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7-2. Reading a Model from a Database
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You want to read the CSDL, MSL, and SSDL definitions for your model from a database table.
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Suppose that you have a model like the one in Figure 7-1.
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Figure 7-1. A model with a Customer entity Our model has just one entity: Customer. The conceptual layer (CSDL), mapping layer (MSL), and storage layer (SSDL) definitions are typically found in the .edmx file in your project. We want to read these definitions from a database. To read these definitions from a database, do the following: 1. Right-click the design surface and view the Properties. Change the Code Generation Strategy to None. We ll use POCO for our Customer class. See 8 for more recipes on using POCO. Create the table shown in Figure 7-2. This table will hold the definitions for our project. Right-click the design surface and view the Properties. Change the Metadata Artifact Processing to Copy to Output Directory. Rebuild your project. The build process will create three files in the output directory: Recipe2.ssdl, Recipe2.csdl, and Recipe2.msl. Insert the contents of these files into the Definitions table in the corresponding columns. Use 1 for the Id column. Follow the pattern in Listing 7-2 to read the metadata from the Definitions table and create a MetadataWorkspace that our application will use.
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Figure 7-2. The Definitions table holds the definitions for our SSDL, CSDL, and MSL. Note that the column data types for the definitions are XML. Listing 7-2. Reading the metadata from the Definitions table using using using using using using using using using using System; System.Collections.Generic; System.Linq; System.Text; System.Data.Metadata.Edm; System.Data.SqlClient; System.Data.EntityClient; System.Xml; System.Data.Mapping; System.Data.Objects;
namespace Recipe2 { class Program { static void Main(string[] args) { RunExample(); } static void RunExample() { using (var context = ContextFactory.CreateContext()) { context.Customers.AddObject( new Customer { Name = "Jill Nickels" }); context.Customers.AddObject( new Customer { Name = "Robert Cole" }); context.SaveChanges(); } using (var context = ContextFactory.CreateContext()) { Console.WriteLine("Customers"); Console.WriteLine("---------"); foreach (var customer in context.Customers)
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{ Console.WriteLine("{0}", customer.Name); } } } } public class Customer { public virtual int CustomerId { get; set; } public virtual string Name { get; set; } } public class EFRecipesEntities : ObjectContext { private ObjectSet<Customer> customers; public EFRecipesEntities(EntityConnection cn) : base(cn) { } public ObjectSet<Customer> Customers { get { return customers (customers = CreateObjectSet<Customer>()); } } } public static class ContextFactory { static string connString = @"Data Source=localhost; Initial Catalog=EFRecipes;Integrated Security=True;"; private static MetadataWorkspace workspace = CreateWorkSpace(); public static EFRecipesEntities CreateContext() { var conn = new EntityConnection(workspace, new SqlConnection(connString)); return new EFRecipesEntities(conn); } private static MetadataWorkspace CreateWorkSpace() { string sql = @"select csdl,msl,ssdl from 7.Definitions"; XmlReader csdlReader = null; XmlReader mslReader = null; XmlReader ssdlReader = null; using (var cn = new SqlConnection(connString)) {
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using (var cmd = new SqlCommand(sql, cn)) { cn.Open(); var reader = cmd.ExecuteReader(); if (reader.Read()) { csdlReader = reader.GetSqlXml(0).CreateReader(); mslReader = reader.GetSqlXml(1).CreateReader(); ssdlReader = reader.GetSqlXml(2).CreateReader(); } } } var workspace = new MetadataWorkspace(); var edmCollection = new EdmItemCollection(new XmlReader[] { csdlReader }); var ssdlCollection = new StoreItemCollection(new XmlReader[] { ssdlReader }); var mappingCollection = new StorageMappingItemCollection( edmCollection, ssdlCollection, new XmlReader[] { mslReader }); workspace.RegisterItemCollection(edmCollection); workspace.RegisterItemCollection(ssdlCollection); workspace.RegisterItemCollection(mappingCollection); return workspace; } } } The following is the output of the code in Listing 7-2: Customers --------Jill Nickels Robert Cole
How It Works
The first part of the code in Listing 7-2 should be very familiar to you by now. We use Entity Framework to create a new context, create a few entities, and call SaveChanges() to persist the entities to the database. To retrieve the entities, we iterate through the collection and display each on the console. The only difference in this part is the call to ContextFactory.CreateContext(). Normally, we would just use the new operator to get a new instance of our EFRecipesEntities context. We ve created the ContextFactory to create our context from the model metadata stored not in the .edmx file, but in a table in a database. We do this in the CreateContext() method. The CreateContext()
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