example.rb

require 'faraday'
require 'multi_json'

# set API key in `MERK_API_TOKEN`
MerkSource.new('cz').find_by_registration_no('28897501')
MerkSource.new('sk').find_by_registration_no('31398871')

merk_source.rb

# For docs on MERK see: 
#   https://api.merk.cz/docs/#!/suggest/Suggest_list
# This operates on endpoint:
#   https://api.merk.cz:443/suggest/?regno=31398871&country_code=sk
# Created in Fakturoid for public use under MIT licence.

class MerkSource
  attr_reader :country
  
  MERK_API_ENDPOINT = "https://api.merk.cz:443"
  MERK_API_TOKEN = "agaca ..." # or Rails.application.secrets.merk_api_key
  MERK_HEADERS = { accept: 'application/json', user_agent: 'RubyMerkSource' }
  
  def initialize(country)
    @country = country.downcase
  end
  
  def find_by_registration_no(registration_no)    
    response = connection.get "/suggest/", regno: registration_no, country_code: country
    parse_response(response)
  end
  
  def find_by_email(email)
    response = connection.get "/suggest/", email: email, country_code: country
    parse_response(response)
  end
  
  def find_by_name(name)
    response = connection.get "/suggest/", name: name, country_code: country
    parse_response(response)
  end
  
private
  
  def connection
    @connection ||= begin
      conn = Faraday.new url: MERK_API_ENDPOINT, headers: MERK_HEADERS
      conn.authorization :Token, MERK_API_TOKEN
      conn
    end
  end
  
  def parse_response(response)
    case response.env.status
      when 200 then merk_data = MultiJson.load(response.env.body)
      when 204 then return nil
      else 
        raise "Merk request failed with code: #{response.env.status} and error: #{response.env.body.to_s}"
    end
    
    if merk_data.is_a?(Array)
      parse_collection(merk_data)
    else
      parse_entity(merk_data) 
    end
  end

  def parse_collection(entities)  
    entities.map do |entity|
      parse_entity(entity)
    end
  end

  def parse_entity(merk_data)
    subject = {}
    subject[:name]              = merk_data['name']
    subject[:registration_no]   = merk_data['regno'].to_s
    subject[:vat_no]            = merk_data['vatno']
    subject[:registration_date] = merk_data['estab_date'].to_date
    subject[:legal_form]        = merk_data['legal_form_code'].to_s
  
    if merk_data['court'].present?
      subject[:court_name]        = merk_data['court']
      subject[:court_reg_no]      = merk_data['court_file_nr']
    end
  
    subject[:street]  = "#{merk_data['address']['street_fixed']} #{merk_data['address']['number']}".rstrip
    
    if merk_data['address']['municipality_part'].include?(merk_data['address']['municipality'])
      subject[:city]  = merk_data['address']['municipality']
    else
      subject[:city]  = "#{merk_data['address']['municipality']} - #{merk_data['address']['municipality_part']}".rstrip
    end
    
    subject[:zip]     = merk_data['address']['postal_code']
    subject[:country] = merk_data['address']['country_code'].presence || subject[:vat_no].try(:[], 0..1)
    
    subject[:bank_accounts] = merk_data['bank_accounts']
    
    subject
  end
end