code: def _build_projection_expression(clean_table_keys):
      
      projection_expression = ''
      for key in clean_table_keys[:-1]:
          projection_expression += ('{},').format(key)
      projection_expression += clean_table_keys[-1]
      return projection_expression
docs: Given cleaned up keys, this will return a projection expression for
      the dynamodb lookup.
  
      Args:
          clean_table_keys (dict): keys without the data types attached
  
      Returns:
          str: A projection expression for the dynamodb lookup.