More complex inheritance in YAML?

How about this? Use multiple anchors.

database: &default
  server: &server
    ip: 192.168.1.5
    port: 2000
  db_name: test
  user: &user
    name: root
    password: root

foo_database:
  <<: *default
  server:
    << : *server
    port: 2001
  db_name: foo
  user:
    << : *user
    password: foo_root

It's just a little extra work, and slightly harder to read than if what you wanted were built into YAML as you suggested (I thought it would work that way too). But overall not bad.

Unfortunately, you can't get the kind of "inheritance" you want to achieve because YAML's "inheritance" is more like a form of "merging hashes".

Expanding out your configuration at the point you use the *default alias, you have:

foo_database:
  server:
    ip: 192.168.1.5
    port: 2000
  db_name: test
  user: 
    name: root
    password: root

If you use hashes with the same keys afterwards, they will completely overwrite the hashes declared earlier, leaving you with (excuse the formatting):

foo_database:

  server:
    ip: 192.168.1.5
    port: 2000
  db_name: test
  user: 
   name: root
   password: root  

  server:
    port: 2001
  db_name: foo
  user:
    password: foo_root

So, in your case, it would seem that since the config is not exactly the same, DRYing up your configuration using anchors and aliases probably isn't the right approach.

More references on this issue below:

• Rake, YAML and Inherited Build Configuration
• Merging hashes in yaml conf files

Edit

If you really wanted to, I think you could reconfigure your YAML as below to get exactly what you want, but in your case, I would say the extra obfuscation isn't worth it:

server_defaults: &server_defaults
  ip: 192.168.1.5
  port: 2000

user_defaults: &user_defaults
  name: root
  password: root

database: &default
  server:
    <<: *server_defaults
  db_name: test
  user: 
    <<: *user_defaults

foo_database:
  <<: *default
  server:
    <<: *server_defaults
    port: 2001
  db_name: foo
  user:
    <<: *user_defaults
    password: foo_root