barcodelib c# Modeling a Many-to-Many Relationship with No Payload in Font

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2-3. Modeling a Many-to-Many Relationship with No Payload
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You have a couple of tables in an existing database that are related to each other via a link or junction table. The link table contains just the foreign keys used to link the two tables together into a many-tomany relationship. You want to import these tables model this many-to-many relationship.
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Let s say your database tables look something like the database diagram in Figure 2-10.
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ENTITY DATA MODELING FUNDAMENTALS
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Figure 2-10. Artists and albums in a many-to-many relationship To create a model and import these tables and relationships, do the following: 1. Add a new model to your project by right-clicking your project and selecting Add New Item. Choose ADO.NET Entity Data Model from the Visual C# Items Data templates. Select Generate from database. Click Next. Use the wizard to select an existing connection to your database or create a new connection. From the Choose Your Database Object dialog box, select the tables Album, LinkTable, and Artist. Leave the Pluralize and Foreign Key options checked. Click Finish.
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The wizard will create the model shown in Figure 2-11.
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Figure 2-11. Our model with a many-to-many relationship between our tables The many-to-many relationship between Album and Artist is represented by a line with the * character on both ends. Because an Album can have many Artists and an Artist can responsible for many Albums, each of these navigation properties is of type EntityCollection.
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How It Works
In Figure 2-11, an artist can be related to many albums, whereas an album can be the work of many artists. Notice that the link table from Figure 2-10 is not represented as an entity in our model. Because our link table has no scalar properties (that is, it has no payload), Entity Framework assumes that its sole purpose is to create the association between Album and Artist. If the link table had scalar properties, Entity Framework would have created a very different model, as we will see in the next Recipe.
ENTITY DATA MODELING FUNDAMENTALS
The code in Listing 2-3 demonstrates how to insert new albums and artists into our model and how to query our model both for artists and their albums and albums with their artists. Listing 2-3. Inserting and querying our artists and albums model through the many-to-many association using (var context = new EFRecipesEntities()) { // add an artist with two albums var artist = new Artist { FirstName = "Alan", LastName = "Jackson" }; var album1 = new Album { AlbumName = "Drive" }; var album2 = new Album { AlbumName = "Live at Texas Stadium" }; artist.Albums.Add(album1); artist.Albums.Add(album2); context.Artists.AddObject(artist); // add an album for two artists var artist1 = new Artist { FirstName = "Tobby", LastName = "Keith" }; var artist2 = new Artist { FirstName = "Merle", LastName = "Haggard" }; var album = new Album { AlbumName = "Honkytonk University" }; artist1.Albums.Add(album); artist2.Albums.Add(album); context.Albums.AddObject(album); context.SaveChanges(); } using (var context = new EFRecipesEntities()) { Console.WriteLine("Artists and their albums..."); var artists = from a in context.Artists select a; foreach (var artist in artists) { Console.WriteLine("{0} {1}", artist.FirstName, artist.LastName); foreach (var album in artist.Albums) { Console.WriteLine("\t{0}", album.AlbumName); } } Console.WriteLine("\nAlbums and their artists..."); var albums = from a in context.Albums select a; foreach (var album in albums) { Console.WriteLine("{0}", album.AlbumName); foreach (var artist in album.Artists) { Console.WriteLine("\t{0} {1}", artist.FirstName, artist.LastName); } } }
ENTITY DATA MODELING FUNDAMENTALS
The output from the code in Listing 2-3 looks like the following: Artists and their albums... Alan Jackson Drive Live at Texas Stadium Tobby Keith Honkytonk University Merle Haggard Honkytonk University
Albums and their artists... Drive Alan Jackson Live at Texas Stadium Alan Jackson Honkytonk University Tobby Keith Merle Haggard After getting an instance of our object context, we create and initialize an instance of an Artist entity type and a couple of instances of the Album entity type. We add the albums to the artist and then add the artist to the Object Context. Next, we create and initialize a couple instances of the Artist entity type and an instance of the Album entity type. Because the two artists collaborated on the album, we add the album to both artists Albums navigation property (which is of type EntityCollection). Adding the album to the Object Context causes the artists to get added as well. Now that the completed object graph is part of the object context, the only thing left to do is to use SaveChanges() to save the whole thing to the database.
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