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Having been the product of one of these two constructors, a Book object would now be used normally, assigning and retrieving values from the properties and executing methods as required When the object has ful lled its purpose, the UpdateDatabase() method should be called Its purpose is to send only new or changed information back to the database To help it decide whether the data is unchanged, altered or new (to the database), two member variables, mvarChanged and mvarBrandNew, are used to track any changes to the object mvarBrandNew is only set to true from the second constructor Adding a new Title to the database requires that a row is inserted into Titles, plus one row for each author into the [Title Author] table SQL queries are built to do the inserts (using the StringFormat() method, which is ideal for inserting variable values into an otherwise literal string) The SQL strings used for these inserts are:
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Insert Into Titles(Title, [Year Published], ISBN, PubID) Values('<>', <>, '<>', <>);
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Insert Into [Title Author](ISBN, Au_ID) Values('<>', <>);
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The StringFormat() method simply makes it easier to handle the lists of variable names, inserting quotes ( ) around the string elds Using a Book object from a form is a simple matter, as the code to retrieve a Book object from the database using a ISBN entered into a text box (Listing 1220) shows:
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Private Sub btnGetBook_Click(ByVal sender As SystemObject, _ ByVal e As SystemEventArgs) _ Handles btnGetBookClick If txtISBNText <> "" Then Dim B As New Book(txtISBNText) txtTitleText = BTitle txtYearText = BYearPublished Dim A As Object For Each A In BAuthors lstAuthorsItemsAdd(A) Next End If End Sub Listing 1220: Retrieving a book s data from the database
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Inserting a new book is similarly trivial (see Listing 1221)
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Private Sub btnAddBook_Click(ByVal sender As SystemObject, _ ByVal e As SystemEventArgs) _ Handles btnAddBookClick Dim B As Book
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125 Data Object Modelling
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B = New Book(txtISBNText, txtTitleText, _ CInt(txtYearText), CInt(txtPubIDText)) Dim A As Object For Each A In lstAuthorsItems BAddAuthor(CType(A, Integer)) Next BUpdateDatabase() End Sub Listing 1221: Inserting a new Book object into the database
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Note that the various member elds for the new book have been entered into text boxes and converted as necessary Author ids have been taken from a list box We can do the same for the Authors table so that an author can be retrieved from the Au_ID number and information updated as necessary (Listing 1222)
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Public Class Author Private Private Private Private Private mvarName As String mvarAuID As Integer mvarYearBorn As Integer mvarChanged As Boolean = False mvarBrandNew As Boolean = False
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'Properties for Name, YearBorn, AuID(read-only) 'go here Public Sub New(ByVal AuID As Integer) mvarAuID = AuID Dim r As SqlClientSqlDataReader r = GetReader( _ "Select * From Authors Where Au_ID = "& _ AuID & ";") rRead() mvarName = rItem("Author")ToString() mvarYearBorn = Val(rItem("Year Born")ToString()) End Sub Public Sub New(ByVal Name As String, _ ByVal YearBorn As Integer) mvarName = Name mvarYearBorn = YearBorn 'To get a AuID value, we'll need to send this to the 'database now Dim Sql As String Sql = StringFormat("Insert Into Authors(Author, " & _ "[Year Born]) Values('{0}',{1});", _ mvarName, mvarYearBorn) 'We need a Select part to this query to retrieve the 'AuID value Sql &= "Select Au_ID From Authors " & _ Where (Au_ID = @@IDENTITY)" mvarAuID = ExecuteCommand(Sql) End Sub
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12 n Databases in Visual Basic NET
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Public Overrides Function ToString() As String Dim s As String s = mvarName 'Best to account for null birth year values If mvarYearBorn > 0 Then s &= ", " & mvarYearBornToString() End If Return s End Function 'Other methods here ' End Class Listing 1222: The Author persistent class
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As a result of these two classes and the associated database functions, we can easily work with persistent Book and Author objects that can be sent to and retrieved from the database at will Use the code in Listing 1223 to retrieve a Book and details of its authors
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Private Sub btnGetBook_Click(ByVal sender As SystemObject, _ ByVal e As SystemEventArgs) _ Handles btnGetBookClick If txtISBNText <> "" Then Dim B As New Book(txtISBNText) txtTitleText = BTitle txtYearText = BYearPublished Dim A As Object For Each A In BAuthors lstAuthorsItemsAdd(New Author(A)ToString()) Next End If End Sub Listing 1223: Using the Book and Author classes
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The main problem with the approach used in these listings is the inef ciency of forming and closing database connections on an object-by-object basis The code could be altered to maintain a single, shared connection for use by the class, but this could result in a database connection being held open over a long period of time whether or not it was necessary One approach to a strategic solution would be to implement the classes to work with a DataSet, since the normal working mode would then be disconnected However, using and de ning a DataSet would bring its own problems: how to determine how many records to retrieve and which ones, how to deal with multiple objects accessing the same DataRows, etc The techniques shown here do no more than provide an example of how you would create and use an object model with database persistence, and they need to be applied with care to make the most effective use of the database
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