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} public class MyFunctions { [EdmFunction("EFRecipesModel", "GetInvoices")] public static IQueryable<Invoice> GetInvoices(IQueryable<Invoice> invoices) { return invoices.Provider.CreateQuery<Invoice>( Expression.Call((MethodInfo)MethodInfo.GetCurrentMethod(), Expression.Constant(invoices, typeof(IQueryable<Invoice>)))); } }
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From the definition of our GetInvoices() function in Listing 11-3, we see that it takes a collection of Invoices and returns a collection of Invoices. On the CLR side, this translates to taking an IQueryable<Invoice> and returning an IQueryable<Invoice>. In the eSQLexpression we use the GetInvoices() function in the from clause. We pass in the unfiltered collection of invoices and our GetInvoices() function returns the filtered collection. We further filter the collection by date and the customer s city using a where clause. Then we use CreateQuery<Invoice>() to build the ObjectQuery<Invoice>. In building the query, we pass in the parameter to filter by city and use the Include() method to include the related customers. See the recipes in 5 for other examples of using the Include() method. Once we have the ObjectQuery<Invoice>, we iterate over the resulting collection and print out the invoices that matched the two filters we applied. For the LINQ query, the story is a little more interesting. Here we build the expression using the GetInvoices() method in the from clause and filter the resulting collection by date and city much like we did with the eSQLexpression. But to use our function in a LINQ query, we need to implement a CLR method that takes an IQueryable<Invoice> and returns an IQueryable<Invoice>. Unlike the stub method in Recipe 11-1, in which the model defined function returned a scalar value, here we have to provide an implementation in the body of the method. Creating this method is often referred to as bootstrapping. Here are some rules for bootstrapping: Bootstrapping is required when a model defined function returns an IQueryable<T>. When a function returns an IQueryable<T>, but does not take an IQueryable<T>, the bootstrapping method must be implemented in a partial class of the ObjectContext.
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The second rule comes about because we can t return an IQueryable<T> that has meaning in our ObjectContext without starting with an IQueryable<T>. If we pass in an IQueryable<T>, then we can perform some operation in our bootstrapping method that returns a related IQueryable<T>. However, we can t manufacture an IQueryable<T> outside of a partial class of our ObjectContext. In our example, we received an IQueryable<T> as a parameter, so we are free to implement the bootstrapping code outside of a partial class of our ObjectContext. In the implementation of our bootstrapping method, we get an instance of IQueryProvider from the IQueryable<Invoice> through the Provider property. IQueryProvider.CreateQuery<Invoice>() allows us
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to tack onto the expression tree for the IQueryable<T>. Here we add in the call to the GetInvoices() function, passing in the collection of invoices we have.
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11-3. Returning a Computed Column from a Model Defined Function
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You want to return a computed column from a model defined function.
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Solution
Suppose we have an Employee entity containing the properties FirstName, LastName, and BirthDate, as shown in Figure 11-3.
Figure 11-3. An Employee entity with a few typical properties We want to create a model defined function that returns the full name of the employee by combining the FirstName and LastName columns. We want to create another model defined function that returns the age of the employee based on the value in the BirthDate column. To create and use these functions, do the following: 1. 2. 3. Right-click the .edmx file in the Solution Explorer and click Open With Editor. This will open the .edmx file in the XML Editor. XML
Insert the code in Listing 11-5 just below the <Schema> tag in the conceptual models section of the .edmx file. This defines the functions in the model. Insert into and query the model using code similar to pattern shown in Listing 11-6.
Listing 11-5. Code for model defined functions <Function Name="FullName" ReturnType="Edm.String"> <Parameter Name="emp" Type="EFRecipesModel.Employee" /> <DefiningExpression> Trim(emp.FirstName) + " " + Trim(emp.LastName) </DefiningExpression>
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