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You have an instance of an entity and you want to defer load two or more related entities in a single query.
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Suppose you have a model like the one in Figure 5-6.
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LOADING ENTITIES AND NAVIGATION PROPERTIES
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Figure 5-6. A a model with an employee, her department, and the department s company In the model shown in Figure 5-6, an Employee is associated with exactly one Department. Each Department is associated with exactly one Company. Given an instance of an Employee, you want to load both his department and the department s company. What makes this problem somewhat unique is that we already have an instance of Employee and we want to avoid going back to the database to get another copy of the Employee just so that we can use the Include() method to obtain the related instances of Company and Department. Perhaps in your real-world problem, Employee is a very expensive entity to retrieve and materialize. We could use the Load() method twice to load the related Department instance and then again to load the related Company instance. However, this would generate two round trips to the database. To load the related instances using just one query, we can either re-query the Employee entity set using the Include() method with a query path including the Department and the Company, or use the CreateSourceQuery() method on the DepartmentReference property. The code in Listing 5-6 shows both approaches. Listing 5-6. Inserting into the model and retrieving the related entities using two slightly different approaches using (var context = new EFRecipesEntities()) { var company = new Company { Name = "Acme Products" }; var acc = new Department { Name = "Accounting", Company = company }; var ship = new Department { Name = "Shipping", Company = company }; var emp1 = new Employee { Name = "Jill Carpenter", Department = acc }; var emp2 = new Employee { Name = "Steven Hill", Department = ship }; context.Employees.AddObject(emp1); context.Employees.AddObject(emp2); context.SaveChanges(); } // first approach using (var context = new EFRecipesEntities()) { // assume we already have an employee var jill = context.Employees.Where(o => o.Name == "Jill Carpenter").First(); // now get Jill's department and company var results = context.Employees.Include("Department.Company")
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.Where(o => o.EmployeeId == jill.EmployeeId).First<Employee>(); Console.WriteLine("{0} works in {1} for {2}", jill.Name, jill.Department.Name, jill.Department.Company.Name); } // more efficient, does not retrieve employee again using (var context = new EFRecipesEntities()) { // assume we already have an employee var jill = context.Employees.Where(o => o.Name == "Jill Carpenter").First(); var moreResults = jill.DepartmentReference.CreateSourceQuery() .Include("Company").First(); context.Attach(moreResults); Console.WriteLine("{0} works in {1} for {2}", jill.Name, jill.Department.Name, jill.Department.Company.Name); } The following is the output of the code in Listing 5-6: Jill Carpenter works in Accounting for Acme Products Jill Carpenter works in Accounting for Acme Products
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If we didn t already have an instance the Employee entity, we could simply use the Include() method with a query path Department.Company . This is essentially the approach we take in the first query. The disadvantage of this approach is that it retrieves all the columns for the Employee entity. In many cases, this might be an expensive operation. Because we already have this object in the context, it seems wasteful to gather these columns again from the database and transmit them across the wire. In the second query, we use the CreateSourceQuery() method available on the DepartmentReference property to retrieve the related instance of the Department entity as well as the instance of the Company entity. This second approach is more efficient because it does not retrieve the Employee columns. Our use of the Attach() method to attach the retrieved Department instance to the object context is not strictly required in this case because of relationship span.
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