How to do Linq Many to Many join with or without navigation properties

问题

this question has been asked several times on stackoverflow, and I have read through atleast half a dozen of those, but I can't get my head around a simple many to many linq join query. This is my database EDMX

I'm simply trying to populate a WPF datagrid with the list of students, along with each student's subjects.

Now I know we can simply use navigation properties , instead of doing a join, but I have been unable to get the right result

so a ( either C#/VB.net )query like

var listOfStudents= // get a list with all students , along with each student's subjects

Thank you for any help, its such a simple query but I'm kind of stuck

回答1:

var listOfStudents = db.Student.Select(x => new { Id = x.Id, Name = x.StudentName, Subjects = x.StudentsSubjects.Select(y => y.Subject) });

回答2:

If you remove Id field from the table StudentsSubject then remove this table from the model and update your model, EF will automatically convert this table in two navigational properties Subjects and Students for Student and Subject entities respectively.

If you must leave StudentsSubject table schema intact you can use Include() method to get both students and their subjects:

var students = dbContext.Students.Include("StudentsSubject.Subject")

With 'normal' navigation properties you could write the following:

var students = dbContext.Students.Include("Subjects")

Sometimes you need to assembly large graphs then Include() and lazy loading can affect performance. There is a small trick for this case:

// switch off for performance
DbContext.Configuration.AutodetectChangesEnabled = false;

// load root entities   
var roots = dbContext.Parents.Where( root => %root_condition%).ToList();

// load entities one level lower
dbContext.DependentEntities.Where( dependent => dependent.Root%root_condition% && %dependent_condition%).ToList();

// detect changes
dbContext.ChangeTracker.DetectChanges();

// enable changes detection.
DbContext.Configuration.AutodetectChangesEnabled = true;

Now root.Dependents collections are filled for roots.

Here is the tradeoff between join redundancy (include or join) and several db requests along with the increasing complexity of requests.

With 'includes' data for joined nodes is duplicated so a chain of includes can produce enormous traffic from DB to client.
With the second approach each level requires filtering conditions of all upper levels in Where() and EF generates the query with N-1 joins for the N-th level, but there is no place for redundancy.

As far as I know EF now works fine with Contains() and conditions for parent nodes can be substituted with Contains():

// load root entities   
var roots = dbContext.Parents.Where( root => %root_condition%).ToList();

var rootIds = new List<int>( roots.Select( root => root.Id));

// load entities one level lower
dbContext.DependentEntities.Where( dependent => %dependent_condition% && rootIds.Contains( dependent.RootId)).ToList();

回答3:

A regular LINQ join should do the trick. Here's a reduced test case:

Public Class Student
  Public Property Id As String
  Public Property StudentName As String
  Public Property GPA As String
End Class

Public Class StudentsSubject
  Public Property SubjectId As String
  Public Property StudentId As String
  Public Property Id As String
End Class

Public Class Subject
  Public Property Id As String
  Public Property SubjectName As String
End Class

Sub Main()
  Dim students As New List(Of Student)
  students.Add(New Student With {.Id = "1", .GPA = "GPA1", .StudentName = "John"})
  students.Add(New Student With {.Id = "2", .GPA = "GPA2", .StudentName = "Peter"})

  Dim subjects As New List(Of Subject)
  subjects.Add(New Subject With {.Id = "100", .SubjectName = "Maths"})
  subjects.Add(New Subject With {.Id = "200", .SubjectName = "Physics"})

  Dim studentsSubject As New List(Of StudentsSubject)
  studentsSubject.Add(New StudentsSubject With {.Id = "10", .StudentId = "1", .SubjectId = "100"})
  studentsSubject.Add(New StudentsSubject With {.Id = "20", .StudentId = "1", .SubjectId = "200"})
  studentsSubject.Add(New StudentsSubject With {.Id = "30", .StudentId = "2", .SubjectId = "100"})

  Dim listOfStudents = From st In students
                       Join ss In studentsSubject On ss.StudentId Equals st.Id
                       Join sb In subjects On ss.SubjectId Equals sb.Id
                       Select st.StudentName, st.GPA, sb.SubjectName
End Sub

来源：https://stackoverflow.com/questions/17766211/how-to-do-linq-many-to-many-join-with-or-without-navigation-properties