Migrations with a Rails multiple database configuration

Question

I'm developing a Rails application in which I will use 2 different databases. Namely one for storing user credentials and another one for storing other type of data. I've setup my database.yml file to accept 2 different databases, for each environment. It looks like this:

<% %w(development test production).each do |env| %>
data_<%= env %>:
  adapter: sqlite3
  database: db/data/<%= env %>.sqlite3
  pool: 5
  timeout: 5000

users_<%= env %>:
  adapter: sqlite3
  database: db/users/<%= env %>.sqlite3
  pool: 5
  timeout: 5000
<% end %>

I also created to 2 level models, one for each database connection, namely:

class UsersBase < ActiveRecord::Base
  establish_connection "users_#{RAILS_ENV}"
end

and

class DataBase < ActiveRecord::Base
  establish_connection "data_#{RAILS_ENV}"
end

I've some issues while generating a model, because it couldn't find the standard "development" database but I've solved that by running the generate model with the --parent option set to the appropriate class.

However, now I have a problem while trying to run rake db:migrate. The rake task doesn't find the default development database (which is by convention should be named development in database.yml). My question is if it is possible to somehow supply this as a parameter to the rake task, or even better to signal Rails somewhere in the configuration script that there are 2 databases, or that the default name of the development database is changed?

ffoeg · Accepted Answer · 2011-11-09 18:08:32Z

1

You are defining these databases:

data_development
users_development
data_test
users_test
data_production
users_production

Rails is looking first for database defns that match the environment name (development, test, production). For you data dbs, just use those names and the data db will be migrated.

try

<% %w(development test production).each do |env| %>
  <%= env %>:
  adapter: sqlite3
  database: db/data/<%= env %>.sqlite3
  pool: 5
  timeout: 5000

users_<%= env %>:
  adapter: sqlite3
  database: db/users/<%= env %>.sqlite3
  pool: 5
  timeout: 5000
<% end %>

all "data_X" dbs will just carry the name of the environment. This is what Rails expects and thus migrations will work on them.

That might leave you out in the cold though w/respect to migrating the users dbs. I don't think Rails migrations were intended to be used this way - migrating 2 dbs in one environment.

That said, I would try it out. Here is the model change needed to align with the database.yml change above:

class DataBase < ActiveRecord::Base
end

In fact you can do away with DataBase altogether and just have former DataBase models extend ActiveRecord::Base directly.

Interested to see what you find if you try this out.

answered Nov 9, 2011 at 18:08

ffoeg

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4 Comments

ffoeg Over a year ago

Afterthought.. why do this database split thing at all? One app that manages 2 separate databases is possible but steps way outside the bounds of the "Rails Conventions". Sure it's possible, but not without the penalty of pain and maintenance headaches. Honestly, if one of my developers proposed this scenario I would send them back to the drawing board.

Victor Blaga Over a year ago

The thing is that the Rails app is part of a much larger system, that involves other components like crawlers, data analysis, system that compile statistics etc... The User database will be practically shared thorugh all the components of the system (namely one could create users outside the Rails web app) and the "data" database will only be used for serving the web side content. By this means i would very much like to keep them separated...

Victor Blaga Over a year ago

Although maybe it would be better to somehow copy the relevant part of the user data from the user database table in the web application database and use that for logging in and out - however maybe in this case I would have synchronization problems...

ffoeg Over a year ago

Sound to me like the user database should be a service that is used by these systems. API or the like. If the user database just captures identity, authentication, and a bit more like roles, I would make an ldap service available for all to consult. The path you have put yourself on introduces pain that never goes away.

sethcall · Accepted Answer · 2012-07-31 11:14:51Z

0

I wrote pg_migrate for this use-case. You can define your schemas outside of your Rails app with this project, but still natively integrate it with your Rails app (pg_migrate can output a 'schema gem', that is just your schemas and a class/cli to migrate it). Usage of this does imply dropping db:migrate as part of your development process.

The web app doesn't necessarily 'own' the schema. Also, you may program in multiple languages.

Postgres-only, though.

answered Jul 31, 2012 at 11:14

sethcall

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