolean IsSatisfiedBy（App.Model.Piece）"方法，并且方法无法转换为存储表达式。

更新

基本规范模式

public class Specification<T> : ISpecification<T> {
    private readonly Expression<Func<T, bool>> _predicate;
    public Specification(Expression<Func<T, bool>> predicate) {
        _predicate = predicate;
    }
    public Expression<Func<T, bool>> Predicate {
        get { return _predicate; }
    }
    public bool IsSatisfiedBy(T entity) {
        return _predicate.Compile().Invoke(entity);
    }
}

更新

如果我做这个，那就很容易了

// call to database
var car = _carRepository
    .Get(CarSpecifications.UsedPiecesGreaterThan(10))
    .FirstOrDefault();
// Whoah! look I'm working, but calling to database again.
var piecesWithColor = _pieceRepository
    .Get(PieceSpecifications.WithColor(car.Color))
    .ToArray();

存储库

// The Get function inside repository accepts ISpecification<T>.
public IQueryable<T> Get(ISpecification<T> specification) {
    return Set.Where(specification.Predicate);
}