Several more or less hacky ways to generate filter parameters as described in the JSON API recommendation (http://jsonapi.org/recommendations/). It looks as if using “filter[name]=value” directly is incompatible with the URI Template specification documented in RFC6570, but “filter%5Bname%5D=value” works. Unsure if that’s expected or not. The Fi…

filter.rb

require 'addressable/template'

class Filter::RawParams
  def initialize(params)
    @params = params
  end

  def encode_value(value)
    Addressable::URI.encode_component(value, Addressable::URI::CharacterClasses::UNRESERVED)
  end

  def to_s
    @params.reduce([]) do |param,(key,val)|
      if val.is_a?(Array)
        value = val.map { |v| encode_value(v) }.join(',')
      else
        value = encode_value(val)
      end

      param << "filter[#{key}]=#{value}"
    end.join('&')
  end
end

template.rb

require 'addressable/template'

class Filter::Template < Addressable::Template

  MAPPER = -> (k) { "filter_.#{k}._" }

  def expand(mapping, processor=nil)
    if mapping.key?(:filter)
      mapping[:filter] = Hash[mapping[:filter].map { |k,v| [MAPPER.call(k), v] }]
    end

    result = self.pattern.dup
    mapping = normalize_keys(mapping)
    result.gsub!( EXPRESSION ) do |capture|
      transform_capture(mapping, capture, processor)
    end
    result.gsub!('_.', '[')
    result.gsub!('._', ']')

    return Addressable::URI.parse(result)
  end
end

template_builder.rb

require 'addressable/template'

class Filter::TemplateBuilder
  def initialize(params)
    @params = params
  end
  
  MAPPER = -> (k) { "filter[#{k}]" }
  
  def to_s
    Addressable::Template('{?filter*}').expand({
      :filter => Hash[@params.map { |k,v| [MAPPER.call(k), v] }]
    }).to_s
  end
end