Entity Framework Code First Map (Linked) Table?

Question

I'm working with the EF code first approach and want to add a link (map) table. I am working off the below example and get the following error:

System.Data.Entity.Edm.EdmEntityType: : EntityType 'EmployeeDepartmentLink' has no key defined. Define the key for this EntityType.

Problem is I dont want a key on this table it just maps the two tables together:

public class Employee
{
    [Key()]
    public int EmployeeID;
    public string Name;
}

public class Department
{
    [Key()]
    public int DepartmentID;
    public string Name;
}

public class EmployeeDepartmentLink
{
    public int EmployeeID;
    public int DepartmentID;
}

I have tried a variety of things like adding the "[Key()]" attribute but it doesn't make sense for it to be used, and which field do I add it to? I am wondering if this sort of table model is even supported?

Check this stackoverflow.com/questions/5482670/…

Sampath
– Sampath

2013-01-21 14:45:00 +00:00
Commented Jan 21, 2013 at 14:45 — Sampath
– Sampath, Commented Jan 21, 2013 at 14:45

lante · Accepted Answer · 2013-01-21 15:34:10Z

15

You are trying to make a "Many to Many" mapping.

To perform this, write this code:

public class Employee
{
    [Key]
    public int EmployeeId;
    public string Name;
    public List<Department> Departments { get; set; }

    public Employee()
    {
        this.Departments = new List<Department>();
    }
}

public class Department
{
    [Key]
    public int DepartmentId;
    public string Name;

    public List<Employee> Employees { get; set; }

    public Department()
    {
        this.Employees = new List<Employee>();
    }
}

then, in your DbContext:

public class YourContext : DbContext
{
    public DbSet<Employee> Employees { get; set; }
    public DbSet<Department> Departments { get; set; }

    public YourContext() : base("MyDb")
    {
    }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Department>().
            HasMany(c => c.Employees).
            WithMany(p => p.Departments).
            Map(
                m =>
                {
                    m.MapLeftKey("DepartmentId");
                    m.MapRightKey("EmployeeId");
                    m.ToTable("DepartmentEmployees");
                });
    }
}

answered Jan 21, 2013 at 15:34

lante

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1 Comment

Rhyous Over a year ago

I knew it would be possible in the fluent API without creating a third object to represent the mapping table.

Community · Accepted Answer · 2017-05-23 12:25:09Z

2

For M:M relationship you have to create your join (link) class is as below.

public class EmployeeDepartmentLink
{
    [Key, Column(Order = 0)]
    public int EmployeeID;

    [Key, Column(Order = 1)]
    public int DepartmentID;

}

For more information check Create code first, many to many

I hope this will help to you.

edited May 23, 2017 at 12:25

CommunityBot

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answered Jan 21, 2013 at 14:50

Sampath

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1 Comment

user1882453 Over a year ago

Just what I was looking for!

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