respond_to causes undefined method `to_sym' for [:json, :html]:Array

Question

In Rails 3.2.8 in Ruby 1.9.3p194 when passing an array to respond_to in an included block of an ActiveSupport::Concern that is included on demand in the class definition by calling an acts_as_... method in module included in a gem results in a:

respond_to causes undefined method `to_sym' for [:json, :html]:Array

and on the next request, I get:

RuntimeError (In order to use respond_with, first you need to declare the formats your controller responds to in the class level):
  actionpack (3.2.8) lib/action_controller/metal/mime_responds.rb:234:in `respond_with'

In the module code, it is just doing the equivalent of:

formats = [:json, :html]
respond_to formats

Where formats is configured elsewhere so it can be applied to all controllers that specify acts_as_....

I know it works when I do this in a class definition:

respond_to :json, :html

So, how can I call respond_to with a variable that is an array of formats?

Answer 1

The related parts of the respond_to method in Rails 3.2.8 are:

def respond_to(*mimes)
  # ...
  mimes.each do |mime|
    mime = mime.to_sym
    # ...
  end
  # ...
end

Because the splat operator is used in respond_to, it is wrapping the incoming array in an array, so mimes is [[:json, :html]], and it is trying to call to_sym on the array.

If calling respond_to with a variable containing an array, you need to use the splat (*) operator, e.g.:

formats = [:json, :html]
respond_to *formats

That will call respond_to as if you were sending in two arguments:

respond_to :json, :html

or:

respond_to(:json, :html)