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5-3. Loading Navigation Properties on Derived Types
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You have a model with one or more derived types that are in a has-a relationship with one or more other entities. You want to eagerly load all the related entities in one round trip to the database.
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LOADING ENTITIES AND NAVIGATION PROPERTIES
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Suppose you have a model like the one in Figure 5-4.
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Figure 5-4. A model for Plumbers with their JobSite and other related entities In this model, the Plumber entity extends the Tradesman entity. A Plumber has a JobSite that is represented by a one-to-many association. The JobSite type extends the Location entity. Location has a Phone, which is represented by a one-to-many association. Finally, a JobSite can have zero or more Foremen. This is also represented by a one-to-many association. Suppose you want to retrieve a plumber, the jobsite she works on, the jobsite s phone number, and all the foremen at the jobsite. You want to retrieve all this in one round trip to the database. The code in Listing 5-4 illustrates one way to use the Include() method to eagerly load the related entities in one query. Listing 5-4. Retrieving related entities in one round trip to the database using eager loading with the Include() method using (var context = new EFRecipesEntities()) { var foreman1 = new Foreman { Name = "Carl Ramsey" }; var foreman2 = new Foreman { Name = "Nancy Ortega" }; var phone = new Phone { Number = "817 867-5309" }; var jobsite = new JobSite { JobSiteName = "City Arena", Address = "123 Main", City = "Anytown",
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LOADING ENTITIES AND NAVIGATION PROPERTIES
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State = "TX", ZIPCode = "76082", Phone = phone }; jobsite.Foremen.Add(foreman1); jobsite.Foremen.Add(foreman2); var plumber = new Plumber { Name = "Jill Nichols", Email = "JNichols@plumbers.com", JobSite = jobsite }; context.Tradesmen.AddObject(plumber); context.SaveChanges(); } using (var context = new EFRecipesEntities()) { var plumber = context.Tradesmen.OfType<Plumber>() .Include("JobSite.Phone") .Include("JobSite.Foremen").First(); Console.WriteLine("Plumber's Name: {0} ({1})", plumber.Name, plumber.Email); Console.WriteLine("Job Site: {0}", plumber.JobSite.JobSiteName); Console.WriteLine("Job Site Phone: {0}", plumber.JobSite.Phone.Number); Console.WriteLine("Job Site Foremen:"); foreach (var boss in plumber.JobSite.Foremen) { Console.WriteLine("\t{0}", boss.Name); } } The following output is produced by code in Listing 5-4: Plumber's Name: Jill Nichols (JNichols@plumbers.com) Job Site: City Arena Job Site Phone: 817 867-5309 Job Site Foremen: Carl Ramsey Nancy Ortega
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Our query starts by selecting instances of the derived type Plumber. To get these, we use the OfType<Plumber>() method. The OfType<>() method select instances of the given subtype from the entity set. From Plumber, we want to load the related JobSite and the Phone for the JobSite. Notice that the JobSite entity does not have a Phone navigation property, but JobSite derives from Location, which does
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have a Phone navigation property. Because Phone is a property of the base entity, it s also available on the derived entity. That s the beauty of inheritance. This makes the query path simply: JobSite.Phone . We used the Include() method again with a query path that references the Foreman entities from the JobSite entity. Here we have a one-to-many association JobSite and Foreman. Notice the navigation property was pluralized by the wizard (from Foreman to Foremen). Finally, we use the First() method to select just the first Plumber instance. The resulting query is somewhat complex, involving several joins and subselects. The alternative, using the Load() method for each related entity, would require several round trips to the database and would result in a performance hit, especially if we retrieved many Plumbers.
5-4. Using Include() with Other LINQ Query Operators
Problem
You have a LINQ query that uses operators such as group by, join, and where; and you want to use the Include() method to eagerly load additional entities.
Solution
Let s say you have a model like the one shown in Figure 5-5.
Figure 5-5. A simple model with a one-to-many association between Club and Event To use the Include() method in combination with a group by clause, form the LINQ expression without the Include() method first; then cast the expression as an ObjectQuery<T> and invoke the Include() method. The code in Listing 5-5 demonstrates this approach. Listing 5-5. Casting to ObjectQuery<T> and Invoking Include() using (var context = new EFRecipesEntities()) { var club = new Club { Name = "Star City Chess Club", City = "New York" }; context.Clubs.AddObject(club); new Event { EventName = "Mid Cities Tournament", EventDate = DateTime.Parse("1/09/2010"), Club = club }; new Event { EventName = "State Finals Tournament",
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