zxing pdf417 c# Figure 3-25. Database diagram for the PTracker database in C#

Create PDF417 in C# Figure 3-25. Database diagram for the PTracker database

Figure 3-25. Database diagram for the PTracker database
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Whenever possible, database access should be performed through stored procedures. Stored procedures offer powerful security control over the database and perhaps most importantly provide an abstraction layer between the physical structure of the database and the logical way in which it is used. The business objects created in s 17 and 18 will make use of stored procedures for their database interaction. I think this is true, even though you may use LINQ to SQL or other technologies to access the database. At the same time, technologies like LINQ to SQL offer flexible query capabilities that are nice to use. So I typically compromise, using LINQ to SQL to query for data, and using stored procedures (called through LINQ to SQL) for insert, update, and delete operations. You can use Server Explorer to add the stored procedures to the database by right-clicking the Stored Procedures node under the database and choosing Add New Stored Procedure. This brings up a designer window in which you can write the stored procedure code. When you close the designer, the stored procedure will be added to the database. Let s look at the stored procedures we ll need, starting with addProject.
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C HA PTER 3 OBJEC T-ORIE NTED APPLIC ATION DES IGN
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The addProject procedure is called to add a record to the Projects table, as follows: CREATE PROCEDURE addProject ( @id uniqueidentifier, @name varchar(50), @started datetime, @ended datetime, @description varchar(MAX), @newLastChanged timestamp output ) AS INSERT INTO Projects (Id,Name,Started,Ended,Description) VALUES (@id,@name,@started,@ended,@description) SELECT @newLastChanged = LastChanged FROM Projects WHERE Id=@id RETURN Note that this only adds the record to the Projects table; a separate stored procedure adds records to the Assignments table. This stored procedure not only includes an INSERT statement, but also a SELECT statement that loads an output parameter value. This is required to support concurrency. Recall that all the tables in the database include a timestamp column, which is incremented automatically each time a row is inserted or updated. As you ll see in 18, the business object must keep track of this value. Since the value changes any time the row changes, the value is returned as the result of any INSERT or UPDATE operation.
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updateProject
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Not only are records added to the Projects table, but the application must allow them to be changed. The updateProject procedure provides this capability, as shown here: CREATE PROCEDURE updateProject ( @id uniqueidentifier, @name varchar(50), @started datetime, @ended datetime, @description varchar(MAX), @lastChanged timestamp, @newLastChanged timestamp output ) AS UPDATE Projects SET Name=@name, Started=@started, Ended=@ended, Description=@description
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WHERE Id=@id AND LastChanged=@lastChanged IF @@ROWCOUNT = 0 BEGIN RAISERROR('Row has been edited by another user', 16, 1) RETURN END SELECT @newLastChanged = LastChanged FROM Projects WHERE Id=@id RETURN Again, this procedure only updates the record in the Projects table; the related records in the Assignments table are updated separately. Notice the @lastChanged parameter required by the procedure. This represents the last known timestamp value for the row. In 18, you ll see how this value is maintained by the business object. When the object attempts to update the row, it provides the last known value for the LastChanged column. If that value hasn t changed in the database, then no other user has updated the row since the object read its data. But if the value has changed in the database, then some other user did change the data in the row since the object read the data. First-write-wins optimistic concurrency specifies that this second update can t be allowed, because it could overwrite changes made by that other user. The UPDATE statement itself uses this parameter in the WHERE clause to ensure that the row is only updated if the value matches. The procedure then checks to see if the row was actually updated. If no rows were updated, it raises an error, which shows up as a database exception in the data access code of the business object. On the other hand, if the update goes through and the row is changed, then a SELECT statement is executed to return the new value of the LastChanged column as an output parameter, so that the object can maintain the new value to allow possible future updates.
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