Adding an existing entity to a child collection of a many-to-many relationship

Question

I have to entities that represent a Message and a Topic .
A Message can have many Topic , and a Topic can belong to many Message

public class Message
{
    [Key]
    public int Id { get; set; }

    public string Text { get; set; }

    public virtual ICollection<Message> Topics { get; set; }
}

public class Topic
{
    public int Id { get; set; }

    public string Value { get; set; }

    public virtual ICollection<Message> Messages { get; set; }
}

In my application, we first save Topic (early on in the application, sometimes seed data etc...)

Later, I need to save a new Message - adding an already existing Topic to the child collection;

var message = new Message();
message.Text = "Hello";
//this topic already exists in the database
message.Topics.Add(new Topic{Id = 6, Value = "val"}); 
dbContext.Messages.Add(message);
dbContext.SaveChanges();

What happens is:

• Message is saved
• New topic is created (with Value of "val")
• ID of new topic is saved in MessageTopics mapping table against the newly created Message ID

What I want to happen is

• Message is saved
• ID of supplied topic (6 in this case) is saved in MessageTopics table against the newly created Message ID

Answer 1

First option

If everything is mapped correctly then the only thing you need to do is to hold reference to topic and set its state to Unchanged :

// This topic already exists in the database.
// No need to set value.
var topic = new Topic{ Id = 6 };
var message = new Message();
message.Text = "Hello"; 
message.Topics.Add(topic); 
dbContext.Messages.Add(message);

// Set state of topic to Unchanged
dbContext.Entry(topic).State = EntityState.Unchanged;
dbContext.SaveChanges();

Second option.

Just get topic from database if there is not too much data.

// Get topic from source
var topic = dbContext.Topics.FirstOrDefault(m => m.Id == 6);
var message = new Message();
message.Text = "Hello";
message.Topics.Add(topic); 
dbContext.Messages.Add(message);
dbContext.SaveChanges();