Linq to SQL Xml Based
Mapping

Published at Sep 27 2007, 12:25
PM by Guy Burstein | 11 comment(s)

Linq to SQL Xml Based Mapping In the last post I talked about Linq
to SQL Attribute Based Mapping that lets you map CLR Objects to database
objects using attributes. Although this approach is very simple and easy, it is
compiled with the code and cannot be changed without recompiling. Another
approach is Linq to SQL Xml Based Mapping that maps entities to
database objects according to an xml file that is loaded in runtime.

So, given the two entities from the previous post, that have no mapping
attributes at all:

class Order{
    public int Id { get; set; }
    public DateTime? OrderDate { get; set; }
    public string CustomerId { get; set; }
    private EntityRef<Customer> _customerRef;
    public Customer Customer
    {
        get { return this._customerRef.Entity; }
        set { this._customerRef.Entity = value; }
   }
}

and:

public class Customer{
    public string CustomerId { get; set; }
}
The Xml Based Mapping schema should look like:

<Database Name="Northwind"           xmlns="http://schemas.microsoft.com/linqtosql/mapping/2007">

    <Type Name="XmlBasedMapping.Customer"> 
              CanBeNull="false"               IsPrimaryKey="true" />

  </Table>  <Table Name="dbo.Orders" Member="Orders">

      <Column Name="OrderID"               Member="Id" IsPrimaryKey="true" IsDbGenerated="true" />

     <Column Name="CustomerID" 

             Member="CustomerId" DbType="NChar(5)" />      <Column Name="OrderDate" 

              Member="OrderDate" DbType="DateTime" />

      <Association Name="Orders_Customers" 

                  Member="Customers" 

                  ThisKey="CustomerId" 

                  OtherKey="CustomerId" 

                  IsForeignKey="true" />

    </Type>

  </Table>

</Database>

The root element is the Database element. The child elements are the database
objects the are included in the mapping - Customers and Orders tables from the
Northwind database. Each table can have child types that are mapped to entities
in the application. This hierarchy sits also with the concept of inheritance in
Linq to SQL since it only supports the Table per Class
Hierarchy strategy. In the above example each table is mapped to a
single entity. Notice that each table column is mapped to a member in the
class.

To work with this mapping source, we should load it from a file / stream /
url or any other resource, and supply it as a parameter for the
DataContext instance we want to work with.

string connectionString 
= "...";

 

// Load the Mapping from a file XmlMappingSource mapping = XmlMappingSource.FromUrl("NorthwindMap.xml");
// Create a DataContext to the database, and supply 
DataContext ctx = new DataContext(connectionString, mapping); 
var query = from order in ctx.GetTable<Order>()  where 
order.CustomerId == "ALFKI"
            select order;
foreach (Order order in query)
{    Console.WriteLine(order.Id + " " + order.OrderDate + " " + order.CustomerId);
}

Enjoy!