barcodelib.barcode.asp.net.dll free download Retrieving a Person and Using Its Role Property in VB.NET

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Listing 2-10. Retrieving a Person and Using Its Role Property
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Dim people = New PeopleDataContext() people.Log = Console.Out
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Dim query = From p In people.People _ Where p.ID = 1 _ Select p For Each Dim row In query Console.WriteLine( _ "Full Name: {0} {1} Role: {2}", _ row.FirstName, _ row.LastName, _ row.Role.RoleDescription) Next
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We don t need to the join the two tables to access the role. LINQ to SQL generates two SQL queries to retrieve both the Person and related Role data. See the two SQL statements in Figure 2-7.
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Figure 2-7. The output of Listing 2-10
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Note in Figure 2-7 that the @p0 parameter is used in both the queries. They are two different queries, so the @p0 parameter has different values. The @p0 parameter used in the first query is the value specified in the LINQ query. The @p0 parameter used in the second query is the value specified with the ThisKey property of the Association attribute used in the Role property decoration. In Listing 2-11 we ll retrieve a role and then use its People property to add a new person.
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Listing 2-11. Adding a New Person to the Database Starting from a Role
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Public Class AddExample Public Shared Function whereCondition(ByVal p As Role) As Boolean Return p.ID = 1 End Function Sub Listing2_11() Dim people = New PeopleDataContext() Dim predicateDelegate As New Func(Of Role, Boolean)(AddressOf whereCondition)
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people.Log = Console.Out Dim role = people.Roles.Single(predicateDelegate) Dim person As New Person() person.FirstName = "From" person.LastName = "Relationship" role.People.Add(person) people.SubmitChanges() End Sub End Class
Since there s a relationship between the two entity classes, we don t have to specify the
IDRole for the Person object. It will be assigned by the Attach_Person delegate function when a new Person object is added to the people collection of the Role entity class. Figure 2-8 shows the INSERT statement generated by LINQ to SQL. Note that a local transaction is automatically created for the INSERT.
Figure 2-8. The INSERT statement generated by LINQ to SQL
Deleting a row and every row related to it is really simple when a relationship is defined between two entity classes. Listing 2-12 deletes a role and all its related Person records.
Listing 2-12. Deleting a Role and All of Its Related Person Records
Public Class DeleteExample Public Shared newID As Integer Public Shared Function whereCondition(ByVal p As Role) As Boolean Return p.ID = newID End Function
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Sub Listing2_12() Dim people = New PeopleDataContext() Dim predicateDelegate As New Func(Of Role, Boolean)(AddressOf whereCondition) people.Log = Console.Out Dim role As New Role() role.RoleDescription = "Administrator" Dim person As New Person() person.FirstName = "From" person.LastName = "Code" role.People.Add(person) people.Roles.Add(role) people.SubmitChanges() newID = role.ID Dim admin = people.Roles.Single(predicateDelegate) people.Roles.Remove(admin) people.SubmitChanges() End Sub End Class
It creates a new role as follows:
Dim role As New Role() role.RoleDescription = "Administrator"
It then adds a new person to it:
Dim person As New Person() person.FirstName = "From" person.LastName = "Code" role.People.Add(person) people.Roles.Add(role)
The code uses the new role s identifier to retrieve the new row added to the database:
Dim admin = people.Roles.Single(predicateDelegate) people.Roles.Remove(admin) people.SubmitChanges()
This behavior is not obtained by the Association attribute and its properties. No reflection technique is used to infer the rows related to the Role entity object. Moreover, there is no check on foreign key constraint rules, such as deleting each related row in a cascade. The secret is in the EntitySet(Of T) constructor. We have provided two delegate functions
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responding to the Add and the Remove events of the Person entity class. In the body of the Detach_Person delegate function we have set the Role value to Nothing, raising a call to the Set accessor of the Role property. It s in the body of Detach_Person that you will find the Remove() method of the Person row related to the role. The Remove() method applied by the parent class will call the Detach delegate function for each child row related to it. This process will be performed once for each Person object related to the Role object.
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