This assume you are using rails 3.0 or above, Warbler, and Tomcat 7.0 above

1. Download latest log4j.jar and put it in your rails project’s lib directory
2. Put your log4j.properties file in the rails root directory
3. Add this line in warble.rb: config.java_classes = FileList["log4j.properties"]
4. Add this line in warble.rb:  config.webxml.jruby.rack.logging = ‘log4j’

Explaination:

Step 2 and 3 will allow warbler to put log4j.properties into the WEB-INF/classes directory

Step 4 will make sure jruby-rack using log4j for logging.

If you run into any issue, check your log4j.properties.

Here is the standard one I use:

log4j.rootLogger=INFO, R

log4j.appender.R=org.apache.log4j.RollingFileAppender
log4j.appender.R.File=${catalina.base}/logs/production.log
log4j.appender.R.MaxFileSize=10000KB
log4j.appender.R.MaxBackupIndex=5
log4j.appender.R.layout=org.apache.log4j.PatternLayout
log4j.appender.R.layout.ConversionPattern=%d [%t] %-5p %c - %m%n


development:

adapter: sqlite3
database: db/development.sqlite3
timeout: 5000

second_development:

adapter: sqlite3
database: db/second_development.sqlite3
timeout: 5000

test:

adapter: sqlite3
database: db/test.sqlite3
timeout: 5000

second_test:

adapter: sqlite3
database: db/second_test.sqlite3
timeout: 5000

production:

adapter: sqlite3
database: db/production.sqlite3
timeout: 5000

second_production:

adapter: sqlite3
database: db/second_production.sqlite3
timeout: 5000

migration for models in first database is like normal migration, example (xxxxxxx_create_user.rb):

def self.up

create_table “users” do |t|

t.string “password”
t.string “email”

t.timestamps

end

end

def self.down

drop_table :users

end

migration for models in second database require slightly modification, example (xxxxxxx_create_dogs.rb)

def self.up

ActiveRecord::Base.establish_connection “second_#{RAILS_ENV}”
create_table :dogs do |t|
t.string :name, :null=>falset.timestamps
end
ActiveRecord::Base.establish_connection “#{RAILS_ENV}”

end

def self.down

ActiveRecord::Base.establish_connection “second_#{RAILS_ENV}”
drop_table :users
ActiveRecord::Base.establish_connection “#{RAILS_ENV}”

end

Note that schema_migration table is located in the first database (default one) only in this case.

Models for the first (default database) is the same as regular rails model.

Models for the second database requires adding the following line in the Models:
establish_connection “second_#{RAILS_ENV}”
Example:

class Dog < ActiveRecord::Base
establish_connection “second_#{RAILS_ENV}”
end

That is it.  Not that complicated.

-module(db).
-export([new/0,destroy/1,write/3,delete/2,read/2,match/2]).
new()->Db=[],
Db.

destroy(Db)->_=Db,
ok.

write(Key,Element,Db)->NewDb = [{Key,Element}|delete(Key,Db)],
NewDb.

delete(Key,Db)->case Db of
[H|T] ->
case H of
{Key,_} -> T;
_-> [H|delete(Key,T)]
end;
[] ->Db
end.

read(Key,Db)->case Db of
[H|T] ->
case H of
{Key,Element} ->Element;
_->read(Key,T)
end;
[] ->{error,instance}
end.
match(Element,Db)->case Db of
[H|T] ->
case H of
{Key,Element}->[Key|match(Element,T)];
_ ->match(Element,T)
end;
[] ->Db
end.
To use it:

1> Db = db:new().
[]
2> Db1 = db:write(book,elrang,Db).
[{book,elrang}]
3> Db2 = db:write(author,francesco,Db1).
[{author,francesco},{book,elrang}]
5> db:read(author,Db2).
francesco
6> db:match(elrang,Db2).
[book]

Not very useful but it shows how to write simple program in erlang.