Question

我有两个表：tbA和tbB。在它们之间我有一个n到n的关系，所以在数据库中生成了一个表tbAB。我首先使用实体框架数据库，然后当我映射这些表时，它不会为tbAB生成特定实体。因此，如果我无法直接调用tbAB，我就无法看到如何创建与这两个表相关的查询。我想在SQL中做的事情如下：

 SELECT * 
 FROM tbA
 INNER JOIN tbAB 
 ON tbAB.idA = tbA.idA
 INNER JOIN tbB
 ON tbB.idB = tbAB.idB

这就是我尝试用Linq做的事情：

  var table = (from l in db.tbA
                                            join k in db.tbB on l.? equals k.?
                                            where ?.IDCONCESSAO == objectTbB.IDCONCESSAO
                                            select l).ToList();

问题是如何在Linq表达式中执行此操作？

提前致谢。

Answer 1

按照@Michal提出的模型，您可以这样做：

var query= from a in db.TableAs
           from b in a.TableBs
           where b.Id==10
           select new{A_Id=a.Id,a.Name, B_Id=b.Id,b.Price,...};

在select中，您可以从两个实体中选择所需的属性（我还从Name选择TableA，从Price选择TableB帮助你更好地理解这个例子。从关系的每个方向，你都不会与junction table进行交互，你只需要跟随每个方向的关系，就好像它是一对多关系一样。我在上面显示的查询将在sql查询中进行转换，其中表之间的连接将以这种方式进行：

{SELECT 
[Extent1].[Id] AS [Id], 
[Extent1].[Name] AS [Name], 
[Join1].[Id] AS [Id1], 
[Join1].[Price] AS [Price]
FROM  [dbo].[TableAs] AS [Extent1]
INNER JOIN  (SELECT [Extent2].[TableA_Id] AS [TableA_Id], [Extent3].[Id] AS [Id], [Extent3].[Price] AS [Price]
    FROM  [dbo].[TableBTableAs] AS [Extent2]
    INNER JOIN [dbo].[TableBs] AS [Extent3] ON [Extent3].[Id] = [Extent2].[TableB_Id] ) AS [Join1] ON [Extent1].[Id] = [Join1].[TableA_Id]
WHERE 10 = [Join1].[Id]}

Answer 2

    public void Test()
    {
        var db = new DbContext();

        // This will automatically do you inner join for you.
        db.TableAs.Include(a => a.TableBs);
    }

上下文：

    public class DbContext
    {
        public IDbSet<TableA> TableAs { get; set; }
        public IDbSet<TableB> TableBs { get; set; }
    }

型号：

    public class TableA
    {
        public int Id { get; set; }
        public virtual List<TableB> TableBs { get; set; }
    }
    public class TableB
    {
        public int Id { get; set; }

        public virtual List<TableA> TableAs { get; set; }
    }

Answer 3

  var table = from a in db.tbA
              join ab in db.tbAB on a.idA  equals ab.idA
              join b in db.tbB on ab.idB  equals b.idB
              where a.Anything = 10
              select a;

  var results = table.ToList();

Linq使用实体框架查询多对多关系

3 个答案: