function db_change_field

Changes a field definition.

IMPORTANT NOTE: To maintain database portability, you have to explicitly recreate all indices and primary keys that are using the changed field.

That means that you have to drop all affected keys and indexes with db_drop_{primary_key,unique_key,index}() before calling db_change_field(). To recreate the keys and indices, pass the key definitions as the optional $keys_new argument directly to db_change_field().

For example, suppose you have:

$schema['foo'] = array(
  'fields' => array(
    'bar' => array(
      'type' => 'int',
      'not null' => TRUE,
  'primary key' => array(

and you want to change to be type serial, leaving it as the primary key. The correct sequence is:

db_change_field('foo', 'bar', 'bar', array(
  'type' => 'serial',
  'not null' => TRUE,
), array(
  'primary key' => array(

The reasons for this are due to the different database engines:

On PostgreSQL, changing a field definition involves adding a new field and dropping an old one which causes any indices, primary keys and sequences (from serial-type fields) that use the changed field to be dropped.

On MySQL, all type 'serial' fields must be part of at least one key or index as soon as they are created. You cannot use db_add_{primary_key,unique_key,index}() for this purpose because the ALTER TABLE command will fail to add the column without a key or index specification. The solution is to use the optional $keys_new argument to create the key or index at the same time as field.

You could use db_add_{primary_key,unique_key,index}() in all cases unless you are converting a field to be type serial. You can use the $keys_new argument in all cases.


$table: Name of the table.

$field: Name of the field to change.

$field_new: New name for the field (set to the same as $field if you don't want to change the name).

$spec: The field specification for the new field.

$keys_new: (optional) Keys and indexes specification to be created on the table along with changing the field. The format is the same as a table specification but without the 'fields' element.

Related topics

57 calls to db_change_field()
aggregator_update_7003 in drupal/modules/aggregator/aggregator.install
Increase the length of {aggregator_feed}.url.
block_update_7003 in drupal/modules/block/block.install
Change the weight column to normal int.
block_update_7005 in drupal/modules/block/block.install
Update the {block_custom}.format column.
block_update_7007 in drupal/modules/block/block.install
Change {block_custom}.format into varchar.
block_update_7009 in drupal/modules/block/block.install
Increase {block}.title length to 255 characters.

... See full list


drupal/includes/database/, line 3075
Core systems for the database layer.


function db_change_field($table, $field, $field_new, $spec, $keys_new = array()) {
  return Database::getConnection()
    ->changeField($table, $field, $field_new, $spec, $keys_new);