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QUERYING AN ENTITY DATA MODEL
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cmd.CommandText = "select value c from EFRecipesEntities.Customers as c"; using (var reader = cmd.ExecuteReader(CommandBehavior.SequentialAccess)) { while (reader.Read()) { Console.WriteLine("{0}'s email is: {1}", reader.GetString(1), reader.GetString(2)); } } } The following is the output from the code in Listing 3-3: Customers... Robert Stevens's email is: rstevens@mymail.com Julia Kerns's email is: julia.kerns@abc.com Nancy Whitrock's email is: nrock@myworld.com
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Customers... Robert Stevens's email is: rstevens@mymail.com Julia Kerns's email is: julia.kerns@abc.com Nancy Whitrock's email is: nrock@myworld.com
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In Listing 3-4, we create three customers, add them to the object context, then call SaveChanges() to save these new customers to the database. After we have these customers in the database, we use two different approaches to retrieve them using Entity SQL. In the first approach, we use the CreateQuery() method on the object context to create an ObjectQuery. When we iterate over the customers, the query is executed in the database and the resulting collection is printed to the console. Because each element in the collection is an instance of our Customer entity type, we can use the properties of the Customer entity type. In the second approach, we use EntityClient in a pattern that is very similar to how we would use SqlClient or any of the other client providers in ADO.NET. We start by creating a connection to the database. With the connection in hand, we create a command object and open the connection. Next we initialize the command object with the text of the Entity SQL statement we want to execute. We execute the command using ExecuteReader() and obtain an EntityDataReader, which is a type of the familiar DbDataReader. We iterate over the resulting collection using the Read() method.
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QUERYING AN ENTITY DATA MODEL
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The Entity SQL statement in Listing 3-3 uses the value keyword. This keyword is useful when we need the entire entity. If our Entity SQL statement forms a projection of the columns (that is, we use some of the columns and/or create columns using Entity SQL expressions), we can dispense with the value keyword. When using Object Services, this means working with a DbDataRecord directly. The code in Listing 3-4 demonstrates this. Listing 3-4. Projecting with both Object Services and EntityClient // using object services without the VALUE keyword using (var context = new EFRecipesEntities()) { Console.WriteLine("Customers..."); string esql = "select c.Name, c.Email from Customers as c"; var records = context.CreateQuery<DbDataRecord>(esql); foreach (var record in records) { var name = record[0] as string; var email = record[1] as string; Console.WriteLine("{0}'s email is: {1}", name, email); } } Console.WriteLine(); // using EntityClient without the VALUE keyword using (var conn = new EntityConnection("name=EFRecipesEntities")) { Console.WriteLine("Customers..."); var cmd = conn.CreateCommand(); conn.Open(); cmd.CommandText = @"select c.Name, C.Email from EFRecipesEntities.Customers as c"; using (var reader = cmd.ExecuteReader(CommandBehavior.SequentialAccess)) { while (reader.Read()) { Console.WriteLine("{0}'s email is: {1}", reader.GetString(0), reader.GetString(1)); } } } When you form a projection in Entity SQL, the results are returned in a DbDataRecord object that contains one element for each column in the projection. With the value keyword, the single object resulting from the query is returned in the first element of the DbDataRecord.
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3-4. Specifying Fully Qualified Names in Entity SQL
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You want to fully qualify an entity type with the correct namespace inside of an Entity SQL statement.
Solution
Let s say you have a simple model using Table per Type inheritance, as shown in Figure 3-5.
Figure 3-5. A model using Table per Type inheritance with the derived entities Teacher and Lawyer In Figure 3-5, we have two entities, Teacher and Lawyer, which are derived from the Person entity type. Because we re using Table per Type inheritance, each of the derived types is represented in a separate table. To query the model for all the Teachers using Object Services, we need to qualify the Teacher entity type with the Recipe4 namespace. This is the CLR namespace that contains our object context and our entities. Because we are interested in only the Teacher entities, we use the Entity SQL OfType() operator passing in the People entity set that contains our Teacher entity and the fully qualified Teacher entity. This is the first query in Listing 3-5. The same query using EntityClient requires that we qualify the entity set with the Entity Container name EFRecipesEntities and the Teacher entity type with the namespace of the conceptual model, EFRecipesModel. This is shown in the second query in Listing 3-5. Listing 3-5. Retrieving the teachers using Object Services and EntityClient using (var context = new EFRecipesEntities()) { context.People.AddObject(new Teacher { Name = "Janet Dietz",
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