I wanted to generate an index of metadata from posts in Jekyll. The field that I wanted to index wasn’t compulsory, it was an additional one that I sometimes set, and sometimes didn’t.

That’s trivial to do by mapping it:

{{posts | map: 'new_field' | uniq}}

However, that always produced one nil in the results.

That can easily be handled by adding an if but I didn’t want to have to add it every time I wanted to generate this index.

Instead, I wanted it to not appear in the first place.

Unfortunately, liquid does not have a where not filter, only a where. Fortunately, it’s simple to add a filter in Jekyll:

# _plugins/compact.rb

module Jekyll
  module Compacter
    module CompacterFilter
      def compact(input)
        return input.compact
      end
    end
  end
end

Liquid::Template.register_filter(Jekyll::Compacter::CompacterFilter)

With this what I can do is:

{{posts | map: 'new_fields' | uniq | compact}}

And the result wont have any nils. Much cleaner then having to if every time.