Entity Framework 使用注意：Where查询条件中用到的关联实体不需要Include

在Entity Framework中，如果实体A关联了实体B，你想在加载实体A的同时加载实体B。通常做法是在LINQ查询中使用Include()。但是，如果你在查询条件中用到了实体B，EF会自动加载实体B，这时Include不仅是多余的，而且还会增加额外的LEFT OUTER JOIN查询，影响性能。 

请看我们在博问开发中遭遇这个问题时的一段代码：

//For q.cnblogs.com
public class QuestionService 
{
    private IRepository<QuestionItem> _questionRepository;

    public QuestionService(IUnitOfWork unitOfWork)
        : base(unitOfWork)
    {
        _questionRepository = new Repository<QuestionItem>(unitOfWork);
    }

    public List<QuestionItem> GetUnsolvedQuestions(int pageIndex, int pageSize)
    {
        return _questionRepository.Entities
            .Include(q => q.User)
            .Where(q => q.IsActive && q.User.IsActive)
            .Skip((pageIndex - 1) * pageSize)
            .Take(pageSize)
            .ToList();
            
    }
}

public class QuestionItem
{
    public int Id { get;set; }
    public string Title { get; set; }
    public bool IsActive { get; set; }
    public int UserId { get; set; }

    public User User { get; set; }
}

public class User
{
    public int UserId { get; set; }
    public bool IsActive {get;set;}
}

在上面的代码中，我们想在GetActiveQuestions()返回List<QuestionItem>时，QuestionItem中要包含User信息，所以在LINQ查询使用了.Include(q => q.User)。

（特别要注意的是：这里把q.User.IsActive作为查询条件之一）

然后我们用SQL Server Profiler发现，Entity Framework生成了如下的SQL语句：

SELECT TOP (25) 
[Filter1].[Id] AS [Id], 
[Filter1].[Title] AS [Title],
[Filter1].[UserId] AS [UserId], 
[Filter1].[UserId1] AS [UserId1], 
[Filter1].[IsActive1] AS [IsActive], 
[Filter1].[UserId2] AS [UserId1],
[Filter1].[IsActive2] AS [IsActive1]
FROM ( SELECT [Extent1].[Id] AS [Id], [Extent1].[Title] AS [Title], 
[Extent1].[UserId] AS [UserId1],[Extent1].[IsActive] AS [IsActive1],
[Extent3].[UserID] AS [UserID2], [Extent3].[IsActive] AS [IsActive2], 
row_number() OVER (ORDER BY [Extent1].[QID] DESC) AS [row_number]
    FROM   [dbo].[question_Item] AS [Extent1]
    INNER JOIN [dbo].[Users] AS [Extent2] ON [Extent1].[UserID] = [Extent2].[UserID]
    LEFT OUTER JOIN [dbo].[Users] AS [Extent3] ON [Extent1].[UserID] = [Extent3].[UserID]
    WHERE ([Extent1].[IsActive] = 1) AND ([Extent2].[IsActive] = 1) 
)  AS [Filter1]
WHERE [Filter1].[row_number] > 0
ORDER BY [Filter1].[Id] DESC

[dbo].[Users]表对应的就是User实体类，上面的SQL中[dbo].[Users]出现了两次JOIN，由于[Users]表数据量比较大，两次JOIN影响了执行计划，查询耗时增加。这显然是要避免的。

在与这个问题一阵激战之后，我们终于找到解决方法 —— 去掉Include，就这么简单！

从这个地方看，Entity Framework还是挺聪明的，但是由于不知道它的这个聪明之处，反而带来了问题。

所以，代码如人，要和她相处好，就要了解她的一切！