Question

I really need someone to help me to fully understand how to do many-to-many relationship with Entity Framework 4 CTP 5, POCO. I need to understand 3 concepts:

1. How to config my model to indicates
   some tables are many-to-many.
2. How to properly do insert.
3. How to properly do update.

Here are my current models:

public class MusicSheet
{
    [Key]
    public int ID { get; set; }
    public string Title { get; set; }
    public string Key { get; set; }

    public virtual ICollection<Author> Authors { get; set; }
    public virtual ICollection<Tag> Tags { get; set; }
}

public class Author
{
    [Key]
    public int ID { get; set; }
    public string Name { get; set; }
    public string Bio { get; set; }

    public virtual ICollection<MusicSheet> MusicSheets { get; set; }
}

public class Tag
{
    [Key]
    public int ID { get; set; }
    public string TagName { get; set; }

    public virtual ICollection<MusicSheet> MusicSheets { get; set; }
}

As you can see, the MusicSheet can have many Authors or Tags, and an Author or Tag can have multiple MusicSheets.

Again, my questions are:

1. What to do on the
   EntityTypeConfiguration to set the
   relationship between them as well as
   mapping to an table/object that
   associates with the many-to-many
   relationship.
2. How to insert a new music sheets
   (where it might have multiple
   authors or multiple tags).
3. How to update a music sheet. For
   example, I might set TagA,
   TagB to MusicSheet1, but later I need to change the tags to TagA
   and TagC. It seems like I need
   to first check to see if the tags
   already exists, if not, insert the
   new tag and then associate it with
   the music sheet (so that I doesn’t
   re-insert TagA?). Or this is
   something already handled by the
   framework?

Thank you very much. I really hope to fully understand it rather than just doing it without fully understand what’s going on. Especially on #3.

Answer 1

1. In the EF4 CTP5 the relationship is done by default convention when you put public virtual ICollection in each of the classes of the many to many relationship, as you already have done, your context class should look like this:

   public class YourContextName : DbContext
   {
       public DbSet<MusicSheet> MusicSheets { get; set; }
       public DbSet<Tag> Tags { get; set; }
       public DbSet<Author> Authors { get; set; }
   }
   

2. Very simple you just create a instance of the MusicSheet class and then add all the instances of you authors and tags to each of the collections of Authors and Tags in your MusicSheet, and then add your instance of MusicSheet to your context collection of MusicSheets and then call SaveChanges:

           MusicSheet musicSheet = new MusicSheet
                                   {
                                       Title = "Music Sheet 1",
                                       Key = "Key",
                                       Authors = new List<Author>
                                                     {
                                                         new Author
                                                             {
                                                                 Name = "Author 1",
                                                                 Bio = "Author 1 biographic text..."
                                                             },
                                                         new Author
                                                             {
                                                                 Name = "Author 2",
                                                                 Bio = "Author 2 biographic text..."
                                                             }
                                                     },
   
                                       Tags = new List<Tag>
                                                  {
                                                      new Tag {TagName = "TagA"},
                                                      new Tag {TagName = "TagC"}
                                                  }
                                   };
   
   
       var context = new YourContextName();
       context.MusicSheets.Add(musicSheet);
       context.SaveChanges();
   

3. To update you have to load your MusicSheet and remove the tags you don’t want and then add the ones you need to add, this is how:

       var context = new YourContextName();
       var myMusicSheet = context.MusicSheets.First();
   
       //The Tag you wnat to remove.
       var tagToRemove = myMusicSheet.Tags.First();
   
       var tagToAdd = new Tag {TagName = "TagX"};
   
       myMusicSheet.Tags.Remove(tagToRemove);
       myMusicSheet.Tags.Add(tagToAdd);
   
       context.Entry(myMusicSheet).State = EntityState.Modified;
       context.SaveChanges();
   

You can also find any author and/or tag that you know that exist and added to your MusicSheet and vice versa, but this is the foundation.

Remember this is for the EF4 CTP5 Code first…

Excuse me my English is not my main language, I hope this can help you, best regards from Dominican Republic.

PS: Don’t forget to add references to EntityFramework and System.Data.Entity, is your responsibility to do anything else like unit test, validation, exception handling…etc

EDIT:

First you need to add a constructor to your models:

public class Tag
{
    [Key]
    public int ID { get; set; }
    public string TagName { get; set; }

    public Tag()
    {
       MusicSheets = new List<MusicSheet>();
    }        

    public virtual ICollection<MusicSheet> MusicSheets { get; set; }
}

…Then you can do something like this:

var context = new YourContextName();
var newMusicSheet = new MusicSheet();
    newMusicSheet.Title = "Newly added Music Sheet";

//Your existing Tag.
var existingTag = contex.Tags.Find(3);        

existingTag.MusicSheets.Add(existingTag);

context.Entry(existingTag).State = EntityState.Modified;
context.SaveChanges();

You can do the same for all your models.

I hope this can help you!