Something we don’t see too much of is talk of using Ruby and ActiveRecord with multiple databases. Turns out it’s really simple. Let’s run through this.
Let’s dive right in. Obviously we want to handle migrations for two databases, so we need two separate Rake tasks to handle that:
desc "Migrate the database through scripts in db/migrate." namespace :db do task :migrate do Rake::Task["db:migrate_db1"].invoke Rake::Task["db:migrate_db2"].invoke end task :migrate_db1 do ActiveRecord::Base.establish_connection DB1_CONF ActiveRecord::Migrator.migrate("db/migrate/db1/") end task :migrate_db2 do ActiveRecord::Base.establish_connection DB2_CONF ActiveRecord::Migrator.migrate("db/migrate/db2/") end end
Our first task here is
db:migrate that delegates out to
Each of those establish a connection to the database and then runs the migrations from their own separate folders. This allows you to store migrations in separate folders so you can easily manage them.
Your migrations are exactly the same as normal.
In order to get those migrations to work, we need to configure the database connections. We’re going to define everything in the
database.yml just like normal, but with a different naming convention:
db1: development: adapter: mysql2 database: db1_dev username: root test: adapter: mysql2 database: db1_test username: root production: adapter: mysql2 database: db1_prod username: root db2: development: adapter: mysql2 database: db2_dev username: root test: adapter: mysql2 database: db2_test username: root production: adapter: mysql2 database: db2_prod username: root
Here we are configuration two separate databases
We need to configure our app to load these now. I typically do this in boot.rb, but if you’re using Rails this may go in your
application.rb or environment file(s).
ENV['ENV'] ||= 'development' db_conf = YAML::load(File.open(File.join(APP_PATH,'config','database.yml'))) DB1_CONF = db_conf["db1"][ENV['ENV']] DB2_CONF = db_conf["db2"][ENV['ENV']]
So here, let’s take a look at what’s going on:
- We set the database configuration to use. Rails users can just use Rails.env here instead of
- Second we load up the database.yml config and parse it with YAML
- Lastly, we grab the configuration from the file for each db and the correct environment that we’re running in.
Connecting Your Models
When you’re working with multiple databases, I like to explicitly setup the connections inside the models themselves instead of inheriting from ActiveRecord::Base and using subclasses.
class Message < ActiveRecord::Base establish_connection DB1_CONF end
And our second model in another database:
class User < ActiveRecord::Base establish_connection DB2_CONF end
That's pretty much as simple as it is. All you really need to do is load the configurations, establish the database connections properly, and setup the migrations to load from a specific folder for each database.
I'm sure there are better ways of handling this, so if you have a suggestion, please let me know in the comments!