c# - Entity Framework - querying a many-to-many relationship table

Question

I have a many-to-many relationship defined like so:

Employees
--------------
EmployeeID (PK)

Roles
--------------
RoleID (PK)

EmployeeRoles
--------------
EmployeeID (PK, FK)
RoleID (PK, FK)

I'm trying to get a list of Employees, given a list or RoleIDs:

private MyDBEntities _entities;

public SqlEmployeesRepository(MyDBEntities entities)
{            
    _entities = entities;
}

public IQueryable<Employee> GetEmployeesForRoles(int[] roleIds)
{
    // get employees
}

But if I try and do _entities.EmployeeRoles, there is no EmployeeRoles object. My edmx looks like this:

So it's recognizing the relationship between the two tables, but it's not creating an entity object for EmployeeRoles.

How can I get a distinct list of Employees given a list of role id's?

Answer 1

The relationship between the tables Role and Employee is represented as a navigation property - each Employees property in the Role entity will only contain the Employees that have this particular role.

Putting it the other way round - every Employee's Roles property only contains the roles that particular employee has.

Given a set of roles roleIds to look for you can use this to get the list of employees that have a role within that set:

public IQueryable<Employee> GetEmployeesForRoles(int[] roleIds)
{
    var employees = _entities.Employees
                             .Where( x=> x.Roles.Any(r => roleIds.Contains(r.RoleID)))
   return employees;
}

Edit:

The other way to get the employees is to attack the problem from the other side of the relationship (starting with the role, not the employee). This is most likely not as efficient as the first approach, since we have to de-duplicate employees (otherwise employees with i.e. two roles would show up twice):

public IQueryable<Employee> GetEmployeesForRoles(int[] roleIds)
{
    var employees = _entities.Roles
                             .Where( r => roleIds.Contains(r.RoleID))
                             .SelectMany( x=> x.Employees)
                             .Distinct()
   return employees;
}