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The edmgen command provides a convenient way to automate some of the build processes and is a useful tool for pregenerating views and generating separate files for the model layers. One restriction of edmgen is that it does not provide a way to generate a model based on a subset of the tables in a database.
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7-7. Working with Dependent Entities in an Identifying Relationship
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You want to insert, update, and delete a dependent entity in an identifying relationship.
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Suppose you have a model like the one in Figure 7-8. The LineItem s entity key is a composite key comprised of InvoiceNumber and ItemNumber. InvoiceNumber is also a foreign key to the Invoice entity.
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Figure 7-8. Invoice and LineItem in an identifying relationship because of the composite entity key in the LineItem entity
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When one of the properties of an entity key is both the primary key and the foreign key, the entity is said to be participating in an identifying relationship. In our model, LineItem s entity key, its identity, is also a foreign key to the Invoice entity. The LineItem entity is referred to as the dependent entity while Invoice is the principal entity. There is a subtle difference in how Entity Framework handles the deletion of dependent entities in an identifying relationship. Because the dependent entity cannot exist without participating in the relationship, simply removing the dependent entity from the principal s collection will result in Entity Framework marking the dependent entity for deletion. Additionally, deleting the principal entity will also mark the dependent for deletion. This is reminiscent of the cascading deletes common in database systems. Of course, Entity Framework allows you to explicitly delete the dependent entity. The code in Listing 7-5 demonstrates all three of these scenarios. Listing 7-5. Deleting the dependent entity static void Main(string[] args) { RunExample(); } static void RunExample() { using (var context = new EFRecipesEntities()) { var invoice1 = new Invoice { BilledTo = "Julie Kerns", InvoiceDate = DateTime.Parse("3/19/2010") }; var invoice2 = new Invoice { BilledTo = "Jim Stevens", InvoiceDate = DateTime.Parse("3/21/2010") }; context.LineItems.AddObject(new LineItem { Cost = 99.29M, Invoice = invoice1 }); context.LineItems.AddObject(new LineItem { Cost = 29.95M, Invoice = invoice1 }); context.LineItems.AddObject(new LineItem { Cost = 109.95M, Invoice = invoice2 }); context.SaveChanges(); // display the line items Console.WriteLine("Original set of line items..."); DisplayLineItems(); // remove a line item from invoice 1's collection var item = invoice1.LineItems.ToList().First(); invoice1.LineItems.Remove(item); context.SaveChanges(); Console.WriteLine("\nAfter removing a line item from an invoice..."); DisplayLineItems(); // remove invoice2 context.DeleteObject(invoice2); context.SaveChanges(); Console.WriteLine("\nAfter removing an invoice...");
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DisplayLineItems(); // remove a single line item context.DeleteObject(invoice1.LineItems.First()); context.SaveChanges(); Console.WriteLine("\nAfter removing a line item..."); DisplayLineItems(); } } static void DisplayLineItems() { bool found = false; using (var context = new EFRecipesEntities()) { foreach (var lineitem in context.LineItems) { Console.WriteLine("Line item: Cost {0}", lineitem.Cost.ToString("C")); found = true; } } if (!found) Console.WriteLine("No line items found!"); } The following is the output of the code in Listing 7-5: Original set of line items... Line item: Cost $99.29 Line item: Cost $29.95 Line item: Cost $109.95
After removing a line item from an invoice... Line item: Cost $29.95 Line item: Cost $109.95
After removing an invoice... Line item: Cost $29.95
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After removing a line item... No line items found!
How It Works
The code in Listing 7-5 deletes line items in three ways. First, it deletes a line item from an invoice s collection. Because a line item is dependent on the invoice for its identity, Entity Framework marks the referenced line item for deletion. Next, it deletes an invoice. Entity Framework marks all the dependent line items for deletion. Finally, the code deletes the last remaining line item directly by calling DeleteObject(). You can modify all the properties of a dependent entity except for properties that participate in the identifying relationship. In our model, we can modify the Cost property in a line item, but we can t change the Invoice navigation property. When a principal object in an identifying relationship is saved to the database, the key that is generated at the database (for store-generated values) is written to the principal entity and to all its dependent entities. This ensures that all are synchronized in the object context. The subtle difference in Entity Framework s treatment of a deleted relationship between two entities in an identifying relationship and two entities in any other relationship is worth noting. For other types of relationships, Entity Framework does not mark an entity for deletion if the entity is removed from the collection of another entity. For an identifying relationship, Entity Framework does mark the dependent entity for deletion.
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