How I set up my Turbogears based development environment...

The start of 2009 brought along a new project for me. It was based on Turbogears 1.0.4.4 with Postgresql 8.3 as the database. Here is a list of steps I followed to setup my development environment.

1. Installed Python 2.4.4.

2. I was working on a windows platform and required support for it so I installed extensions for windows. PyWin32 is a collection of modules for advanced windows-specific support. These include utilities for COM, Win32 API calls, Registry, Event Log and MFC user interfaces.

3. Installed setuptools, so as to be able to build and distribute packages. It includes a utility easy_install, which lets you automatically download, build, install, and manage Python packages and their dependencies.

4. Installed the Turbogears web application framework by running the command:

>easy_install-2.4 -f http://files.turbogears.org/ "TurboGears==1.0.4.4"

5. Installed win-psycopg, a windows port of the psycopg python-postgresql database interface.

6. Installed SQLObject, an Object Relational Manager which provides an object interface to your database, with tables as classes, rows as instances, and columns as attributes. Used the following command to install it:

>easy_install-2.4 "SQLObject==0.10.2"

7. Installed other packages like PyPDF 1.9, ElementTree 1.2.6, ElementTidy and PyLucene for my text based application.

8. Installed TinyMCE 1.0.6, a JavaScript HTML WYSIWYG editor control, using the following command:

>easy_install-2.4 "TurboTinyMCE==1.0.6"

9. Downloaded the ExtJS 2.2 Library. It has some excellent built-in components ready to use.

10. Installed Eclipse 3.2 IDE and added the PyDev plugin using the manual and configured it accordingly.

There I was, ready to play!

Remote access to PostgreSQL Server

Error: "could not connect to Server: Connection refused"

The resolution is a five step process : -

1. Firstly, check if your firewall allows the port (default is 5432) to establish a connection.

2. By default, PostgreSQL does not allow remote incoming connections. This has been implemented in this way, because of security concerns. You need to enable incoming TCP/IP connection requests from the client. This can be done by adding

#
listen_addresses = '*'
#

in the configuration file postgresql.conf. This configuration is valid for postgresql 8.x.

3. You will have to inform Postgresql who can connect to the server. This has to be done by modifying the configuration file pg_hba.conf

host all all 127.0.0.1/32 trust
host all all 192.168.0.0/16 trust

This would allow any user on the local network to access all the databases on the server in "trust" mode.

4. After you have saved the configuration files you will have to restart the server. Remember, these configuration files are read on server startup.

5. Test your config. Go to the command prompt and run the following command.


>psql -h serverIP -U postgresUser -d postgresdb


And, you should be set for remote access to postgresql server.

JSON

JSON is an acronym for JavaScript Object Notation. It is a means of transferring serialized JavaScript objects so that a JavaScript application can evaluate them and transform them into JavaScript objects which, the application can utilize to perform its operations. In short, JSON is a data interchange format. It is easy to parse and generate in almost any language.

A sample JSON:-

{"continents": {
             "asia":[{"country": "india",
                      "code": "101"
                     },
                     {"country": "japan",
                      "code": "102"
                     }],
             "europe":[{"country": "france",
                        "code": "401"
                     }]
                  }
}

The most notable features of JSON are:-

1. It is basically a collection of name-value pairs ("key": "value") separated by a colon.
2. It is not only easy to read for humans but also easy to parse by the machines.
3. Values can be a string, an integer, an object or an array.
    Standard literals like True, False, null are also acceptable.
4. Each object is placed between {...}.
5. Arrays are place between [...] and are comma separated.
6. It can be passed using httpWebRequest.
7. Reserved keywords from JavaScript are not allowed in JSON.
8. JSON can be parsed using the eval() method in JavaScript.
9. It uses Unicode.


Common errors like the "missing } after property list" can be corrected
using a JSON validator like www.jsonlint.com

Existing database tables and SQLObject

Define the existing table "Bookmark" as a class in python:-

class Bookmark(SQLObject):

_fromDatabase = True # the table structure will be loaded from the database
BookmarkName = StringCol (dbName="bookmark_name")
# dbName is the name of the column in the database, BookmarkName is the name reference in code.
ProjectId = IntCol(dbName="project_id")

(Make sure the indentation is correct)

Connecting to an existing database using TurboGears framework

Your application would need a DB connection to access the database. A connection is how you tell SQLObject to locate your database. SQLObject is an object relational mapper, a software layer that maps data stored in a relational database to an object model in an object-oriented programming language. The connection string resembles a URI in the following format:

scheme://[user[:password]@]host[:port]/database[?parameters]
For instance:
postgres://user:pwd@localhost:5432/db

SQLObject provides a debug mechanism, which can be turned on easily by appending ?debug=True to the end of your database URI in the dev.cfg file.

postgres://user:pwd@localhost:5432/db?debug=True
This would publish the query generated onto the console.
              ..........

Define the existing table Bookmark as a class in model.py:-

class Bookmark(SQLObject):

_fromDatabase = True # the table schema be loaded from the database

BookmarkName = StringCol (dbName="bookmark_name")

ProjectId = IntCol(dbName="project_id")

# dbName is the name of the column in the database, BookmarkName is the name reference in code.
..........
Using the following code in controller.py you should be able to retrieve the second record from the Bookmark table

b=Bookmark.select()
print ":  %s " % b[2].BookmarkName