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CHAPTER 6 s OBJECT-ORIENTED APPLICATION DESIGN
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the database. They provide a level of indirection between the business objects and the underlying table structures, and thus they reduce coupling between the data management and business layers in your application. In short, stored procedures help make applications more maintainable over time. That said, you ll notice that none of these stored procedures are complex, and every effort is made to keep business logic out of the database and in the business objects. Putting the business logic in both the objects and the database is just another way to duplicate business logic, which increases maintenance costs for the application as a whole.
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Creating the Databases
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The PTracker database will contain tables and stored procedures to persist the data for the business objects in the object model designed earlier in the chapter. This is a SQL Server 2005 Express database, and so you can think of it as being just another file in your project. To create the database, open Visual Studio and create a new Class Library project named PTDB. I won t have you build this project at any point, so the project settings and Class1.cs file can be ignored. The purpose of this project is just so you can use Visual Studio to set up the database. Choose Project Add New Item, and choose the SQL Database option. As shown in Figure 6-16, name the file and click Add.
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Figure 6-16. Adding the PTracker database in Visual Studio
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Visual Studio will force you to walk through the process of creating a DataSet for the new database. You can walk through or cancel that wizard as you choose. It is not required for anything covered in this book. Repeat the process to add a Security.mdf database as well. The end result is that you ll have two databases in the project and more importantly, in the Server Explorer window, as shown in Figure 6-17.
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CHAPTER 6 s OBJECT-ORIENTED APPLICATION DESIGN
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Figure 6-17. The PTracker and Security databases in Server Explorer
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Table creation can also be done within Server Explorer: just right-click the Tables node under the database, and choose New Table. This will bring up a table designer in VS .NET, with which you can define the columns for the new table. Once the columns, keys, and indexes have been set up, save the changes by closing the designer or clicking the Save button in the toolbar. At this point, you ll be prompted to provide a name for the table, and it will be added to the database.
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PTracker Database
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Follow this process to add each of the following four tables to the database.
Roles
The Roles table will store the list of possible roles a resource can fill when assigned to a project it simply contains an Id value and the name of the role. Figure 6-18 shows the VS .NET designer with these columns added, and the Id column configured as the primary key.
Figure 6-18. Design of the Roles table
Notice that none of the columns allow null values. There s no business requirement to differentiate between an empty value and one that was never entered, so null values would make no sense. The table also has a LastChanged column, which will be used to implement optimistic, first-writewins concurrency in 8. It is of type timestamp, and so provides a unique, auto-incrementing
CHAPTER 6 s OBJECT-ORIENTED APPLICATION DESIGN
value every time a row is inserted or updated. All the tables in the PTracker database will have this type of column.
Projects
The Projects table will contain the data for each project in the system. The columns for this table are shown in Figure 6-19.
Figure 6-19. Design of the Projects table
The Id column is set up as the primary key, and it s of type uniqueidentifier, which is a Guid type in .NET. There are many ways to create primary key columns in tables, including using auto-incrementing numeric values or user-assigned values. However, the use of a uniqueidentifier is particularly powerful when working with object-oriented designs. Other techniques don t assign the identifier until the data is added to the database, or they allow the user to provide the value, which means that you can t tell if it collides with an existing key value until the data is added to the database. With a uniqueidentifier, however, the business developer can write code to assign the primary key value to an object as the object is created. There s no need to wait until the object is inserted into the database to get or confirm the value. If the value isn t assigned ahead of time, the database will supply the value. Notice that the two datetime fields allow null values. The null value is used here to indicate an empty value for a date. The Description column is also allowed to be null. This isn t because of any business requirement, but rather because it is quite common for database columns to allow null values in cases in which they re meaningless. 8 will illustrate how to easily ignore any null values in this column. The Description column is of type varchar(MAX), so that it can hold a blob of text data. This field allows the user to enter a lengthy description of the project, if so desired.
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