function db_change_field

8.x db_change_field($table, $field, $field_new, $spec, $keys_new = array())

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('bar')

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('bar')));

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

20 calls to db_change_field()
block_update_8002 in drupal/core/modules/block/block.install
Replace serial role IDs with machine name strings.
block_update_8003 in drupal/core/modules/block/block.install
Increase {block}.title length to 255 characters.
comment_update_8000 in drupal/core/modules/comment/comment.install
Renames {comment}.language to {comment}.langcode.
comment_update_8001 in drupal/core/modules/comment/comment.install
Create a UUID column for comments.
comment_update_8002 in drupal/core/modules/comment/comment.install
Make *id fields unsigned.

... See full list


drupal/core/includes/, line 889
Core systems for the database layer.


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