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Querying Across Relationships (LINQ to SQL)

References to other objects or collections of other objects in your class definitions directly correspond to foreign-key relationships in the database. You can use these relationships when you query by using dot notation to access the relationship properties and navigate from one object to another. These access operations translate to more complex joins or correlated subqueries in the equivalent SQL.

For example, the following query navigates from orders to customers as a way to restrict the results to only those orders for customers located in London.

    Dim db As New Northwnd("c:\northwnd.mdf")
    Dim londonOrderQuery = _
From ord In db.Orders _
Where ord.Customer.City = "London" _
Select ord

If relationship properties did not exist you would have to write them manually as joins, just as you would do in a SQL query, as in the following code:

    Dim db As New Northwnd("c:\northwnd.mdf")
    Dim londOrderQuery = _
From cust In db.Customers _
Join ord In db.Orders On cust.CustomerID Equals ord.CustomerID _
Select ord

You can use the relationship property to define this particular relationship one time. You can then use the more convenient dot syntax. But relationship properties exist more importantly because domain-specific object models are typically defined as hierarchies or graphs. The objects that you program against have references to other objects. It is only a happy coincidence that object-to-object relationships correspond to foreign-key-styled relationships in databases. Property access then provides a convenient way to write joins.

With regard to this, relationship properties are more important on the results side of a query than as part of the query itself. After the query has retrieved data about a particular customer, the class definition indicates that customers have orders. In other words, you expect the Orders property of a particular customer to be a collection that is populated with all the orders from that customer. That is in fact the contract you declared by defining the classes in this manner. You expect to see the orders there even if the query did not request orders. You expect your object model to maintain an illusion that it is an in-memory extension of the database with related objects immediately available.

Now that you have relationships, you can write queries by referring to the relationship properties defined in your classes. These relationship references correspond to foreign-key relationships in the database. Operations that use these relationships translate to more complex joins in the equivalent SQL. As long as you have defined a relationship (using the AssociationAttribute attribute), you do not have to code an explicit join in LINQ to SQL.

To help maintain this illusion, LINQ to SQL implements a technique called deferred loading. For more information, see Deferred versus Immediate Loading (LINQ to SQL).

Consider the following SQL query to project a list of CustomerID-OrderID pairs:

SELECT t0.CustomerID, t1.OrderID
FROM   Customers AS t0 INNER JOIN
          Orders AS t1 ON t0.CustomerID = t1.CustomerID
WHERE  (t0.City = @p0)

The following illustration shows the table relationship graphically.

Querying across relationships

To obtain the same results by using LINQ to SQL, you use the Orders property reference already existing in the Customer class. The Orders reference provides the necessary information to execute the query and project the CustomerID-OrderID pairs, as in the following code:

    Dim db As New Northwnd("c:\northwnd.mdf")
    Dim idQuery = _
From cust In db.Customers, ord In cust.Orders _
Where cust.City = "London" _
Select cust.CustomerID, ord.OrderID

You can also do the reverse. That is, you can query Orders and use its Customer relationship reference to access information about the associated Customer object. The following code projects the same CustomerID-OrderID pairs as before, but this time by querying Orders instead of Customers.

    Dim db As New Northwnd("c:\northwnd.mdf")
    Dim idQuery = _
From ord In db.Orders _
Where ord.Customer.City = "London" _
Select ord.CustomerID, ord.OrderID

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