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LOADING ENTITIES AND NAVIGATION PROPERTIES
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We use the Include() method to eagerly load the project together with its related manager in the original query. After the query, we check whether the manager instance is loaded using the project.ManagerReference.IsLoaded property. Because this is an entity reference, the IsLoaded property is available on ManagerReference property rather than on Manager property, which is null. Next, we check whether the Contractor entity collection is loaded. It is not loaded because we didn t eagerly load it with the Include() method nor did we load it directly (yet) with the Load() method. Once we use the Load() method, IsLoaded is set to true. If lazy loading is enabled on the object context by setting DeferredLoadingEnabled to true, then IsLoaded is set to true when the entity or entity collection is referenced. The DeferredLoadingEnabled flag causes Entity Framework to automatically load the entity or entity collection when referenced. When you use the CreateSourceQuery() method to grab the query for loading the entity or entity collection, Entity Framework will not set IsLoaded when the query is executed. The exact meaning of IsLoaded can be a little more confusing than it seems it should be. IsLoaded is set by the results of a query, by calling the Load() method, or implicitly by the span of relationship keys. When you query for an entity, there is an implicit query for the key of the related entity. If the result of this implicit query is a null key value, then IsLoaded is set to true, indicating there is no related entity in the database. This is the same value for IsLoaded we would expect if we did an explicit load on the relationship and found no related entity.
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5-9. Loading Related Entities Explicitly
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You want to directly load related entities.
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Let s say you have a model like the one in Figure 5-10.
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Figure 5-10. A model for doctors, their patients, and appointments
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The model depicted in Figure 5-10 represents doctors, their patients, and appointments. To explicitly load related entities, follow the pattern in Listing 5-11. Listing 5-11. Using the Load() method using (var context { var doc1 = new var doc2 = new var pat1 = new var pat2 = new var pat3 = new var app1 = new = new EFRecipesEntities())
Doctor { Name = "Joan Meyers" }; Doctor { Name = "Steven Mills" }; Patient { Name = "Bill Rivers" }; Patient { Name = "Susan Stevenson" }; Patient { Name = "Roland Marcy" }; Appointment { Date = DateTime.Today, Doctor = doc1, Fee = 109.92M, Patient = pat1, Reason = "Checkup" }; var app2 = new Appointment { Date = DateTime.Today, Doctor = doc2, Fee = 129.87M, Patient = pat2, Reason = "Arm Pain" }; var app3 = new Appointment { Date = DateTime.Today, Doctor = doc1, Fee = 99.23M, Patient = pat3, Reason = "Back Pain" }; context.Doctors.AddObject(doc1); context.Doctors.AddObject(doc2); context.SaveChanges();
} using (var context = new EFRecipesEntities()) { var doc = context.Doctors.First(o => o.Name == "Joan Meyers"); if (!doc.Appointments.IsLoaded) { doc.Appointments.Load(); Console.WriteLine("Dr. {0}'s appointments were lazy loaded.", doc.Name); } Console.WriteLine("Dr. {0} has {1} appointment(s).", doc.Name, doc.Appointments.Count().ToString()); foreach (var app in context.Appointments) { if (!app.DoctorReference.IsLoaded) { app.DoctorReference.Load(); Console.WriteLine("Dr. {0} was lazy loaded.", app.Doctor.Name); } else Console.WriteLine("Dr. {0} was already loaded.", app.Doctor.Name); } Console.WriteLine("There are {0} appointments for Dr. {1}", doc.Appointments.Count().ToString(), doc.Name); doc.Appointments.Clear(); Console.WriteLine("Collection clear()'ed");
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Console.WriteLine("There are now {0} appointments for Dr. {1}", doc.Appointments.Count().ToString(), doc.Name); doc.Appointments.Load(); Console.WriteLine("Collection loaded()'ed"); Console.WriteLine("There are now {0} appointments for Dr. {1}", doc.Appointments.Count().ToString(), doc.Name); doc.Appointments.Load(MergeOption.OverwriteChanges); Console.WriteLine("Collection loaded()'ed with MergeOption.OverwriteChanges"); Console.WriteLine("There are now {0} appointments for Dr. {1}", doc.Appointments.Count().ToString(), doc.Name); } The output of the code in Listing 5-11 is the following: Dr. Joan Meyers's appointments were lazy loaded. Dr. Joan Meyers has 2 appointment(s). Dr. Steven Mills was lazy loaded. Dr. Joan Meyers was already loaded. Dr. Joan Meyers was already loaded. There are 2 appointments for Dr. Joan Meyers Collection clear()'ed There are now 0 appointments for Dr. Joan Meyers Collection loaded()'ed There are now 0 appointments for Dr. Joan Meyers Collection loaded()'ed with MergeOPtion.OverwriteChanges There are now 2 appointments for Dr. Joan Meyers
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