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The key to making the query in a Table per Type inheritance model more efficient is to explicitly tell Entity Framework the type of the expected result. This allows Entity Framework to generate code that limits the search to the specific tables that hold the values for the base type and the derived type. Without this information, Entity Framework has to generate a query that pulls together all the results from all the tables holding derived type values and then determines the appropriate type for materialization. Depending on the number of derived types and the complexity of your model, this may require substantially more work than necessary. Of course, this assumes that you know exactly what derived type the query will return.
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13-2. Retrieving a Single Entity Using an Entity Key
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You want to retrieve a single entity using an entity key.
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Suppose you have a model with an entity type representing a painting. The model might look like the one in Figure 13-2.
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Figure 13-2. The Painting entity type in our model To retrieve a particular entity using an entity key can be more efficient than using a where clause. To retrieve an entity using an entity key, use the GetObjectByKey() method, as shown in Listing 13-2. Listing 13-2. Retrieving an entity using an entity key using (var context = new EFRecipesEntities()) { context.Paintings.AddObject(new Painting { AccessionNumber = "PN001", Name = "Sunflowers", Artist = "Rosemary Golden", LastSalePrice = 1250M }); context.Paintings.AddObject(new Painting { AccessionNumber = "PN002", Name = "Red River", Artist = "Alex Jones", LastSalePrice = 1800M }); context.SaveChanges(); } using (var context = new EFRecipesEntities()) { // let's assume we already know the key for the painting var p = context.GetObjectByKey( new System.Data.EntityKey( "EFRecipesEntities.Paintings", "AccessionNumber", "PN001")); Painting painting = (Painting) p;
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Console.WriteLine("The painting with accession number {0}", painting.AccessionNumber); Console.WriteLine("\tName: {0}", painting.Name); Console.WriteLine("\tArtist: {0}", painting.Artist); Console.WriteLine("\tSale Price: {0}", painting.LastSalePrice.ToString("C")); } The following is the output of the code in Listing 13-2: The painting with accession number PN001 Name: Sunflowers Artist: Rosemary Golden Sale Price: $1,250.00
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At first, using the GetObjectByKey() method seems like a lot more work than using a simple LINQ expression with a where clause. The call to GetObjectByKey() is more noisy, and the returned object must be cast to our entity type. The one advantage that GetObjectByKey() has is that it can retrieve the entity from the object context without making a round trip to the database. Using a LINQ expression with a where clause and a first() or single() method call will always cause a round trip to the database even if the entity has been previously retrieved and is in the object context. Bypassing the database and retrieving the entity directly from the object context is the primary performance benefit we might see from the GetObjectByKey() method. This assumes, of course, that some previous operation has already fetched the entity from the database and placed it into the object context. GetObjectByKey() will throw an exception if an entity with the given entity key cannot be found in either the object context or in the database. The TryGetObjectByKey() method does the same thing as the GetObjectByKey() method but will not throw an exception if the entity can t be found. There are a couple of important things to remember about the GetObjectByKey() method. First, it won t return entities from the object context if they are in the inserted state. However, it will return objects that are marked for deletion. If MergeOption.NoTracking is set, calls to GetObjectByKey() will make a round trip to the database because the MergeOption.NoTracking will keep objects from being inserted into the object context after they are materialized. If the objects are not in the object context, then GetObjectByKey() has no choice but to retrieve the objects from the database.
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