def SELECT_ONE(src_column):
  """
  Builtin aggregator for groupby which selects one row in the group.

  Example: Get one rating row from a user.

  >>> sf.groupby("user",
  ...            {'rating':tc.aggregate.SELECT_ONE('rating')})

  If multiple columns are selected, they are guaranteed to come from the
  same row. for instance:
  >>> sf.groupby("user",
  ...            {'rating':tc.aggregate.SELECT_ONE('rating')},
  ...            {'item':tc.aggregate.SELECT_ONE('item')})

  The selected 'rating' and 'item' value for each user will come from the
  same row in the SFrame.

  """
  return ("__builtin__select_one__", [src_column])