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Listing 6-6. The GetSubCategories() stored procedure that returns subcategories for a given CategoryId create proc chapter6.GetSubCategories (@categoryid int) as begin with cats as ( select c1.* from chapter6.Category c1 where CategoryId = @categoryid union all select c2.* from cats join chapter6.Category c2 on cats.CategoryId = c2.ParentCategoryId ) select * from cats where CategoryId != @categoryid end With the GetSubCategories() stored procedure imported into the conceptual model, Entity Framework now exposes a GetSubCategories() method on the object context. We can use this method to materialize our entire graph of categories and subcategories. The code in Listing 6-7 demonstrates the use of the GetSubCategories() method. Listing 6-7. Retrieving the entire hierarchy using the GetSubCategories() method using (var context = new EFRecipesEntities()) { var book = new Category { Name = "Books" }; var fiction = new Category { Name = "Fiction", ParentCategory = book }; var nonfiction = new Category { Name = "Non-Fiction", ParentCategory = book }; var novel = new Category { Name = "Novel", ParentCategory = fiction }; var history = new Category { Name = "History", ParentCategory = nonfiction }; context.Categories.AddObject(book); context.SaveChanges(); } using (var context = new EFRecipesEntities()) { var root = context.Categories.Where(o => o.Name == "Books").First(); Console.WriteLine("Parent category is {0}, subcategories are:", root.Name); foreach (var sub in context.GetSubCategories(root.CategoryId)) { Console.WriteLine("\t{0}", sub.Name); } } The output from the code in Listing 6-7 is the following:
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Parent category is Books, subcategories are: Fiction Non-Fiction History Novel
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Entity Framework supports self-referencing associations, as we have seen in Recipes 6.2 and 6.3. In these recipes, we directly loaded the entity references and collections using the Load() method. We cautioned, however, that each Load() results in a round trip to the database to retrieve an entity or entity collection. For larger object graphs, this database traffic may consume too many resources. In this recipe, we demonstrated a slightly different approach. Rather than explicitly using Load() to materialize each entity or entity collection, we pushed the work off to the storage layer by using a stored procedure to recursively enumerate all the subcategories and return the collection. We used a Common Table Expression in our stored procedure to implement the recursive query. In our example, we chose to enumerate all the subcategories. You could, of course, modify the stored procedure to selectively enumerate elements of the hierarchy. To use our stored procedure, we first imported it into the model. Then, using the Add Function Import, we added the imported stored procedure to the conceptual layer. Once added, the stored procedure was mapped by Entity Framework to a new method, GetSubCategories(), which was available in the data context. On the conceptual side, the stored procedure is represented in the code snippet shown in Listing 6-8. Listing 6-8. GetSubCategories() store procedure represented in the conceptual layer <Function Name="GetSubCategories" Aggregate="false" BuiltIn="false" NiladicFunction="false" IsComposable="false" ParameterTypeSemantics="AllowImplicitConversion" Schema="6"> <Parameter Name="categoryid" Type="int" Mode="In" /> </Function> Based on the signature of the stored procedure represented in the FunctionImport tag, Entity Framework will generate a method in the object context to make the stored procedure available to the application.
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6-6. Mapping Null Conditions in Derived Entities
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You have a column in a table that allows null. You want to create a model using Table per Hierarchy inheritance with one derived type representing instances in which the column has a value and another derived type representing instances in which the column is null.
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Solution
Let s say you have a table describing experimental medical drugs. The table contains a column indicating when the drug was accepted for production. Until the drug is accepted for production, it is considered experimental. Once accepted, it is considered a Medicine. We ll start with the Drug table in the database diagram in Figure 6-16.
Figure 6-16. Drug table with the nullable discriminator column, AcceptedDate To create a model using the Drug table, do the following: 1. 2. Add a new ADO.NET Entity Data Model to your project and import the Drug table. Create the Experimental derived entity by right-clicking the design surface and selecting Add Entity. Name the entity Experimental. Select Drug as the base type. Repeat this step to create the Medicine entity. Move the PrincipalResearcher property from the Drug entity to the Experimental entity. Move the TargetPrice and AcceptedDate properties from the Drug entity to the Medicine entity. You can use Cut/Paste to move properties between entities. Mark the Drug entity as abstract. Right-click the Drug entity and select Properties. Set the Abstract property to true. Select the Medicine entity. In the Mapping Details window, map the entity to the Drug table by selecting Add a Table or View and choosing the Drug table. Select Add a Condition and add the AcceptedDate is Not Null condition. Repeat step 5 for the Experimental entity. This time, set the condition to AcceptedDate Is Null.
4. 5.
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