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Although it s certainly true that large and complex applications often have complex relationships between classes and objects, those complex relationships should always be carefully reviewed. As a general rule, if relationship lines are crossing each other or wrapping around each other in a diagram like Figure 6-1, you should review those relationships to see if they need to be so complex. Sometimes, it s just the way things have to be, but more often, this is a sign that the object model needs some work. Though relying on the aesthetics of a diagram may sound a bit odd, it is a good rule of thumb. In this case, there s a pretty complex relationship between Project, ProjectResources, Resource, and ResourceAssignments. It is, in fact, a circular relationship, in which all these objects refer to the other objects in an endless chain. In a situation like this, you should always be looking for a way to simplify the relationships. What you ll often find is that the object model is missing a class: one that doesn t necessarily flow directly from the use cases, but is required to make the object model workable. The specific problem caused by the circular relationship in Figure 6-1 becomes very apparent when an object is to be loaded from the database. At that time it will typically also load any child objects it contains. With an endless loop of relationships, that poses a rather obvious problem!

Running this command tells plutil to convert the .plist file to text ( xml1 ) and send the output ( -o ) to standard out. If you wanted to store the file, you could write the output to a file on disk.

Summary

There must be some way to short-circuit the process, and the best way to do this is to introduce another object into the mix. In the object model thus far, what s missing is a class that actually represents the assignment of a resource to a project. At this point, there s no object responsible for assigning a resource to a project, so there s an entire behavior from the use cases that s missing in the object model. Additionally, there s data described in the use cases that isn t yet reflected in the object model, such as the role of a resource on a particular project, or the date that the resource was assigned to a project. These data fields can t be kept in the Project object, because a project will have many resources filling many different roles at different times. Similarly, they can t be kept in the Resource object, because a resource may be assigned to many projects at different times and in different roles.

In this chapter, I have presented some of the more complex database internal technologies. You learned about how queries are represented internally within the server and how they are executed. More importantly, you discovered how MySQL can be used to conduct your own database internals experiments. The knowledge of these technologies should provide you with a greater understanding of why and how the MySQL system was built and how it executes. In the next chapter, I ll show you more about internal query representation through an example implementation of a query tree structure. The next chapter begins a series of chapters designed as a baseline for you to implement your own query optimizer and execution engine. If you ve ever wondered what it takes to build a database system, the next chapters will show you how to get started on your own query engine.

The need for another object an Assignment object is clear. This object s responsibility is to assign a resource to a project. Figure 6-2 shows an updated diagram, including the changes thus far.

his chapter presents the first part of the advanced database technologies for the database experiment project (DBXP). I begin by introducing the concept of the query tree structure used for storing a query in memory. Next I ll present the query tree structure used for the project and the first in a series of short projects for implementing the DBXP code. The chapter concludes with a set of exercises you can use to learn more about MySQL and query trees.

However, we re still not done. The Assignment class itself just became overly complex, because it s used within two different contexts: from the list of resources assigned to a project, and from the list of projects to which a resource is assigned. This is typically problematic. Having a single object as a child of two different collections makes for very complicated implementation and testing, and should be avoided when possible. Beyond that, think about its responsibility in the diagram in Figure 6-2. Assignment is now responsible for assigning a resource to a project AND for associating a project with a resource. When used from ProjectResources, it has the first responsibility, and when used from ResourceAssignments,

plutil can also lint a file----that is, check it for consistency and basic errors. What it cannot do is verify that your key-names and data are correct. Running a lint check is as simple as using the -lint switch: