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Some of you might find YAML’s merge key convenient, especially in your database.yml files within Rails. The merge key basically mixes two hashes together, just like the Hash#merge method. You just place a << key in a hash with a value which is a hash or an array of hashes.

 death masks are?:
   sad: 2
   <<: {magnificent: 4}

Is just like:

 death masks are?:
   sad: 2
   magnificent: 4

So, if you are using the same login for different databases (which is likely very common):

 login: &login
   adapter: mysql
   host: localhost
   username: portly
   password: 666HELL666

 development:
   database: rails_dev
   <<: *login

 test:
   database: rails_test
   <<: *login

 production:
   database: rails_prod
   <<: *login

The login anchor is set up first and then its anchor is used throughout. I don’t think having a login entry in the file does any harm, does it?