Because invoices, time "tickets," and projects are all predicated to some extent on clients, you first create the Client object. Some incarnation of a client or customer object is present in most applications, so the structure presented in Figure 1 is probably a familiar concept.

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Figure 1. The Client object structure

This Client object has the standard informational fields, such as an address and contact information, as well as an extra field-rate. The rate property allows users to set different rates for different clients.

Your next object is the Project, and its structure is represented in Figure 2.

Figure 2. The Project object structure

The Project object is associated with its owner through the client_id property. It also has a code property that can be used to store a client's internal reference or project number.

The Ticket object, depicted in Figure 3, is the time- tracking object. It contains the time slips, or tickets, that users submit using the AIR client.

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Figure 3. The Ticket object structure

The Ticket is the most complex object so far, and it contains several key attributes to help you store, classify, and present them:

• account_id: Links a ticket to a user
• category: Used to track how time is spent-for example, development, on-site, or research
• start_time: A UNIX time stamp indicating when the ticket was first opened
• end_time: A UNIX time stamp indicating when the ticket was last closed

A ticket's timer can be paused and restarted, so the start and end values refer to the first clock-in and final clock- out for a particular ticket, respectively. All times are represented as UNIX time stamps, or seconds elapsed since midnight, January 1, 1970.

Finally, the Invoice object represents the invoices that will be generated. Its structure is shown in Figure 4.

Figure 4. The Invoice object structure