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672 ChAPTER 11
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Connecting to and Querying Data with LINQ
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FigURE 11-5 Using the Entity Data Model Wizard to create an Entity Framework model of an existing
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FigURE 11-6 The Entity Data Model as laid out in Visual Studio.
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Lesson 2: LINQ and ADO.NET
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ChAPTER 11
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You can use the Visual Studio tools to edit this model and the related mapping. For example, by default, the object model is generated as a 1:1 mapping with the database. The object model includes the name of classes and properties. Figure 11-7 shows the Author class selected with the mapping editor open. Notice that the class has been renamed along with all the properties of the class. However, the map is still defined to connect this object and these properties to their counterparts in the database.
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FigURE 11-7 Using the entity model editor to change the structure of an application model but maintain the mapping back to the database.
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These tools allow you to do several additional things, such as generate a database from the model, validate the model mapping against a data source, map to stored procedures, and update the database portion of the model from an actual database. Notice the Model Browser in Figure 11-7 (on the right side). You can clearly see here that the model (top) is independent of the database information (bottom).
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674 ChAPTER 11
Connecting to and Querying Data with LINQ
Writing LINQ to Entities Queries
You can use what you ve learned thus far to write LINQ to Entities queries. You simply create a new instance of your entity model, and then you can write queries against the data it represents. An entity model inherits from System.Data.Objects.ObjectContext. This context object provides access to your application model and provides functionality to map that model to the data source. The following code shows a basic example of creating a new instance of an entity model and then writing a query against the Authors collection. Note that the ObjectContext is wrapped in a using statement. This is not required but is good practice, to make sure that the context is properly disposed the data retrieval is finished.
Sample of Visual Basic Code Using pubs As New PubsModel.pubsEntities() Dim authQuery = From a In pubs.Authors Where a.State = "CA" Order By a.LastName Select a GridView1.DataSource = authQuery GridView1.DataBind() End Using Sample of C# Code using (PubsModel.pubsEntities pubs = new PubsModel.pubsEntities()) { var authQuery = from a in pubs.Authors where a.state == "CA" orderby a.LastName select a; GridView1.DataSource = authQuery; GridView1.DataBind(); }
Of course, you can write more complex queries with LINQ to Entities (as you can with all LINQ data sources). You can also take advantage of the entity model relationships between objects. You will see an example of this in the upcoming practice.
Lesson 2: LINQ and ADO.NET
ChAPTER 11
Quick Check
1 . When writing a LINQ query against a standard DataSet, how do you access the
DataTable and the column or columns you want to query against
2. What are three ways to create an O/R map by using LINQ to SQL 3. From what class does a LINQ to Entities model inherit
Quick Check Answers
1 . Because a standard DataSet is not strongly typed, you must use the DataSet.Tables
and DataTable.Field properties to access the table and its columns in a LINQ query. You pass the names (as string values) to these methods.
2. You can use the LINQ to SQL Classes template to create a DBML file and
drag tables onto the design surface. You can also use the command-line tool, SqlMetal. Finally, you can code your entities manually as class files that use the System.Data.Linq.Mapping attributes to identify tables and columns.
3. An entity model inherits from the System.Data.Objects.ObjectContext class. This
class provides the mapping from an application model to a data source.
Pr ActIcE
Working with LinQ to Entities
In this practice, you create a website and define an entity model. You then use LINQ to Entities to work with this data on a webpage.
oN the CoMpaNIoN MedIa
If you encounter a problem completing an exercise, you can find the completed projects in the samples installed from this book s companion CD. For more information about the project files and other content on the CD, see Using the Companion Media in this book s Introduction.
E xErcIsE 1 Creating the ASP.NET Website and Entity Model
In this exercise, you create the website, define the XML file, and write LINQ queries to work with the XML.
1. 2. 3.
Open Visual Studio and create a new ASP.NET website called LinqEntitymodelExample by using either C# or Visual Basic. Add the pubs.mdf file to the App_Data directory of your solution. You can get this file from the CD for this book. Add a new ADO.NET Entity Data Model to your solution. You can do so by right-clicking the website and choosing Add New Item. Name this model PubsEntitymodel.edmx.
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