动态包含在查询预先加载报表 - EF 4.3.1

Question

我有这样的方法：

public CampaignCreative GetCampaignCreativeById(int id) 
     { 
      using (var db = GetContext()) 
      { 
       return db.CampaignCreatives 
        .Include("Placement") 
        .Include("CreativeType")      
        .Include("Campaign") 
        .Include("Campaign.Handshake") 
        .Include("Campaign.Handshake.Agency") 
        .Include("Campaign.Product") 
        .AsNoTracking() 
        .Where(x => x.Id.Equals(id)).FirstOrDefault(); 
      } 
     } 

我想就包括动态列表。我试过了：

public CampaignCreative GetCampaignCreativeById(int id, string[] includes) 
     { 
      using (var db = GetContext()) 
      { 
       var query = db.CampaignCreatives; 

       foreach (string include in includes) 
       { 
        query = query.Include(include); 
       } 

       return query.AsNoTracking() 
        .Where(x => x.Id.Equals(id)).FirstOrDefault();      
      } 
     } 

但是没有编译。我得到这个错误：

不能隐式地将类型'System.Data.Entity.Infrastructure.DbQuery'转换为'System.Data.Entity.DbSet'。存在明确的转换（您是否缺少演员？）

有谁知道如何制作包含动态列表？

感谢

Answer 1

充分利用query变量可查询：

public CampaignCreative GetCampaignCreativeById(int id, string[] includes) 
{ 
    using (var db = GetContext()) 
    { 
     var query = db.CampaignCreatives.AsQueryable(); 
     foreach (string include in includes) 
     { 
      query = query.Include(include); 
     } 

     return query 
      .AsNoTracking() 
      .Where(x => x.Id.Equals(id)) 
      .FirstOrDefault();      
    } 
} 

Answer 2

通过使用IQueryable<CampaignCreative>代替var将工作太给编译器的提示。

IQueryable<CampaignCreative> query = db.CampaignCreatives; 
// or 
DbQuery<CampaignCreative> query = db.CampaignCreatives; 

当使用var编译器推断DbSet<T>为query其比Include返回（这是DbQuery<T>（=基类的DbSet<T>）实施IQueryable<T>），所以不能将结果赋予类型更具体的query变量了。因此编译器错误在query = query.Include(include)行。

Answer 3

我更喜欢定义包含的非字符串表达方式。主要是因为它不依赖于魔术字符串。

对于示例代码，它会是这个样子：

public CampaignCreative GetCampaignCreativeById(int id) { 
    using (var db = GetContext()) { 
     return db.CampaignCreatives 
      .Include(cc => cc.Placement) 
      .Include(cc => cc.CreativeType)      
      .Include(cc => cc.Campaign.Select(c => 
       c.Handshake.Select(h => h.Agency))) 
      .Include(cc => cc.Campaign.Select(c => c.Product) 
      .AsNoTracking() 
      .Where(x => x.Id.Equals(id)) 
      .FirstOrDefault(); 
    } 
} 

而为了让那些充满活力，这是如何做到这一点：

public CampaignCreative GetCampaignCreativeById(
    int id, 
    params Expression<Func<T, object>>[] includes 
) { 
    using (var db = GetContext()) { 
     var query = db.CampaignCreatives; 
     return includes 
      .Aggregate(
       query.AsQueryable(), 
       (current, include) => current.Include(include) 
      ) 
      .FirstOrDefault(e => e.Id == id); 
    } 
} 

这是用这样的：

var c = dataService.GetCampaignCreativeById(
    1, 
    cc => cc.Placement, 
    cc => cc.CreativeType, 
    cc => cc.Campaign.Select(c => c.Handshake.Select(h => h.Agency)), 
    cc => cc.Campaign.Select(c => c.Product 
);