Easy Database Development Using Rekall
After developing for a while on the Linux version of Rekall I downloaded as source, I tested my application on the version I purchased as a Windows install. My application looked identical in both environments. In fact, porting to Windows was a breeze: I simply performed a backup of the PostgreSQL database on my Linux box and copied that file, as well as the Rekall project file, to the Windows box. There, I restored the backup file to the Windows version of PostgreSQL 8.0.2 and ran the project file in the Windows version of Rekall.
The Windows install is reasonably sized at about 7MB. You have to be careful, though, to install the right version of Python before you install Rekall. For Rekall 2.2.3, the current version, you need any Python version in the 2.3 line.
Compiling under Linux was fairly straightforward. Under CentOS 3, the compile produced no errors and installed cleanly. After upgrading to CentOS 4, I received compile errors during the install, but the application did install cleanly. The compile process took about an hour on my relatively old 900MHz Athlon-equipped CPU with 512MB of RAM.
The first time you run Rekall, it presents you with a series of dialog boxes to configure certain behaviors of the development environment. Among these behaviors are the verification of record updates and deletes as well as the layout of the development environment. After this occurs the first time, it doesn't happen again unless the user's Rekall configuration file is deleted.
To demonstrate the ease of development in Rekall, I put together a small application that tracks philosophers and their writings. This example provides a good examination of both the ease of development and the Python scripting capabilities of Rekall. The end goal was an application similar to what you see in Figure 1—a small data-entry window with two tabs, a report and a status bar at the bottom describing the current philosopher.
Before you begin, you need to configure PostgreSQL to work well with Rekall. Configure PostgreSQL to support connections from local applications, as is shown in the Configuring PostgreSQL sidebar. After this is done, add a user philosophy_major with a password and create the philosophers database. This all must be done before the initial connection by Rekall, because Rekall does not have the capability to add users or databases. After a reload of the PostgreSQL server, it is ready for you to connect.
Now, we need to create the project. Open a terminal window and make a directory for the project. Then, fire up Rekall by typing rekall on the command line. After installation, the rekall executable should be under the /usr/bin directory.
Starting a project is straightforward. After clicking on the New button, you are presented with a project creation wizard. On the first screen, you enter the directory where your information should be stored, as well as the name of the project, the Database Name). In the next dialog window, tell it where to store the forms, reports and other XML structures. These items can be stored either in the database, in a special Rekall Objects table or as files in the filesystem. After you choose that, select which database platform you're using. You can have multiple data sources, each running a different driver. This particular dialog controls the main database.
The next two dialogs allow you to enter the database host and port as well as the user name/password pair you want to use to connect. Then, upon a successful connection, you specify which database you want to use.
After you select the database, a screen similar to the one shown in Figure 2 appears. Now you can start building your application. The first step in application building is to create the tables. To do so, click on the Tables item in the Objects tree. Then, select the correct server and double-click on Create new table. The table builder, as shown in Figure 3, then appears.
Rekall can't use tables that already exist. It would be an interesting exercise to write a utility to convert database schemas to Rekall definition files. Right-clicking your project name listed in the Tables tree reveals a menu that allows you to import a table definition, and this is contained in an XML file that could be built from SQL table definitions.
Next, it's time to build some forms from the database schemas you've created. Fortunately, Rekall provides some easy tools with which to do so. When you click on the Forms item in the Objects tree and expand the item named after your project, you can create a form or create a form with a wizard. Even when creating complex applications that use things such as tabbed pages, it's helpful first to create each page as a separate form, copy those objects and then paste them into the appropriate blocks on your form.
In Figure 4, you can see how I've used the wizard to create a form based on the philosophers table. You have a large amount of control over your forms, even when you use the wizard. Not only does it ask you for a table and fields to use, you also can specify multiple records per page, field formats and the tools that should be added automatically to the page. In the form in Figure 4, the wizard added the buttons you see along the bottom of the screen, as well as the small navigation tool at the very bottom. Called a Nav. Tool, this widget allows you to navigate through the database on a record-by-record basis.
After creating the forms for Philosophers and their Publications, as seen in Figure 5, I need to combine them into one window. This is where it is advantageous to create a form by hand, so that I can to add components at will. Before I do that, however, we need to examine the concept of blocks.
In a Rekall form, data is represented in an entity called a block. There are several types of blocks, the simplest being a table block. Other types of blocks include query blocks and SQL blocks, which display data retrieved from a query. A form can have any number of blocks. These blocks can exist side by side or in any configuration on the same visual plane. Alternatively, they can exist as pages in a notebook by way of a tabbed-page control. This is what I exploit to get to my one-window goal.
To create my notebook, I create a new form without using the wizard. After I select that command, I get the Form Attribute window shown in Figure 6. As you can see, I've configured a basic window. It's not stretchable, there is no status bar and the top-level block type is null— it's called a menu block in the selection dialog. Setting the top-level block type to menu only or null is what allows us to place blocks and controls arbitrarily on the form.
After going through each attribute by clicking on it, making a selection and pressing the Accept button at the bottom of the dialog, I am presented with a Block attribute window which that much like the Form Attribute page. Because this is simply a menu block, I safely can accept many of the default values, as most values are appropriate only for table or query blocks. That action nets me a blank page, which I can resize appropriately and add the tabber control.
On each page of the tabber control, I simply create a table block corresponding to the correct table. Then, I copy all of the contents from the block contained in the Philosophers form to the corresponding block in the Philosophers tabber page, and I repeat the action for the Publications page. After doing so, I dress up the fields with their correct names, modify the size of the Abstract field on the Publications page and my notebook is complete.
Now, I need to find a way for the Philosopher page to set the key that is used to look up the publications. I do this through the use of the status label at the bottom of the main form, underneath the tabber page control. This label's text value is set whenever a database lookup is performed or a new entry is created. The label is set when the code, shown in Listing 1, is run by way of a callback set by the On Display event for the Philosophers block. When the Publications block is shown, the On Display event is called for that block, which sets a user filter on the data shown in that block. The user filter code can be seen in Listing 2.
- New Products
- New Products
- Integrating Trac, Jenkins and Cobbler—Customizing Linux Operating Systems for Organizational Needs
- Tech Tip: Really Simple HTTP Server with Python
- Dialog: An Introductory Tutorial
- RSS Feeds
- Non-Linux FOSS: Remember Burning ISOs?
- Returning Values from Bash Functions
- EdgeRouter Lite