Sets the relation_type property to relation_type,
such as GCONTACT_EXTERNAL_ID_NETWORK or
GCONTACT_EXTERNAL_ID_ACCOUNT.
If relation_type is null, the relation type will be unset. When the
GContactExternalID is used in a query, however, exactly one of
relation_type and
label must be null.
| this | |
| relation_type |
the new type for the external ID, or |