Is there anyway to define defaults for multidimensional attribute trees in Chef?

Typical use case of default attributes

Recipe attributes:

default['human']['jack']['arms'] = 2
default['human']['jack']['legs'] = 2
default['human']['jack']['heads'] = 1

In node/role:

override['human']['jack']['legs'] = 1

Also in recipe:

node.override['human']['jack']['legs'] = 1

My use case with dynamic collections

So what if my recipe doesn't know Jack will exist for a node/role and I want a dynamic collection for lots of entries. What is a good strategy to define or merge in defaults?

I don't want to suggest a solution so I'll use a made up wildcard for my example where Jack and Jill are different but I don't have to define defaults (like two arms and a head) every time I define a new instance.

Recipe attributes:

default['human'][*] = { 
    "arms" => 2,
    "legs" => 2,
    "heads" => 1
}

In node/role:

default['human']['jack'] = { 
    "legs" => "1"
}
default['human']['jill'] = { 
    "superpower" => "flying"
}

Solution

Yes, chef attributes use deep merge and you can do this in the recipe, see http://docs.opscode.com/essentials_node_object_deep_merge.html

Do this in the recipe:

node['nginx']['sites'].each_key do |site|
  node.default['nginx']['sites'][site] = node['nginx']['site_defaults']
end
log JSON.pretty_generate(node['nginx']['sites'])

Cookbook attributes:

default['nginx']['site_defaults']['listen'] = [80]
default['nginx']['site_defaults']['location'] = '/'
default['nginx']['site_defaults']['index'] = ['index.html','index.htm']
default['nginx']['sites']['api']['index'] = 'api.cgi'

In node/role:

"nginx" : {
  "sites" : {
    "blog" : {
      "location" : "/blog/",
      "listen" : [443]
    },
    "wiki" : {
      "index" : ["index.php"] 
    }
  }
},

Produces

  * log[{
  "api": {
    "listen": [
      80
    ],
    "location": "/",
    "index": [
      "index.html",
      "index.htm"
    ]
  },
  "blog": {
    "listen": [
      443
    ],
    "location": "/blog/",
    "index": [
      "index.html",
      "index.htm"
    ]
  },
  "wiki": {
    "listen": [
      80
    ],
    "location": "/",
    "index": [
      "index.php"
    ]
  }
}] action write

Note that if you do this, the recipe default has higher precedence over the attributes default so the api index was overridden. The following would work.

force_default['nginx']['sites']['api']['index'] = 'api.cgi'

Also be very careful of Arrays, sometimes they are merged other times replaced (above).