Support Multiple Databases with Ruby on Rails

Add a new DB configuration

config/database.yml

my_app_development:
    adapter: mysql2
    encoding: utf8
    reconnect: false
    database: my_app_development
    pool: 5
    username: root
    password: my_secret
    socket: /tmp/mysql.sock

Add the test DB my_app_test and production DB my_app_production configuration similar as above

For the models requiring the non-default databases

class Member < ActiveRecord::Base
   establish_connection "my_app_#{RAILS_ENV}"
end

Override the DB table name in Ruby on Rails

Define a model Member to access the legacy DB table t_members

class Member < ActiveRecord::Base
  set_table_name :t_members
end

Use a legacy primary key name for a DB table in Ruby on Rails

Override the default primary key column name id with the primary key name member_id of a legacy table

class Member < ActiveRecord::Base
  set_primary_key :member_id
end

Use a non-integer for the primary key

Add a column to be used as primary key in the migration file

create_table :groups, {:id => false} do |t|
  t.string :gp_id
  ...
end
execute "ALTER TABLE groups ADD PRIMARY KEY (gp_id);"

Set the primary key name manually

class Group < ActiveRecord::Base
  set_primary_key :gp_id
  ...
end

Access to legacy DB column name

class Member < ActiveRecord::Base
 alias_attribute :name, :legacy_member_name
end

@member.name or @member.name = 'John' will access the DB column legacy_member_name

Associate a new DB table using Ruby on Rails with a Legacy DB table

A new DB table is referencing a legacy DB table

Create a new model (Member)
```
% rails generate model Member ...
```
Run the migration code to create the new DB table
```
rake db:migrate
```
Create a new model class for the legacy DB table (groups)
app/models/company.rb
```
class Group < ActiveRecord::Base
  has_many :members
end
```
Add the association in Member
```
class Member < ActiveRecord::Base
   belongs_to :group, :primary_key => :g_id
end
```
- Use :primary_key to define the primary key name of the legacy DB table
  - :g_id is the primary key for the legacy DB table group

Access the data with the association

    @group = Member.find(params[:id]).group

    @members = Group.find(params[:id]).members

A legacy DB table is referencing another legacy DB table in Ruby on Rails

app/models/member.rb

class Member < ActiveRecord::Base
   belongs_to :group, :foreign_key => :g_fkid, :primary_key => :g_id
end

:g_fkid is the foreign key column name in the legacy DB table members
:g_id is the primary key for the legacy DB table group

Ruby on Rails 3 Model Working with Legacy Database