schema.inc

<?php


/**
 * @file
 * Generic Database schema code.
 */
require_once dirname(__FILE__) . '/query.inc';

/**
 * @defgroup schemaapi Schema API
 * @{
 * API to handle database schemas.
 *
 * A Drupal schema definition is an array structure representing one or
 * more tables and their related keys and indexes. A schema is defined by
 * hook_schema(), which usually lives in a modulename.install file.
 *
 * By implementing hook_schema() and specifying the tables your module
 * declares, you can easily create and drop these tables on all
 * supported database engines. You don't have to deal with the
 * different SQL dialects for table creation and alteration of the
 * supported database engines.
 *
 * hook_schema() should return an array with a key for each table that
 * the module defines.
 *
 * The following keys are defined:
 *   - 'description': A string in non-markup plain text describing this table
 *     and its purpose. References to other tables should be enclosed in
 *     curly-brackets. For example, the node_revisions table
 *     description field might contain "Stores per-revision title and
 *     body data for each {node}."
 *   - 'fields': An associative array ('fieldname' => specification)
 *     that describes the table's database columns. The specification
 *     is also an array. The following specification parameters are defined:
 *     - 'description': A string in non-markup plain text describing this field
 *       and its purpose. References to other tables should be enclosed in
 *       curly-brackets. For example, the node table vid field
 *       description might contain "Always holds the largest (most
 *       recent) {node_revision}.vid value for this nid."
 *     - 'type': The generic datatype: 'char', 'varchar', 'text', 'blob', 'int',
 *       'float', 'numeric', or 'serial'. Most types just map to the according
 *       database engine specific datatypes. Use 'serial' for auto incrementing
 *       fields. This will expand to 'INT auto_increment' on MySQL.
 *     - 'mysql_type', 'pgsql_type', 'sqlite_type', etc.: If you need to
 *       use a record type not included in the officially supported list
 *       of types above, you can specify a type for each database
 *       backend. In this case, you can leave out the type parameter,
 *       but be advised that your schema will fail to load on backends that
 *       do not have a type specified. A possible solution can be to
 *       use the "text" type as a fallback.
 *     - 'serialize': A boolean indicating whether the field will be stored as
 *       a serialized string.
 *     - 'size': The data size: 'tiny', 'small', 'medium', 'normal',
 *       'big'. This is a hint about the largest value the field will
 *       store and determines which of the database engine specific
 *       datatypes will be used (e.g. on MySQL, TINYINT vs. INT vs. BIGINT).
 *       'normal', the default, selects the base type (e.g. on MySQL,
 *       INT, VARCHAR, BLOB, etc.).
 *       Not all sizes are available for all data types. See
 *       DatabaseSchema::getFieldTypeMap() for possible combinations.
 *     - 'not null': If true, no NULL values will be allowed in this
 *       database column. Defaults to false.
 *     - 'default': The field's default value. The PHP type of the
 *       value matters: '', '0', and 0 are all different. If you
 *       specify '0' as the default value for a type 'int' field it
 *       will not work because '0' is a string containing the
 *       character "zero", not an integer.
 *     - 'length': The maximal length of a type 'char', 'varchar' or 'text'
 *       field. Ignored for other field types.
 *     - 'unsigned': A boolean indicating whether a type 'int', 'float'
 *       and 'numeric' only is signed or unsigned. Defaults to
 *       FALSE. Ignored for other field types.
 *     - 'precision', 'scale': For type 'numeric' fields, indicates
 *       the precision (total number of significant digits) and scale
 *       (decimal digits right of the decimal point). Both values are
 *       mandatory. Ignored for other field types.
 *     - 'binary': A boolean indicating that MySQL should force 'char',
 *       'varchar' or 'text' fields to use case-sensitive binary collation.
 *       This has no effect on other database types for which case sensitivity
 *       is already the default behavior.
 *     All parameters apart from 'type' are optional except that type
 *     'numeric' columns must specify 'precision' and 'scale', and type
 *     'varchar' must specify the 'length' parameter.
 *  - 'primary key': An array of one or more key column specifiers (see below)
 *    that form the primary key.
 *  - 'unique keys': An associative array of unique keys ('keyname' =>
 *    specification). Each specification is an array of one or more
 *    key column specifiers (see below) that form a unique key on the table.
 *  - 'foreign keys': An associative array of relations ('my_relation' =>
 *    specification). Each specification is an array containing the name of
 *    the referenced table ('table'), and an array of column mappings
 *    ('columns'). Column mappings are defined by key pairs ('source_column' =>
 *    'referenced_column'). This key is for documentation purposes only; foreign
 *    keys are not created in the database, nor are they enforced by Drupal.
 *  - 'indexes':  An associative array of indexes ('indexname' =>
 *    specification). Each specification is an array of one or more
 *    key column specifiers (see below) that form an index on the
 *    table.
 *
 * A key column specifier is either a string naming a column or an
 * array of two elements, column name and length, specifying a prefix
 * of the named column.
 *
 * As an example, here is a SUBSET of the schema definition for
 * Drupal's 'node' table. It show four fields (nid, vid, type, and
 * title), the primary key on field 'nid', a unique key named 'vid' on
 * field 'vid', and two indexes, one named 'nid' on field 'nid' and
 * one named 'node_title_type' on the field 'title' and the first four
 * bytes of the field 'type':
 *
 * @code
 * $schema['node'] = array(
 *   'description' => 'The base table for nodes.',
 *   'fields' => array(
 *     'nid'       => array('type' => 'serial', 'unsigned' => TRUE, 'not null' => TRUE),
 *     'vid'       => array('type' => 'int', 'unsigned' => TRUE, 'not null' => TRUE,'default' => 0),
 *     'type'      => array('type' => 'varchar','length' => 32,'not null' => TRUE, 'default' => ''),
 *     'language'  => array('type' => 'varchar','length' => 12,'not null' => TRUE,'default' => ''),
 *     'title'     => array('type' => 'varchar','length' => 255,'not null' => TRUE, 'default' => ''),
 *     'uid'       => array('type' => 'int', 'not null' => TRUE, 'default' => 0),
 *     'status'    => array('type' => 'int', 'not null' => TRUE, 'default' => 1),
 *     'created'   => array('type' => 'int', 'not null' => TRUE, 'default' => 0),
 *     'changed'   => array('type' => 'int', 'not null' => TRUE, 'default' => 0),
 *     'comment'   => array('type' => 'int', 'not null' => TRUE, 'default' => 0),
 *     'promote'   => array('type' => 'int', 'not null' => TRUE, 'default' => 0),
 *     'moderate'  => array('type' => 'int', 'not null' => TRUE,'default' => 0),
 *     'sticky'    => array('type' => 'int', 'not null' => TRUE, 'default' => 0),
 *     'tnid'      => array('type' => 'int', 'unsigned' => TRUE, 'not null' => TRUE, 'default' => 0),
 *     'translate' => array('type' => 'int', 'not null' => TRUE, 'default' => 0),
 *   ),
 *   'indexes' => array(
 *     'node_changed'        => array('changed'),
 *     'node_created'        => array('created'),
 *     'node_moderate'       => array('moderate'),
 *     'node_frontpage'      => array('promote', 'status', 'sticky', 'created'),
 *     'node_status_type'    => array('status', 'type', 'nid'),
 *     'node_title_type'     => array('title', array('type', 4)),
 *     'node_type'           => array(array('type', 4)),
 *     'uid'                 => array('uid'),
 *     'tnid'                => array('tnid'),
 *     'translate'           => array('translate'),
 *   ),
 *   'unique keys' => array(
 *     'vid' => array('vid'),
 *   ),
 *   // For documentation purposes only; foreign keys are not created in the
 *   // database.
 *   'foreign keys' => array(
 *     'node_revision' => array(
 *       'table' => 'node_revision',
 *       'columns' => array('vid' => 'vid'),
 *      ),
 *     'node_author' => array(
 *       'table' => 'users',
 *       'columns' => array('uid' => 'uid'),
 *      ),
 *    ),
 *   'primary key' => array('nid'),
 * );
 * @endcode
 *
 * @see drupal_install_schema()
 */

/**
 * Base class for database schema definitions.
 */
abstract class DatabaseSchema implements QueryPlaceholderInterface {
    
    /**
     * The database connection.
     *
     * @var DatabaseConnection
     */
    protected $connection;
    
    /**
     * The placeholder counter.
     */
    protected $placeholder = 0;
    
    /**
     * Definition of prefixInfo array structure.
     *
     * Rather than redefining DatabaseSchema::getPrefixInfo() for each driver,
     * by defining the defaultSchema variable only MySQL has to re-write the
     * method.
     *
     * @see DatabaseSchema::getPrefixInfo()
     */
    protected $defaultSchema = 'public';
    
    /**
     * A unique identifier for this query object.
     */
    protected $uniqueIdentifier;
    public function __construct($connection) {
        $this->uniqueIdentifier = uniqid('', TRUE);
        $this->connection = $connection;
    }
    
    /**
     * Implements the magic __clone function.
     */
    public function __clone() {
        $this->uniqueIdentifier = uniqid('', TRUE);
    }
    
    /**
     * Implements QueryPlaceHolderInterface::uniqueIdentifier().
     */
    public function uniqueIdentifier() {
        return $this->uniqueIdentifier;
    }
    
    /**
     * Implements QueryPlaceHolderInterface::nextPlaceholder().
     */
    public function nextPlaceholder() {
        return $this->placeholder++;
    }
    
    /**
     * Get information about the table name and schema from the prefix.
     *
     * @param
     *   Name of table to look prefix up for. Defaults to 'default' because thats
     *   default key for prefix.
     * @param $add_prefix
     *   Boolean that indicates whether the given table name should be prefixed.
     *
     * @return
     *   A keyed array with information about the schema, table name and prefix.
     */
    protected function getPrefixInfo($table = 'default', $add_prefix = TRUE) {
        $info = array(
            'schema' => $this->defaultSchema,
            'prefix' => $this->connection
                ->tablePrefix($table),
        );
        if ($add_prefix) {
            $table = $info['prefix'] . $table;
        }
        // If the prefix contains a period in it, then that means the prefix also
        // contains a schema reference in which case we will change the schema key
        // to the value before the period in the prefix. Everything after the dot
        // will be prefixed onto the front of the table.
        if (($pos = strpos($table, '.')) !== FALSE) {
            // Grab everything before the period.
            $info['schema'] = substr($table, 0, $pos);
            // Grab everything after the dot.
            $info['table'] = substr($table, ++$pos);
        }
        else {
            $info['table'] = $table;
        }
        return $info;
    }
    
    /**
     * Create names for indexes, primary keys and constraints.
     *
     * This prevents using {} around non-table names like indexes and keys.
     */
    function prefixNonTable($table) {
        $args = func_get_args();
        $info = $this->getPrefixInfo($table);
        $args[0] = $info['table'];
        return implode('_', $args);
    }
    
    /**
     * Build a condition to match a table name against a standard information_schema.
     *
     * The information_schema is a SQL standard that provides information about the
     * database server and the databases, schemas, tables, columns and users within
     * it. This makes information_schema a useful tool to use across the drupal
     * database drivers and is used by a few different functions. The function below
     * describes the conditions to be meet when querying information_schema.tables
     * for drupal tables or information associated with drupal tables. Even though
     * this is the standard method, not all databases follow standards and so this
     * method should be overwritten by a database driver if the database provider
     * uses alternate methods. Because information_schema.tables is used in a few
     * different functions, a database driver will only need to override this function
     * to make all the others work. For example see includes/databases/mysql/schema.inc.
     *
     * @param $table_name
     *   The name of the table in question.
     * @param $operator
     *   The operator to apply on the 'table' part of the condition.
     * @param $add_prefix
     *   Boolean to indicate whether the table name needs to be prefixed.
     *
     * @return QueryConditionInterface
     *   A DatabaseCondition object.
     */
    protected function buildTableNameCondition($table_name, $operator = '=', $add_prefix = TRUE) {
        $info = $this->connection
            ->getConnectionOptions();
        // Retrieve the table name and schema
        $table_info = $this->getPrefixInfo($table_name, $add_prefix);
        $condition = new DatabaseCondition('AND');
        $condition->condition('table_catalog', $info['database']);
        $condition->condition('table_schema', $table_info['schema']);
        $condition->condition('table_name', $table_info['table'], $operator);
        return $condition;
    }
    
    /**
     * Check if a table exists.
     *
     * @param $table
     *   The name of the table in drupal (no prefixing).
     *
     * @return
     *   TRUE if the given table exists, otherwise FALSE.
     */
    public function tableExists($table) {
        $condition = $this->buildTableNameCondition($table);
        $condition->compile($this->connection, $this);
        // Normally, we would heartily discourage the use of string
        // concatenation for conditionals like this however, we
        // couldn't use db_select() here because it would prefix
        // information_schema.tables and the query would fail.
        // Don't use {} around information_schema.tables table.
        return (bool) $this->connection
            ->query("SELECT 1 FROM information_schema.tables WHERE " . (string) $condition, $condition->arguments())
            ->fetchField();
    }
    
    /**
     * Find all tables that are like the specified base table name.
     *
     * @param $table_expression
     *   An SQL expression, for example "simpletest%" (without the quotes).
     *   BEWARE: this is not prefixed, the caller should take care of that.
     *
     * @return
     *   Array, both the keys and the values are the matching tables.
     */
    public function findTables($table_expression) {
        $condition = $this->buildTableNameCondition($table_expression, 'LIKE', FALSE);
        $condition->compile($this->connection, $this);
        // Normally, we would heartily discourage the use of string
        // concatenation for conditionals like this however, we
        // couldn't use db_select() here because it would prefix
        // information_schema.tables and the query would fail.
        // Don't use {} around information_schema.tables table.
        return $this->connection
            ->query("SELECT table_name AS table_name FROM information_schema.tables WHERE " . (string) $condition, $condition->arguments())
            ->fetchAllKeyed(0, 0);
    }
    
    /**
     * Finds all tables that are like the specified base table name. This is a
     * backport of the change made to findTables in Drupal 8 to work with virtual,
     * un-prefixed table names. The original function is retained for Backwards
     * Compatibility.
     * @see https://www.drupal.org/node/2552435
     *
     * @param string $table_expression
     *   An SQL expression, for example "cache_%" (without the quotes).
     *
     * @return array
     *   Both the keys and the values are the matching tables.
     */
    public function findTablesD8($table_expression) {
        // Load all the tables up front in order to take into account per-table
        // prefixes. The actual matching is done at the bottom of the method.
        $condition = $this->buildTableNameCondition('%', 'LIKE');
        $condition->compile($this->connection, $this);
        $individually_prefixed_tables = $this->connection
            ->getUnprefixedTablesMap();
        $default_prefix = $this->connection
            ->tablePrefix();
        $default_prefix_length = strlen($default_prefix);
        $tables = array();
        // Normally, we would heartily discourage the use of string
        // concatenation for conditionals like this however, we
        // couldn't use db_select() here because it would prefix
        // information_schema.tables and the query would fail.
        // Don't use {} around information_schema.tables table.
        $results = $this->connection
            ->query("SELECT table_name AS table_name FROM information_schema.tables WHERE " . (string) $condition, $condition->arguments());
        foreach ($results as $table) {
            // Take into account tables that have an individual prefix.
            if (isset($individually_prefixed_tables[$table->table_name])) {
                $prefix_length = strlen($this->connection
                    ->tablePrefix($individually_prefixed_tables[$table->table_name]));
            }
            elseif ($default_prefix && substr($table->table_name, 0, $default_prefix_length) !== $default_prefix) {
                // This table name does not start the default prefix, which means that
                // it is not managed by Drupal so it should be excluded from the result.
                continue;
            }
            else {
                $prefix_length = $default_prefix_length;
            }
            // Remove the prefix from the returned tables.
            $unprefixed_table_name = substr($table->table_name, $prefix_length);
            // The pattern can match a table which is the same as the prefix. That
            // will become an empty string when we remove the prefix, which will
            // probably surprise the caller, besides not being a prefixed table. So
            // remove it.
            if (!empty($unprefixed_table_name)) {
                $tables[$unprefixed_table_name] = $unprefixed_table_name;
            }
        }
        // Convert the table expression from its SQL LIKE syntax to a regular
        // expression and escape the delimiter that will be used for matching.
        $table_expression = str_replace(array(
            '%',
            '_',
        ), array(
            '.*?',
            '.',
        ), preg_quote($table_expression, '/'));
        $tables = preg_grep('/^' . $table_expression . '$/i', $tables);
        return $tables;
    }
    
    /**
     * Check if a column exists in the given table.
     *
     * @param $table
     *   The name of the table in drupal (no prefixing).
     * @param $name
     *   The name of the column.
     *
     * @return
     *   TRUE if the given column exists, otherwise FALSE.
     */
    public function fieldExists($table, $column) {
        $condition = $this->buildTableNameCondition($table);
        $condition->condition('column_name', $column);
        $condition->compile($this->connection, $this);
        // Normally, we would heartily discourage the use of string
        // concatenation for conditionals like this however, we
        // couldn't use db_select() here because it would prefix
        // information_schema.tables and the query would fail.
        // Don't use {} around information_schema.columns table.
        return (bool) $this->connection
            ->query("SELECT 1 FROM information_schema.columns WHERE " . (string) $condition, $condition->arguments())
            ->fetchField();
    }
    
    /**
     * Returns a mapping of Drupal schema field names to DB-native field types.
     *
     * Because different field types do not map 1:1 between databases, Drupal has
     * its own normalized field type names. This function returns a driver-specific
     * mapping table from Drupal names to the native names for each database.
     *
     * @return array
     *   An array of Schema API field types to driver-specific field types.
     */
    public abstract function getFieldTypeMap();
    
    /**
     * Rename a table.
     *
     * @param $table
     *   The table to be renamed.
     * @param $new_name
     *   The new name for the table.
     *
     * @throws DatabaseSchemaObjectDoesNotExistException
     *   If the specified table doesn't exist.
     * @throws DatabaseSchemaObjectExistsException
     *   If a table with the specified new name already exists.
     */
    public abstract function renameTable($table, $new_name);
    
    /**
     * Drop a table.
     *
     * @param $table
     *   The table to be dropped.
     *
     * @return
     *   TRUE if the table was successfully dropped, FALSE if there was no table
     *   by that name to begin with.
     */
    public abstract function dropTable($table);
    
    /**
     * Add a new field to a table.
     *
     * @param $table
     *   Name of the table to be altered.
     * @param $field
     *   Name of the field to be added.
     * @param $spec
     *   The field specification array, as taken from a schema definition.
     *   The specification may also contain the key 'initial', the newly
     *   created field will be set to the value of the key in all rows.
     *   This is most useful for creating NOT NULL columns with no default
     *   value in existing tables.
     * @param $keys_new
     *   (optional) Keys and indexes specification to be created on the
     *   table along with adding the field. The format is the same as a
     *   table specification but without the 'fields' element. If you are
     *   adding a type 'serial' field, you MUST specify at least one key
     *   or index including it in this array. See db_change_field() for more
     *   explanation why.
     *
     * @throws DatabaseSchemaObjectDoesNotExistException
     *   If the specified table doesn't exist.
     * @throws DatabaseSchemaObjectExistsException
     *   If the specified table already has a field by that name.
     */
    public abstract function addField($table, $field, $spec, $keys_new = array());
    
    /**
     * Drop a field.
     *
     * @param $table
     *   The table to be altered.
     * @param $field
     *   The field to be dropped.
     *
     * @return
     *   TRUE if the field was successfully dropped, FALSE if there was no field
     *   by that name to begin with.
     */
    public abstract function dropField($table, $field);
    
    /**
     * Set the default value for a field.
     *
     * @param $table
     *   The table to be altered.
     * @param $field
     *   The field to be altered.
     * @param $default
     *   Default value to be set. NULL for 'default NULL'.
     *
     * @throws DatabaseSchemaObjectDoesNotExistException
     *   If the specified table or field doesn't exist.
     */
    public abstract function fieldSetDefault($table, $field, $default);
    
    /**
     * Set a field to have no default value.
     *
     * @param $table
     *   The table to be altered.
     * @param $field
     *   The field to be altered.
     *
     * @throws DatabaseSchemaObjectDoesNotExistException
     *   If the specified table or field doesn't exist.
     */
    public abstract function fieldSetNoDefault($table, $field);
    
    /**
     * Checks if an index exists in the given table.
     *
     * @param $table
     *   The name of the table in drupal (no prefixing).
     * @param $name
     *   The name of the index in drupal (no prefixing).
     *
     * @return
     *   TRUE if the given index exists, otherwise FALSE.
     */
    public abstract function indexExists($table, $name);
    
    /**
     * Add a primary key.
     *
     * @param $table
     *   The table to be altered.
     * @param $fields
     *   Fields for the primary key.
     *
     * @throws DatabaseSchemaObjectDoesNotExistException
     *   If the specified table doesn't exist.
     * @throws DatabaseSchemaObjectExistsException
     *   If the specified table already has a primary key.
     */
    public abstract function addPrimaryKey($table, $fields);
    
    /**
     * Drop the primary key.
     *
     * @param $table
     *   The table to be altered.
     *
     * @return
     *   TRUE if the primary key was successfully dropped, FALSE if there was no
     *   primary key on this table to begin with.
     */
    public abstract function dropPrimaryKey($table);
    
    /**
     * Add a unique key.
     *
     * @param $table
     *   The table to be altered.
     * @param $name
     *   The name of the key.
     * @param $fields
     *   An array of field names.
     *
     * @throws DatabaseSchemaObjectDoesNotExistException
     *   If the specified table doesn't exist.
     * @throws DatabaseSchemaObjectExistsException
     *   If the specified table already has a key by that name.
     */
    public abstract function addUniqueKey($table, $name, $fields);
    
    /**
     * Drop a unique key.
     *
     * @param $table
     *   The table to be altered.
     * @param $name
     *   The name of the key.
     *
     * @return
     *   TRUE if the key was successfully dropped, FALSE if there was no key by
     *   that name to begin with.
     */
    public abstract function dropUniqueKey($table, $name);
    
    /**
     * Add an index.
     *
     * @param $table
     *   The table to be altered.
     * @param $name
     *   The name of the index.
     * @param $fields
     *   An array of field names.
     *
     * @throws DatabaseSchemaObjectDoesNotExistException
     *   If the specified table doesn't exist.
     * @throws DatabaseSchemaObjectExistsException
     *   If the specified table already has an index by that name.
     */
    public abstract function addIndex($table, $name, $fields);
    
    /**
     * Drop an index.
     *
     * @param $table
     *   The table to be altered.
     * @param $name
     *   The name of the index.
     *
     * @return
     *   TRUE if the index was successfully dropped, FALSE if there was no index
     *   by that name to begin with.
     */
    public abstract function dropIndex($table, $name);
    
    /**
     * Change 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:
     * @code
     * $schema['foo'] = array(
     *   'fields' => array(
     *     'bar' => array('type' => 'int', 'not null' => TRUE)
     *   ),
     *   'primary key' => array('bar')
     * );
     * @endcode
     * and you want to change foo.bar to be type serial, leaving it as the
     * primary key. The correct sequence is:
     * @code
     * db_drop_primary_key('foo');
     * db_change_field('foo', 'bar', 'bar',
     *   array('type' => 'serial', 'not null' => TRUE),
     *   array('primary key' => array('bar')));
     * @endcode
     *
     * 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.
     *
     * @param $table
     *   Name of the table.
     * @param $field
     *   Name of the field to change.
     * @param $field_new
     *   New name for the field (set to the same as $field if you don't want to change the name).
     * @param $spec
     *   The field specification for the new field.
     * @param $keys_new
     *   (optional) Keys and indexes specification to be created on the
     *   table along with changing the field. The format is the same as a
     *   table specification but without the 'fields' element.
     *
     * @throws DatabaseSchemaObjectDoesNotExistException
     *   If the specified table or source field doesn't exist.
     * @throws DatabaseSchemaObjectExistsException
     *   If the specified destination field already exists.
     */
    public abstract function changeField($table, $field, $field_new, $spec, $keys_new = array());
    
    /**
     * Create a new table from a Drupal table definition.
     *
     * @param $name
     *   The name of the table to create.
     * @param $table
     *   A Schema API table definition array.
     *
     * @throws DatabaseSchemaObjectExistsException
     *   If the specified table already exists.
     */
    public function createTable($name, $table) {
        if ($this->tableExists($name)) {
            throw new DatabaseSchemaObjectExistsException(t('Table @name already exists.', array(
                '@name' => $name,
            )));
        }
        $statements = $this->createTableSql($name, $table);
        foreach ($statements as $statement) {
            $this->connection
                ->query($statement);
        }
    }
    
    /**
     * Return an array of field names from an array of key/index column specifiers.
     *
     * This is usually an identity function but if a key/index uses a column prefix
     * specification, this function extracts just the name.
     *
     * @param $fields
     *   An array of key/index column specifiers.
     *
     * @return
     *   An array of field names.
     */
    public function fieldNames($fields) {
        $return = array();
        foreach ($fields as $field) {
            if (is_array($field)) {
                $return[] = $field[0];
            }
            else {
                $return[] = $field;
            }
        }
        return $return;
    }
    
    /**
     * Prepare a table or column comment for database query.
     *
     * @param $comment
     *   The comment string to prepare.
     * @param $length
     *   Optional upper limit on the returned string length.
     *
     * @return
     *   The prepared comment.
     */
    public function prepareComment($comment, $length = NULL) {
        return $this->connection
            ->quote($comment);
    }

}

/**
 * Exception thrown if an object being created already exists.
 *
 * For example, this exception should be thrown whenever there is an attempt to
 * create a new database table, field, or index that already exists in the
 * database schema.
 */
class DatabaseSchemaObjectExistsException extends Exception {

}

/**
 * Exception thrown if an object being modified doesn't exist yet.
 *
 * For example, this exception should be thrown whenever there is an attempt to
 * modify a database table, field, or index that does not currently exist in
 * the database schema.
 */
class DatabaseSchemaObjectDoesNotExistException extends Exception {

}

/**
 * @} End of "defgroup schemaapi".
 */