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Why Shape Your Data?
The ability to display hierarchical or parent-child relationships is key to a wide variety of applications. You use these types of relationships whenever you need to list all orders associated with a product, all products associated with a category, all files contained in a folder, or customers located in all cities. Each of these examples can be represented in a parentchild relationship. Although we could set up a series of stored procedures or commands that query a set of data based upon a parent’s unique key in order to return the filtered set of children, this method can create a tremendous amount of overhead.
In ADO.NET, we can use the Relations() method of the DataSet object to identify and enforce parent-child relationships between disparate sets of data, even if those data sets do not have an enforced relationship in the source database. As long as there is a way to connect to distinct data stores through a common key field, then we can enforce a parent-child relationship.
DataSet Object
If you are an ADO programmer, you may look at a DataSet object as a disconnected RecordSet object that contains one or more tables of data. In effect a DataSet is similar to a hierarchical RecordSet that is created using the MSDataShape ADO Provider and then building a SELECT command using the shape syntax as we discussed previously. If you are an XML developer, you’re more likely to see the DataSet object as a specific Document Object Model (DOM), which is set to represent tables of data where each table contains a set of elements with a consistent strongly typed structure. Either of these views provides an accurate representation of the DataSet object.
The DataSet object in ADO.NET provides the ability to hold an unlimited number of data tables. We add data tables to the DataSet object by creating a DataAdapter and supplying a stored procedure or SQL syntax to fill the data table as shown in Listing 25-2:
Listing 25-2 Example of using the DataAdapter and DataSet commands
<%@ Import Namespace=”System.Data.OleDB”%> <%@ Import Namespace=”System.Data” %>
<%@ Page Language=”VB” Debug=”true” %> <HTML>
<SCRIPT LANGUAGE=”VB” RUNAT=”server”>
Sub Page_Load()
Dim myConnection as new OleDBConnection(“provider=sqloledb;Data Source=localhost;Initial Catalog=pubs;User ID=sa;pwd=”)
Dim myDataSet as New DataSet()
Dim StoresDSCommand as new OleDBDataAdapter(“select * from stores”, myConnection)
StoresDSCommand.Fill(myDataSet,”Stores”)
Dim SalesDSCommand as OleDBDataAdapter
SalesDSCommand = new OleDBDataAdapter(“select *, title from sales inner join titles on sales.title_id=titles.title_id”, myConnection)
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SalesDSCommand.Fill(myDataSet,”Sales”)
myDataSet.Relations.Add(“StoreSale”,myDataSet.Tables(“Stores”).Columns(“stor_id”),myDataS et.Tables(“Sales”).Columns(“stor_id”))
Dim Store as DataRow
Dim Sale as DataRow
Msg.Text=Msg.Text & “<H2>List of Titles Sold By Store</H2>” for each Store in myDataSet.Tables(“Stores”).Rows
Msg.Text=Msg.Text & “<BR><B><I>”
Msg.Text=Msg.Text & “Store Name: “ & Store(“stor_name”).ToString() Msg.Text=Msg.Text & “</B></I>”
Msg.Text=Msg.Text & “<p>Address: “ & Store(“stor_address”) & “</p>” Msg.Text=Msg.Text & “<blockquote>”
Msg.Text=Msg.Text & “<p><i>List of Titles sold by “ & Store(“stor_name”)
&”</i></p>”
for each Sale in Store.GetChildRows(myDataSet.Relations(“StoreSale”)) Msg.Text=Msg.Text & “<p>Title = “ & Sale(“title”).ToString() & “</p>” next
Msg.Text=Msg.Text &”</blockquote>”
next
end sub
</SCRIPT>
<BODY>
<ASP:LABEL ID=”Msg” RUNAT=”server” /> </BODY>
</HTML>
This code fills the myDataSet object with the contents of the stores table.
SalesDSCommand.Fill(myDataSet,”Sales”)
In order to describe relationships between these tables, so we can easily display or report on any hierarchical relationships that may exist, we need to use the Relations() method of the DataSet object. We will cover this in the next section.
Shaping Data with the Relations Method
The Relations method enables you to set the parent-child relationship between two tables by supplying the name of the relation, the table and column, that should be the parent relation and the table and column that is the child relation. Looking at the code in Listing 25-2 from the previous section, we set up a relationship between the Stores table and the Sales table in myDataSet using the following code segment:
myDataSet.Relations.Add(“StoreSale”,myDataSet.Tables(“Stores”).Columns(“stor_id”),myDataS et.Tables(“Sales”).Columns(“stor_id”))
This will establish a relationship where the Stores table represents the parent and is joined via the stor_id column to the Sales table. Using this relationship, you can quickly loop through each of the stores and provide a list of titles sold to each store. The following
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segment of Listing 25-2, illustrates how you can loop through each of the stores using a simple For...Next loop:
Dim Store as DataRow
Dim Sale as DataRow
Msg.Text=Msg.Text & “<H2>List of Titles Sold By Store</H2>”
for each Store in myDataSet.Tables(“Stores”).Rows Msg.Text=Msg.Text & “<BR><B><I>”
Msg.Text=Msg.Text & “Store Name: “ & Store(“stor_name”).ToString() Msg.Text=Msg.Text & “</B></I>”
Msg.Text=Msg.Text & “<p>Address: “ & Store(“stor_address”) & “</p>” Msg.Text=Msg.Text & “<blockquote>”
Msg.Text=Msg.Text & “<p><i>List of Titles sold by “ & Store(“stor_name”)
&”</i></p>”
In order to access each of the child records, we use the Relation method established earlier and a For...Next loop as illustrated in the following example:
for each Sale in Store.GetChildRows(myDataSet.Relations(“StoreSale”)) Msg.Text=Msg.Text & “<p>Title = “ & Sale(“title”).ToString() & “</p>”
next
Msg.Text=Msg.Text &”</blockquote>”
next end sub
The resulting output of Listing 25-2 can be seen in Figure 25-3.
Figure 25-3 Resulting output of using the relations method in ASP.NET
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REVIEW
The use of the Relations method enables you to quickly navigate hierarchical relationships between datasets. This common but powerful technique allows you as a developer to robustly handle complex relationships with ease.
QUIZ YOURSELF
1.How can parent-child relationships be established between data stores from different data sources? (See “Why Shape Your Data?”)
2.Describe the differences between the ADO Shape Command and the ADO.Net Relations method. (See “What Is Data Shaping?” and “Why Shape Your Data?”)
3.What limits are there in establishing tables and relationships for a specified DataSet object? (See “Shaping Data with the Relations Method.”)
S E S S I O N
26
Handling ADO.NET Errors
Session Checklist
Understanding the OLEDBException class and its interaction with the
OLEDBError object
Implementing custom OLEDBError handling in your application
Implementing a generic event handler that writes errors to the Event Log
ADO.NET provides a framework similar to ADO for handling errors that occur when calling ADO.NET components. When the ADO.NET adapter encounters an error, it throws an Exception which can then be evaluated for any contained errors.
There are two types of exceptions that can be thrown: an OLEDBException and a a SQLException. An OLEDBException can be thrown by any OLE DB data source, while a SQLException is thrown by SQL Server. In this session, we will focus on handling the OLEDBException, however the approach for handling a SQLException is nearly identical except you have access to a broader range of properties to evaluate as shown in Table 26-1.
Table 26-1 SQLException Properties not contained in OLEDBException
SQLException Property |
Description |
Class |
Gets the severity level of the error returned from the SQL |
|
Server .NET Data Provider. |
|
|
LineNumber |
Gets the line number within the Transact-SQL command |
|
batch or stored procedure that generated the error. |
|
|
Number |
Gets a number that identifies the type of error. |
|
|
Continued
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Table 26-1 |
Continued |
SQLException Property |
Description |
Procedure |
Gets the name of the stored procedure or remote procedure |
|
call (RPC) that generated the error. |
|
|
Server |
Gets the name of the computer running an instance of SQL |
|
Server that generated the error. |
|
|
State |
Gets the number modifying the error to provide additional |
|
information. |
|
|
The Try . . . Catch . . . Finally error-handling functionality of ASP.NET is very useful for evaluating ADO.NET errors. Let’s start by creating a function, GetRecords(). GetRecords() is a simple function that attempts to connect to a database, fill a dataset, and then return the dataset. In the next section we will implement OLEDBErrorHandler(), a generic error handler that will evaluate the errors that are established during calls in the GetRecords() function and then write those errors to the browser and to the System Event Log.
The code listings 26-1, 26-2, 26-3 are each segments of code extracted from the file errorhandle.aspx in the Session 26 folder of the CD.
Note
Listing 26-1 OLEDBError handling code segment from errorhandle.aspx
<%@ Import Namespace=”System.Diagnostics”%> <%@ Import Namespace=”System.Exception” %> <%@ Import Namespace=”System.Data.OleDB” %> <%@ Import Namespace=”System.Data” %>
<%@ Page Language=”VB” Debug=”False” Trace=”False” %> <HTML>
<SCRIPT LANGUAGE=”VB” RUNAT=”server”>
Sub ExecuteDBCodeBtn_Click(Sender As Object, E as EventArgs) Try
Dim MyDataset As DataSet = GetRecords() grid1.DataSource = MyDataset.Tables(0).DefaultView grid1.DataBind()
Catch ex as Exception
Dim oLabel as new Label() Dim sMess as String oLabel.Text=””
oLabel.Id=”ExecuteDBCodeBtn_ErrorLabel” Page.Controls.Add(oLabel)
sMess = sMess & “<b><p>Code Error Occurred</p></b>” sMess = sMess & “<ul><li>” & ex.message & “</li></ul>” oLabel.Text = oLabel.Text & sMess
End Try