class Restforce::Client

def upsert!(sobject, field, attrs)

Raises an error if something bad happens.
Returns the Id of the newly created record if the record was created.
Returns true if the record was found and updated.

See .upsert 
def upsert!(sobject, field, attrs)
  external_id = attrs.has_key?(field.to_sym) ? attrs.delete(field.to_sym) : attrs.delete(field.to_s)
  response = api_patch "sobjects/#{sobject}/#{field.to_s}/#{external_id}", attrs
  (response.body && response.body['id']) ? response.body['id'] : true
end