Faraday PreserveKeysEncoder: Only decodes url parameter values and leaves keys intact.

preserve_keys_encoder.rb

## PreserveKeysEncoder doesn't encode parameter keys (which is standard Faraday behaviour).
## Usage:
## Faraday.new(url, request: { params_encoder: PreserveKeysEncoder.new })
## Modes:
##  :nested { '$product_id' =>' [10,20,30] } becomes $product_id[1]=10&$product_id[1]=20&$product_id[1]=30
##  :flat and $product_id=10&$product_id=20&$product_id=30 in :flat mode
class Faraday::PreserveKeysEncoder
  MODES = [:nested, :flat]

  delegate :decode, :escape, to: :super_encoder, allow_nil: true
  attr_reader :mode
  MODES.each{ |mode_name| define_method("#{mode_name}?"){ mode == mode_name } }

  def initialize(mode: :nested)
    @mode = mode.intern
  end

  def encode(params)
    return nil if params.nil?
    params.to_a.map{ |param| param_to_string(param) }.compact.join('&')
  end

  private

  def super_encoder
    case mode
    when :flat then Faraday::FlatParamsEncoder
    when :nested then Faraday::NestedParamsEncoder
    end
  end

  def param_to_string(param)
    key, value = param # Input should be [key, param].
    case value
    when nil, '' then nil
    when Array
      value.each_with_index.map do |e, i|
        sub_key = nested? ? "#{key}#{[i+1]}" : key
        param_to_string([sub_key, e])
      end.join('&')
    else "#{key}=#{escape(value)}"
    end
  end
end