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LOADING ENTITIES AND NAVIGATION PROPERTIES
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The bottom line is that Include(), used carefully, can improve performance over piecemeal loading of the entities. Keep in mind the extra memory footprint and the extra work done at the database layer and in Entity Framework. Listing 5-2. The SQL query resulting from our use of the Include() method SELECT [Project1].[CustomerId] AS [CustomerId], [Project1].[Name] AS [Name], [Project1].[CustomerTypeId] AS [CustomerTypeId], [Project1].[CustomerTypeId1] AS [CustomerTypeId1], [Project1].[Description] AS [Description], [Project1].[C1] AS [C1], [Project1].[CustomerEmailId] AS [CustomerEmailId], [Project1].[CustomerId1] AS [CustomerId1], [Project1].[Email] AS [Email] FROM ( SELECT [Extent1].[CustomerId] AS [CustomerId], [Extent1].[Name] AS [Name], [Extent1].[CustomerTypeId] AS [CustomerTypeId], [Extent2].[CustomerTypeId] AS [CustomerTypeId1], [Extent2].[Description] AS [Description], [Extent3].[CustomerEmailId] AS [CustomerEmailId], [Extent3].[CustomerId] AS [CustomerId1], [Extent3].[Email] AS [Email], CASE WHEN ([Extent3].[CustomerEmailId] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1] FROM [5].[Customer] AS [Extent1] LEFT OUTER JOIN [5].[CustomerType] AS [Extent2] ON [Extent1].[CustomerTypeId] = [Extent2].[CustomerTypeId] LEFT OUTER JOIN [5].[CustomerEmail] AS [Extent3] ON [Extent1].[CustomerId] = [Extent3].[CustomerId] ) AS [Project1] ORDER BY [Project1].[CustomerId] ASC, [Project1].[CustomerTypeId1] ASC, [Project1].[C1] ASC
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Figure 5-2. Redundant data resulting from the Include() method
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LOADING ENTITIES AND NAVIGATION PROPERTIES
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5-2. Loading a Complete Object Graph
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You have a model with several related entities and you want to load the complete object graph of all the instances of each entity in a single query.
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Suppose you have a conceptual model like the one in Figure 5-3. Each course has several sections. Each section is taught by an instructor and has several students.
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Figure 5-3. A model with a few related entities To retrieve all the courses, sections, instructors, and students represented in the database in a single query, use the Include() method with a query path parameter, as shown in Listing 5-3. Listing 5-3. Retrieving an entire object graph in a single query using (var context = new EFRecipesEntities()) { var course = new Course { Title = "Biology 101" }; var fred = new Instructor { Name = "Fred Jones" }; var julia = new Instructor { Name = "Julia Canfield" }; var section1 = new Section { Course = course, Instructor = fred };
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LOADING ENTITIES AND NAVIGATION PROPERTIES
var section2 = new Section { Course = course, Instructor = julia }; var jim = new Student { Name = "Jim Roberts" }; jim.Sections.Add(section1); var jerry = new Student { Name = "Jerry Jones" }; jerry.Sections.Add(section2); var susan = new Student { Name = "Susan O'Reilly" }; susan.Sections.Add(section1); var cathy = new Student { Name = "Cathy Ryan" }; cathy.Sections.Add(section2); context.Courses.AddObject(course); context.SaveChanges(); } using (var context = new EFRecipesEntities()) { var graph = context.Courses .Include("Sections.Instructor") .Include("Sections.Students"); Console.WriteLine("Courses"); Console.WriteLine("======="); foreach (var course in graph) { Console.WriteLine("{0}", course.Title); foreach (var section in course.Sections) { Console.WriteLine("\tSection: {0}, Instrutor: {1}", section.SectionId.ToString(), section.Instructor.Name); Console.WriteLine("\tStudents:"); foreach (var student in section.Students) { Console.WriteLine("\t\t{0}", student.Name); } Console.WriteLine("\n"); } } } The code in Listing 5-3 produces the following output: Courses ======= Biology 101 Section: 7, Instructor: Fred Jones Students:
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Susan O'Reilly Jim Roberts
Section: 8, Instructor: Julia Canfield Students: Cathy Ryan Jerry Jones
How It Works
A query path is a string parameter to the Include() method. A query path represents the entire path of the object graph that is loaded by the Include() method. The Include() method extends the query to include the entities referenced along the query path. In Listing 5-3, we use the Include() method twice. Include() is invoked first with a query path parameter that includes the part of the graph extending through Section to Instructor. This modifies the query to include all the Sections and their Instructors. The second invocation includes a path extending through Section to Student. This modifies the query to include Sections and their Students. The result is a materialization of the complete object graph including all the Course entities and the entities on each end of all associations in the model. You can use query paths that use navigation properties to any depth. This gives you a lot of flexibility in partial or complete object graph loading. Entity Framework attempts to optimize the final query generation by pruning off overlapping or duplicate query paths. The syntax and semantics of the Include() method is deceptively simple. Don t let the simplicity fool you into thinking that there is no performance price to be paid when using the Include() method. Eager loading with several Include() method invocations can rapidly increase the complexity of the query sent to the database and dramatically increase the amount of data returned from the database. The complex queries generated can lead to poor performance plan generation and the large amount of returned data can cause Entity Framework to spend an inordinate amount of time removing duplicate data.
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