Saving to a many to many relationship - entity-framework-4

Question

The problem am facing is, anytime I call events.Tags.Add(tag) and call Save changes on the context, it ends up creating a new tag info in the Tags table instead of just inserting the EventId and TagId into just the EventTags Table.
Base on the data below how do I add an event and tag into the EventTags Table. Lets say I want to add Event with Id=2 and and tag with Id =1 to the EventTags table.
I have the following entities.
public class Event
{
public int Id { get; set; }
public string Name { get; set; }
public virtual ICollection<Tag> Tags { get; set; }
}
public class Tag
{
public int Id { get; set; }
public string Name { get; set; }
public virtual ICollection<Event> Events { get; set; }
}
public class EventConfiguration : EntityTypeConfiguration<Event> {
public EventConfiguration () {
ToTable("Events");
HasKey(x => x.Id).Property(x => x.Id).HasColumnName("Id").IsRequired();
Property(x => x.Name).HasColumnName("Name").IsRequired();
HasMany(x => x.Tags)
.WithMany(x => x.Events)
.Map(m => {
m.ToTable("EventTags");
m.MapLeftKey("EventId");
m.MapRightKey("TagId");
});
}
}
public class TagConfiguration : EntityTypeConfiguration<Tag> {
public TagConfiguration () {
ToTable("Tags");
HasKey(x => x.Id).Property(x => x.Id).HasColumnName("Id").IsRequired();
Property(x => x.Name).HasColumnName("Name").IsRequired();
}
}
/*
These are the records in my many to many tables
--------------
Events Table
--------------
Id Name
1 Test1
2 Test2
--------------
EventTags
-------------
EventId TagId
1 2
-------------
Tags
------------
Id Name
1 MVC
2 C#
*/

Answer 1

Lets say I want to add Event with Id=2 and and tag with Id =1 to the
EventTags table.
using (var context = new MyContext())
{
// Load the event
var theEvent = context.Events.Find(2);
// Load the tag
var theTag = context.Tags.Find(1);
// Add tag to Tags collection of the event
theEvent.Tags.Add(theTag);
// Save
context.SaveChanges();
}
It's enough to update one of the two collections. EF takes care of the other collection automatically.
Edit
Another option without loading the entities from the DB if you know the existing Ids:
using (var context = new MyContext())
{
// Create an event with Id 2
var theEvent = new Event { Id = 2, Tags = new HashSet<Tag>() };
// Create a tag with Id 1
var theTag = new Tag { Id = 1 };
// Attach both event and tag to the context
context.Events.Attach(theEvent);
context.Tags.Attach(theTag);
// Add tag to Tags collection of the event
theEvent.Tags.Add(theTag);
// Save
context.SaveChanges();
}
You can also mix both approaches, for example load event from DB, create and attach the tag.