Simply On Rails - Part 4: Quick and Easy Default Data Migrations

Posted by h3rald Sat, 15 Sep 2007 19:10:00 GMT

In the last post of this series I tried to find a DRY solution to deal with tables storing “ancillary” data, i.e. names of user roles, predefined categories, page state names and other similar things. I personally chose to put this kind of data to make my application more dynamic, although I could have decided to use ENUMs or simply ordinary varchar fields—that would have been easier, but less flexible. For now, I’m sticking with my original choice.

The data in these tables is kind of a prerequisite for the application to run: I must be able to have a status to assign to a user when creating it, and the same applies to roles. Sure, I could spend 20 minutes populating these tables manually, but it would be nice if there was a less tedious way, wouldn’t it?

There is indeed. The inspiration came from a technique described in the book (which I highly recommend) Agile Web Development With Rails, in which the author outlines how it would be possible to use Rails’ fixtures and migrations to load data in the database automatically from YAML files. All you have to do is create a migration to load the specified YAML files and you’re all set.

I wanted to take a little step further, allowing the migration to load data from all YAML files in a specific directory, automatically.

Let’s start creating the YAML files then and place them all in one directory of the application like /db/migrate/defaults. Here’s the one I used for user roles, for example:

visitor:
    id: 1
    name: Visitor
    level: 0

user:
    id: 2
    name: User
    level: 10

contributor:
    id: 3
    name: Contributor
    level: 20

provider:
    id: 4
    name: Provider
    level: 50

operator:
    id: 5
    name: Operator
    level: 100

administrator:
    id: 6
    name: Administrator
    level: 500

webmaster:
    id: 7
    name: Webmaster
    level: 1000

The important thing to remember is to provide a unique string to identify each record, before specifying each fiels. The other files look similar, so I won’t bother listing them here.

And here’s the simple code for the migration:

require 'active_record/fixtures'

class LoadDefaults < ActiveRecord::Migration

    def self.up
        down
        models = self.default_models
        models.each do |m|   
            Fixtures.create_fixtures(self.default_directory, m)
        end
    end

    def self.down
        models = self.default_models
        models.each do |m|
            eval("#{m.singularize.capitalize}.delete_all")
        end
    end

    def self.default_directory
        File.join(File.dirname(__FILE__), "defaults" )
    end

    def self.default_models
        files, names = Dir.glob("#{self.default_directory}/*.yml"), []
        unless files.blank?
            files.each { |f| names << File.basename(f, '.yml') }
            names
        else
            []
        end
    end

end

Basically the migration will look in a directory named “defaults” for some YAML files named after a particular database table, and it will attempt to load all the records defined in each one of them. The down method of the migration deletes all the data in the specified tables, so use with care…