Empty JSON response when using cache and collection contains only one element

[ryreitsma]

If caching is on and the collection which is rendered contains only one element, I get an empty JSON repsonse in some cases. I have these three templates in my Rails 3.2 application:

message.rabl

object @message
cache @message

attribute :id, :url, :created_at

node :file_shares do |message|
  partial "drive/file_shares/file_shares", object: message.file_shares
end

file_shares.rabl

collection @file_shares, object_root: false
cache @file_shares

extends "drive/file_shares/file_share"

file_share.rabl

object @file_share
cache @file_share

attribute :url, :file_name, :file_type

The message.rabl template is always rendered first. This works fine, I get the json reponse including a node containing one file_share. However, if the file_shares.rabl is rendered in the next request, I get an empty json response.
When the file_shares.rabl is rendered, the extends method gets a cache hit on:

rabl/drive/file_shares/1012-20130620132506//hash

However, the result of that cache hit is an array containing a hash of the file_share. But an array is not what the extends method expects:

lib/rabl/builder.rb:118:

# Extends an existing rabl template with additional attributes in the block
# extends("users/show") { attribute :full_name }
def extends(file, options={}, &block)
  options = @options.slice(:child_root).merge(:object => @_object).merge(options)
  result = self.partial(file, options, &block)
  @_result.merge!(result) if result.is_a?(Hash)
end

The result from self.partial is not a hash, which results in an empty response. However, I think the cause of this bug is in the construction of the cache key in the Rabl engine:

lib/rabl/engine.rb:277

result_cache_key = Array(cache_key) + [@_options[:root_name], @_options[:format]]

The cache_key variable contains either an object or a collection of objects. However, by using Array(cache_key) the result_cache_key for one object is equal to the result_cache_key for the same object used as the sole object in an array.

file_share = FileShare.create
result_cache_key_1 = Array(file_share) + [@_options[:root_name], @_options[:format]]

result_cache_key_2 = Array([file_share]) + [@_options[:root_name], @_options[:format]]

result_cache_key_1 == result_cache_key_2

A possible solution could be to add information about whether the cache key is for an object or for a collection into the result_cache_key:

result_cache_key = [cache_key, cache_key.class, @_options[:root_name], @_options[:format]]

I don't know how to create a failing test case for this in the Rabl test suite. If anyone can, I'd love to help solve this issue.