how to create model from different databases?

I just started learning ROR. And came across this DOUBT.

I have defined multiple databases in my

database.yml

file. Like this

development:
  adapter: mysql2
  encoding: utf8
  reconnect: true
  database: mydb1
  username: root
  password: root
  host: localhost

custom:
  adapter: mysql2
  encoding: utf8
  reconnect: true
  database: customdb
  username: root
  password: root
  host: localhost

Now if i create a model usig rails generate model MyModel it will create a migration script to generate a table called my_modles in mydb1.

My question is "How can i create/specify a model, that to create a table under customdb?"

Do i need to change my rails environment or is there are any other ways. I am just curious. I can do this manually by creating a table in customdb and creating a model without migration.