LINQ - Conditional Join - entity-framework

Question

I have a condition where joining table have a condition. Let's say I have a table called Mapper Student Teacher where Mapper table have a column named AcNoId which contains a id from both table Student and Teacher. The table structure is
Mapper
Student
Teacher
TestOption is a enum and is defined as a
public enum TestOption
{
Teacher = 1,
Student = 2
}
Now I have a condition where if TestOption is a type of Student it should perform a join with Student table and if is a type of Teacher it should perform a join with Teacher table
This is how I have tried so far
(from m in _context.Mapper
where m.TestOption == TestOption.Student
join s in _context.Student
on m.AcNoId equals s.Id into tempStudent
from st in tempStudent.DefaultIfEmpty()
where m.TestOption == TestOption.Teacher
join t in _context.Teacher
on m.AcNoId equals t.Id into tempTeacher
from ta in tempTeacher.DefaultIfEmpty()
select new
{
Type = m.TestOption.ToString(),
Student = st.StudentName ?? string.Empty,
Teacher = ta.TeacherName ?? string.Empty
}).ToList();
Instead of conditional join this query perform a following query on SQL Profiler
exec sp_executesql N'SELECT [m].[TestOption], COALESCE([s].[StudentName], #__Empty_0) AS [Student], COALESCE([t].[TeacherName], #__Empty_1) AS [Teacher]
FROM [Mapper] AS [m]
LEFT JOIN [Student] AS [s] ON [m].[AcNoId] = [s].[Id]
LEFT JOIN [Teacher] AS [t] ON [m].[AcNoId] = [t].[Id]
WHERE ([m].[TestOption] = 2) AND ([m].[TestOption] = 1)',N'#__Empty_0 nvarchar(4000),#__Empty_1 nvarchar(4000)',#__Empty_0=N'',#__Empty_1=N''
How can I do this????

Answer 1

You can use the below code, with no need to use join or where statements on _context.Mapper:
(from m in _context.Mapper
select new
{
Type = m.TestOption.ToString(),
Student = _context.Student
.FirstOrDefault(s =>
m.TestOption == TestOption.Student &&
s.Id == m.AcNoId) ?? string.Empty,
Teacher = _context.Teacher
.FirstOrDefault(t =>
m.TestOption == TestOption.Teacher &&
t.Id == m.AcNoId) ?? string.Empty,
})
.ToList();