Index: field.api.php
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RCS file: /cvs/drupal/drupal/modules/field/field.api.php,v
retrieving revision 1.71
diff -r1.71 field.api.php
1668a1669,1685
> 
> /**
>  * Allows a module to throw an Exception if they object to a 
>  * field definition update.
>  *
>  * @param $field
>  *   The updated field structure that is attempting to be saved.
>  * @param $prior_field
>  *   The field structure that is currently being used.
>  * @param $has_data
>  *   If the field has data in the storage
>  */
> function hook_field_update_forbid($field, $prior_field, $has_data) {
>   if ($has_data && $field['columns'] != $prior_field['columns']) {
>     throw new FieldUpdateForbiddenException("field_sql_storage cannot change the schema for an existing field with data.");
>   }
> }