Projects in VB.NET

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The Projects table will contain the data for each project in the system. The columns for this table are shown in Figure 6-19.
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Figure 6-19. Design of the Projects table
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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 autoincrementing 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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Resources
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The Resources table will hold the data for the various resources that can be assigned to a project. The columns for this table are shown in Figure 6-20.
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CHAPTER 6 s OBJECT-ORIENTED APPLICATION DESIGN
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Figure 6-20. Design for the Resources table
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Once again, the Id column is the primary key it s an int that is configured as an identity column using the Column Properties window, as shown in Figure 6-21.
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Figure 6-21. Making the Id column an identity column
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This table has now been given an identity key; the code in 8 will demonstrate how to support this concept within your business objects. As with the Description field in the Projects table, the LastName and FirstName columns allow null values even though they have no business meaning. Again, this is merely to illustrate how to build business objects to deal with real-world database designs and their intrinsic flaws.
Assignments
Finally, there s the Assignments table. A many-to-many relationship exists between projects and resources a project can have a number of resources assigned to it, and a resource can be assigned to a number of projects. The way you can represent this relationally is to create a link table that contains the primary keys of both tables. In this case, it will also include information about the relationship, including the date of the assignment and the role that the resource plays in the project, as shown in Figure 6-22.
Figure 6-22. Design for the Assignments table
CHAPTER 6 s OBJECT-ORIENTED APPLICATION DESIGN
The first two columns here are the primary keys from the Projects and Resources tables; when combined, they make up the primary key in the link table. Though the Assigned column is of datetime type, null values are not allowed. This is because this value can t be empty a valid date is always required. The Role column is also a foreign key, linking back to the Roles table. The data in this table will be used to populate the ProjectResource and ResourceAssignment objects discussed earlier in the chapter. This really drives home the fact that a relational model isn t the same as an object-oriented model. The many-to-many relational design doesn t match up to the object model that represents much of the same data. The objects are designed around normalization of behavior, while the data model is designed around normalization of data.
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