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

h3rald — Sat, 15 Sep 2007 13:10:00 -0600

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…

"Simply On Rails - Part 4: Quick and Easy Default Data Migrations" by Electrical Repair

Sat, 22 Dec 2007 02:47:22 -0700

Migrations would make the application much more flexible in terms of moving it among different DB servers and more.

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

Tue, 27 Nov 2007 16:36:01 -0700

Cool code and post. I second the idea of using Migrations with such application.

Thanks

"Simply On Rails - Part 4: Quick and Easy Default Data Migrations" by Fabio Cevasco

Sun, 16 Sep 2007 02:40:48 -0600

Indeed, indeed you are right. You have to be careful when you prepare the YAML files, and in certain situation (and for certain kind of data) a more standard approach is recommendable.

"Simply On Rails - Part 4: Quick and Easy Default Data Migrations" by Pei Mei

Sat, 15 Sep 2007 23:41:35 -0600

Very nice. I still prefer to include my migrations as rails code User.create() in my migrations to avoid having developers create dodgy yaml files though, especially with pks.

H3RALD: Simply On Rails - Part 4: Quick and Easy Default Data Migrations

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

"Simply On Rails - Part 4: Quick and Easy Default Data Migrations" by Electrical Repair

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

"Simply On Rails - Part 4: Quick and Easy Default Data Migrations" by Fabio Cevasco

"Simply On Rails - Part 4: Quick and Easy Default Data Migrations" by Pei Mei