creating AR access to an external, readonly database

Based on excellent suggestions by Rob Biedenharn
<Rob@AgileConsultingLLC.com> in

  http://www.ruby-forum.com/topic/213083

...here's a recipe for setting up access to an external readonly
database.

Motivation: My application has several very large tables that are
constant (from the point of my Rails app). In fact, there are external
scripts that will occasionally update the large tables, but such updates
are infrequent. I want my Rails app to access but not modify these
tables.

Overall approach: we create an external database ('myapp_external') and
a database user ('readonly_user') that is granted only SELET privileges
on the database. We then create a subclass of ActiveRecord
('ReadonlyDB') that connects to the database through readonly_user, so
any attempts to modify the database will raise an error.

=== Step 1: extend config/database.yml with a new entry specifying a
database named 'myapp_external' and a username of "readonly_user". For
mysql, it might look like this:

readonly_db:
  adapter: mysql
  encoding: utf8
  reconnect: false
  database: myapp_external # see note 1
  pool: 5
  username: readonly_user
  password: readonly_pass # see note 2
  socket: /tmp/mysql.sock

[note 1]: You could, in fact specify "myapp_development" or
"myapp_production", in which case you'd have a readonly window onto your
development or production database. In our case, we want a separate
database altogether.

[note 2]: Naturally, you can pick any username and password that you
want for the readonly user.

=== Step 2: create the external database and readonly user.

% mysql -uroot -pxxxxx

create database myapp_external;

Query OK, 1 row affected (0.15 sec)

CREATE USER 'readonly_user'@'localhost' IDENTIFIED BY

'readonly_pass';
Query OK, 0 rows affected (0.82 sec)

GRANT SELECT ON *.* TO 'readonly_user'@'localhost' IDENTIFIED BY

'readonly_pass';
Query OK, 0 rows affected (0.00 sec)