实体框架和多对多查询不可用?

因为它在EFV1中,我们将 不 有 和 角色

解决方案1:

Persons.Include("Role").Include("Location") // Include to load Role and Location
       .Where(p => p.Role.Any(r => r.description == "Student") 
       && p.Location.Any(l => l.description == "Amsterdam")).ToList();  

而且它的性能还可以。

这里有一些分类。

   // Find out all persons in the role
   // Return IQuerable<Person> 
  var students = Roles.Where(r => r.description == "Student")
                      .SelectMany(r => r.Person);

  // Find out all persons in the location
  // Return IQuerable<Person>  
  var personsInAmsterdam = Locations.Where(l=> l.description == "Amsterdam")
                                    .SelectMany(l=>l.Person);

  // Find out the intersection that gives us students in Admsterdam.
  // Return List<Person>
     var AdmsterdamStudents = students.Intersect(personsInAmsterdam).ToList();

 //Return List<Person>
 var AdmsterdamStudents = Roles.Where(r => r.description == "Student")
                              .SelectMany(r => r.Person)
                              .Intersect
                              ( 
                                Locations
                                .Where(l=> l.description == "Amsterdam")
                                .SelectMany(l=>l.Person)
                               ).ToList();

在Lambda:

    var persons = Persons.Where(p=>(p.PersonLocations.Select(ps=>ps.Location)
   .Where(l=>l.Description == "Amsterdam").Count() > 0)
    && (p.PersonRoles.Select(pr=>pr.Role)
   .Where(r=>r.Description == "Student").Count() > 0));

查询结果:

SELECT [t0].[personId] AS [PersonId], [t0].[description] AS [Description]
FROM [Persons] AS [t0]
WHERE (((
    SELECT COUNT(*)
    FROM [personlocations] AS [t1]
    INNER JOIN [Locations] AS [t2] ON [t2].[locationid] = [t1].[locationid]
    WHERE ([t2].[description] = @p0) AND ([t1].[personid] = [t0].[personId])
    )) > @p1) AND (((
    SELECT COUNT(*)
    FROM [PersonRoles] AS [t3]
    INNER JOIN [Roles] AS [t4] ON [t4].[roleid] = [t3].[roleid]
    WHERE ([t4].[description] = @p2) AND ([t3].[personid] = [t0].[personId])
    )) > @p3)

var persons = Persons
            .Where(p=>(p.Personlocations.Select(ps=>ps.Location)
            .Select(l=>l.Description).Contains("Amsterdam")) && 
            (p.PersonRoles.Select(pr=>pr.Role)
            .Select(r=>r.Description).Contains("Student")));

查询结果:

SELECT [t0].[personId] AS [PersonId], [t0].[description] AS [Description]
FROM [Persons] AS [t0]
WHERE (EXISTS(
    SELECT NULL AS [EMPTY]
    FROM [personlocations] AS [t1]
    INNER JOIN [Locations] AS [t2] ON [t2].[locationid] = [t1].[locationid]
    WHERE ([t2].[description] = @p0) AND ([t1].[personid] = [t0].[personId])
    )) AND (EXISTS(
    SELECT NULL AS [EMPTY]
    FROM [PersonRoles] AS [t3]
    INNER JOIN [Roles] AS [t4] ON [t4].[roleid] = [t3].[roleid]
    WHERE ([t4].[description] = @p1) AND ([t3].[personid] = [t0].[personId])
    ))

使用join():

var persons = Persons
        .Join(Personlocations, p=>p.PersonId, ps=>ps.Personid,
(p,ps) => new {p,ps})
.Where(a => a.ps.Location.Description =="Amsterdam")
        .Join(PersonRoles,
pr=> pr.p.PersonId, r=>r.Personid,(pr,r) => new {pr.p,r})
.Where(a=>a.r.Role.Description=="Student")
        .Select(p=> new {p.p});

查询结果:

SELECT [t0].[personId] AS [PersonId], [t0].[description] AS [Description]
FROM [Persons] AS [t0]
INNER JOIN [personlocations] AS [t1] ON [t0].[personId] = [t1].[personid]
INNER JOIN [Locations] AS [t2] ON [t2].[locationid] = [t1].[locationid]
INNER JOIN [PersonRoles] AS [t3] ON [t0].[personId] = [t3].[personid]
INNER JOIN [Roles] AS [t4] ON [t4].[roleid] = [t3].[roleid]
WHERE ([t4].[description] = @p0) AND ([t2].[description] = @p1)

朱利亚诺·莱姆斯

找到了有关这方面的更多信息,以及实体处理多对多的方式和原因:

ASP.NET Blog - Many to Many Mappings in Entity Framework