Introducing MultiConfig gem for ActiveRecord

September 18th, 2012

If you need your model to use a different database from the default databases(production, development etc.) then you have to call method establish_connection in the model. The parameters are either complete database config or a key name as specified in database.yml like other_test or other_production etc. If u need it to dynamically change according to the environment then you write:

MyModel < ActiveRecord::Base
    establish_connection "other_#{Rails.env}"
end

This messes the database.yml a bit as you end up with a database.yml with configs it should not have, since the other configs are not really environments. This gave me an idea to create a gem which allows you to specify separate config files for models which access other dbs. It’s this simple:

# In your Gemfile
require 'multi_config'

# In your model
MyModel > ActiveRecord::Base
    self.config_file = 'other'
end

Your config file config/other.yml just needs to have similar environments to database.yml. You can see it is much simpler to maintain and cleaner.

This is how it works:

ActiveRecord stores the configurations read from database.yml in ActiveRecord::Base.configurations hash (copied from Rails.application.config.database_configuration).
When establish_connection connection is called with a string argument, it loads the config from the configurations hash having key same as the argument.
Multi config extends this hash. It prefixes filename_ as a namespace in the keys to avoid collisions. Then it calls establish_connection with the namespaced key for the current environment.

That’s the core really; rest of the code deals with railties initialization, error conditions and the majority is tests. I have been absolutely obsessive about tests and documentation. I have added comments to every detail like Bundler.require etc so that it can serve as goto guide for writing gems for myself and maybe for you too. Similarly CHANGELOG and README are created as per best practices. For continous integration I am using travis which is really easy to integrate with github.

One interesting thing I did:

# In your environments config file (e.g. development.rb)
ActiveRecord::Base.config_file = 'other' Now, all your models will use the new config file. Though this can also be done by setting **DATABASE_URL** environment variable

There are some caveats of course, like migrations. I may add support for those as well in future. Meanwhile you can do this : Migration om different DB

I will be glad if you download/install the gem, fork the repo or share this article. Thanks for reading. view source here

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