soaparray.rb

# extensions
# …
  module SOAPArray
    def to_soap_array(item_name = "messageId")
      result=Array.new
      self.each do |val|
        result.push({item_name => val.to_i})
      end
      result
    end
  end

  Array.send :include, SOAPArray
# …

# client
  class Client
#   …
    def get_messages
      do_request("GetMessagesRequest").message.presence || Hashie::Mash.new
    end

    def received_messages(ids)
      do_request "ReceivedMessagesRequest", ids.to_soap_array
    end

    private
    def do_request(method, body = nil)
      response_hash = @client.request(:wsdl, method) do
        soap.body = body
      end.to_hash
      Hashie::Mash.new(response_hash)
    end

  end

client = Client.new
client.received_messages([14,2,3])

#  produces XML like this:
# <?xml version="1.0" encoding="UTF-8"?>
# <env:Envelope xmlns:wsdl="somewebserviceprovider.de" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:env="http://schemas.xmlsoap.org/soap/envelope/">
#  <env:Body>
#    <wsdl:ReceivedMessagesRequest>messageId14messageId2messageId3</wsdl:ReceivedMessagesRequest>
#  </env:Body>
#</env:Envelope>


# correct XML should be:
# <?xml version="1.0" encoding="UTF-8"?>
# <env:Envelope xmlns:wsdl="somewebserviceprovider.de" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:env="http://schemas.xmlsoap.org/soap/envelope/">
#   <env:Body>
#     <wsdl:ReceivedMessagesRequest>
#       <messageId>14</messageId>
#       <messageId>2</messageId>
#       <messageId>3</messageId>
#     </wsdl:ReceivedMessagesRequest>
#   </env:Body>
# </env:Envelope>

there you go:

client.request :wsdl, "ReceivedMessagesRequest" do
  soap.body = { :message_id => [14, 2, 3] }
end

works perfect, thx for your help.

you're welcome. btw. if you're only using using the body method inside the request block, you could also use a somewhat "secret" little shortcut: https://github.com/rubiii/savon/blob/v0.8.5/lib/savon/client.rb#L112

client.request :wsdl, "ReceivedMessagesRequest", :body => { :message_id => [14, 2, 3] }