How to get categories of a post for an Octopress plugin in class Liquid::Tag

Question

I have this code:

module Jekyll
  class ConnexeTag < Liquid::Tag
    def render(context)
      categories = get_categories(context)
      categories.class.name # => "Array"
      # categories # => "category1category2"
      # categories.join(',') # => Error !
      # categories.size # => Error !
    end

    private

    def get_categories(context)
      context.environments.first["page"]["categories"]
    end
  end
end

It outputs Array, and that's ok. But when I try some methods on categories, like size or each I get this error:

Building site: source -> public
Liquid Exception: undefined method `size' for nil:NilClass in atom.xml
/home/xavier/octopress/plugins/connexe_tag.rb:25:in `render'

I can't apply any methods on categories. Does anybody could tell me what I am doing wrong here ?

Answer 1

Happily, the fix is simple. The problem is that your code assumes every page will have an array of categories. That isn't the case for atom.xml so context.environments.first["page"]["categories"] will return nil which, of course, doesn't have the method 'size'. You can set it to only output if get_categories returns a value and you're all set.

module Jekyll
  class ConnexeTag < Liquid::Tag
    def render(context)
      categories = get_categories(context)

      # return a list of categories for pages which have them
      categories.join(', ') if categories
    end

    private

    def get_categories(context)
      context.environments.first["page"]["categories"]
    end
  end
end


Liquid::Template.register_tag('connexe_tag', Jekyll::ConnexeTag)

That should do it.