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nashvegas

The purpose of this app is to enable a plug and play method for managing database changes.

Database migrations is a large topic with a lot of different approaches. This approach worked well for my needs and maybe it will for you as well.

Installation

  • pip install nashvegas
  • Add the application to your INSTALLED_APPS list in your settings.py file.

Settings

You can set the NASHVEGAS_MIGRATIONS_DIRECTORY to whatever absolute path you are using to store your migrations. It defaults to migrations/ at the same level as your settings.py.

Usage

nashvegas ships with two management commands, upgradedb and comparedb. The first, upgradedb, will manage the creation, listing, and execution of individual migrations. The second, comparedb, is currently an experimental command that attempts to help you discover missing migrations.

  • Execute the command line:

    $ ./manage.py upgradedb --create|--list|--execute $ ./manage.py comparedb

Options for upgradedb

  • --create - Compares database with current models in apps that are installed and outputs the sql for them so that you can easily pipe the contents to a migration.
  • --list - Lists all the scripts that will need to be executed.
  • --execute - Executes all the scripts that need to be executed.
  • --seed - Populates Migration model with scripts that have already been applied to your database and effectively want to skip execution. Provide a migration id to stop at. For instance, running ./manage.py upgradedb --seed 005 will skip migrations 000 to 005 but not 006.

Conventions

Part of the simplicity of this solution is based on the naming conventions of the sql scripts. They should be named in a manner that enforces order. Some examples include:

0001_short_comment_about_migration.sql
0001.sql

The model, nashvegas.Migration will get synced into your database if it doesn't exist when you go to execute any of the upgradedb commands. In this model the scripts that have been executed will be recorded, effectively versioning your database.

In addition to sql scripts, --execute will also execute python scripts that are in the directory. This are run in filename order interleaved with the sql scripts. For example:

0001.sql
0002.py
0003.sql

The Python script will be executed 2nd between 0000.sql and 0003.sql. The script will only be executed if the module contains a callable named migrate. It is a good idea to put all your executing code within a class or series of functions or within a singe migrate() function so as to avoid code executing upon import.

For example, your script might light like this if you need to update all your product codes on next release:

from store.models import Product

def migrate():
    for product in Product.objects.all():
        product.code = "NEW-%s" % product.code
        product.save()

Configuration for comparedb

The comparedb command is available only for advanced system administrators. It proceeds as such:

  • get the SQL structure dump of the current database
  • create a new database, the "compare" database
  • syncdb in the "compare" database,
  • get the SQL structure dump of the "compare" database
  • output the diff

It executes a few raw shell commands which you might need to customize to add user credentials, encoding or specify database templates. This can be done through the NASHVEGAS dictionnary in your setting.

Example for PostgreSQL

By default, nashvegas executes raw createdb, dropdb or pg_dump commands, example customisation:

NASHVEGAS = {
    "createdb": "createdb -U postgres -T template0 -E UTF8 {dbname}",
    "dropdb": "dropdb -U postgres {dbname}",
    "dumpdb": "pg_dump -U postgres {dbname}",
}

If you add a field "test" on model "Foo", comparedb will output:

>>> ./manage.py comparedb
Getting schema for current database...
Getting schema for fresh database...
Outputing diff between the two...
---
+++
@@ -515,7 +515,8 @@

 CREATE TABLE testapp_foo (
     id integer NOT NULL,
-    bar character varying(100)
+    bar character varying(100),
+    test character varying(100)
 );

Example for MySQL

MySQL is not supported by default thought such settings do work:

NASHVEGAS = {
    "createdb": "mysql -u root -p -e \"create database {dbname}\"",
    "dropdb": "mysql -u root -p -e \"drop database {dbname}\"",
    "dumpdb": "mysqldump -u root -p {dbname}",
}

If you add a field "test" on model "Foo", comparedb will output:

>>> ./manage.py comparedb
Getting schema for current database...
Enter password:
Getting schema for fresh database...
Enter password:
Enter password:
Enter password:
Outputing diff between the two...
---
+++
@@ -1,6 +1,6 @@
 -- MySQL dump 10.13  Distrib 5.1.58, for debian-linux-gnu (x86_64)
 --
--- Host: localhost    Database: testproject
+-- Host: localhost    Database: testproject_compare
 -- ------------------------------------------------------
 -- Server version  5.1.58-1ubuntu1

@@ -419,6 +419,7 @@
 CREATE TABLE `testapp_foo` (
   `id` int(11) NOT NULL AUTO_INCREMENT,
   `bar` varchar(100) DEFAULT NULL,
+  `test` varchar(100) DEFAULT NULL,
   PRIMARY KEY (`id`)
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1;
 /*!40101 SET character_set_client = @saved_cs_client */;
@@ -441,4 +442,4 @@
 /*!40101 SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION */;
 /*!40111 SET SQL_NOTES=@OLD_SQL_NOTES */;

--- Dump completed on 2012-03-07 12:58:15
+-- Dump completed on 2012-03-07 12:58:18

Typicall customisation would be to setup a $HOME/.my.cnf that contains credentials allowing to run this command without password prompt.

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