php-monstra/libraries/Idiorm/ORM.php

<?php

/**
 *
 * Idiorm
 *
 * http://github.com/j4mie/idiorm/
 *
 * A single-class super-simple database abstraction layer for PHP.
 * Provides (nearly) zero-configuration object-relational mapping
 * and a fluent interface for building basic, commonly-used queries.
 *
 * BSD Licensed.
 *
 * Copyright (c) 2010, Jamie Matthews
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 * * Redistributions of source code must retain the above copyright notice, this
 *   list of conditions and the following disclaimer.
 *
 * * Redistributions in binary form must reproduce the above copyright notice,
 *   this list of conditions and the following disclaimer in the documentation
 *   and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 */

class ORM
{
    // ----------------------- //
    // --- CLASS CONSTANTS --- //
    // ----------------------- //

    // Where condition array keys
    const WHERE_FRAGMENT = 0;
    const WHERE_VALUES = 1;

    // ------------------------ //
    // --- CLASS PROPERTIES --- //
    // ------------------------ //

    // Class configuration
    protected static $_config = array(
        'connection_string' => 'sqlite::memory:',
        'id_column' => 'id',
        'id_column_overrides' => array(),
        'error_mode' => PDO::ERRMODE_EXCEPTION,
        'username' => null,
        'password' => null,
        'driver_options' => null,
        'identifier_quote_character' => null, // if this is null, will be autodetected
        'logging' => false,
        'caching' => false,
    );

    // Database connection, instance of the PDO class
    protected static $_db;

    // Last query run, only populated if logging is enabled
    protected static $_last_query;

    // Log of all queries run, only populated if logging is enabled
    protected static $_query_log = array();

    // Query cache, only used if query caching is enabled
    protected static $_query_cache = array();

    // --------------------------- //
    // --- INSTANCE PROPERTIES --- //
    // --------------------------- //

    // The name of the table the current ORM instance is associated with
    protected $_table_name;

    // Alias for the table to be used in SELECT queries
    protected $_table_alias = null;

    // Values to be bound to the query
    protected $_values = array();

    // Columns to select in the result
    protected $_result_columns = array('*');

    // Are we using the default result column or have these been manually changed?
    protected $_using_default_result_columns = true;

    // Join sources
    protected $_join_sources = array();

    // Should the query include a DISTINCT keyword?
    protected $_distinct = false;

    // Is this a raw query?
    protected $_is_raw_query = false;

    // The raw query
    protected $_raw_query = '';

    // The raw query parameters
    protected $_raw_parameters = array();

    // Array of WHERE clauses
    protected $_where_conditions = array();

    // LIMIT
    protected $_limit = null;

    // OFFSET
    protected $_offset = null;

    // ORDER BY
    protected $_order_by = array();

    // GROUP BY
    protected $_group_by = array();

    // The data for a hydrated instance of the class
    protected $_data = array();

    // Fields that have been modified during the
    // lifetime of the object
    protected $_dirty_fields = array();

    // Is this a new object (has create() been called)?
    protected $_is_new = false;

    // Name of the column to use as the primary key for
    // this instance only. Overrides the config settings.
    protected $_instance_id_column = null;

    // ---------------------- //
    // --- STATIC METHODS --- //
    // ---------------------- //

    /**
     * Pass configuration settings to the class in the form of
     * key/value pairs. As a shortcut, if the second argument
     * is omitted, the setting is assumed to be the DSN string
     * used by PDO to connect to the database. Often, this
     * will be the only configuration required to use Idiorm.
     */
    public static function configure($key, $value=null)
    {
        // Shortcut: If only one argument is passed,
        // assume it's a connection string
        if (is_null($value)) {
            $value = $key;
            $key = 'connection_string';
        }
        self::$_config[$key] = $value;
    }

    /**
     * Despite its slightly odd name, this is actually the factory
     * method used to acquire instances of the class. It is named
     * this way for the sake of a readable interface, ie
     * ORM::for_table('table_name')->find_one()-> etc. As such,
     * this will normally be the first method called in a chain.
     */
    public static function for_table($table_name)
    {
        self::_setup_db();

        return new self($table_name);
    }

    /**
     * Set up the database connection used by the class.
     */
    protected static function _setup_db()
    {
        if (!is_object(self::$_db)) {
            $connection_string = self::$_config['connection_string'];
            $username = self::$_config['username'];
            $password = self::$_config['password'];
            $driver_options = self::$_config['driver_options'];
            $db = new PDO($connection_string, $username, $password, $driver_options);
            $db->setAttribute(PDO::ATTR_ERRMODE, self::$_config['error_mode']);
            self::set_db($db);
        }
    }

    /**
     * Set the PDO object used by Idiorm to communicate with the database.
     * This is public in case the ORM should use a ready-instantiated
     * PDO object as its database connection.
     */
    public static function set_db($db)
    {
        self::$_db = $db;
        self::_setup_identifier_quote_character();
    }

    /**
     * Detect and initialise the character used to quote identifiers
     * (table names, column names etc). If this has been specified
     * manually using ORM::configure('identifier_quote_character', 'some-char'),
     * this will do nothing.
     */
    public static function _setup_identifier_quote_character()
    {
        if (is_null(self::$_config['identifier_quote_character'])) {
            self::$_config['identifier_quote_character'] = self::_detect_identifier_quote_character();
        }
    }

    /**
     * Return the correct character used to quote identifiers (table
     * names, column names etc) by looking at the driver being used by PDO.
     */
    protected static function _detect_identifier_quote_character()
    {
        switch (self::$_db->getAttribute(PDO::ATTR_DRIVER_NAME)) {
            case 'pgsql':
            case 'sqlsrv':
            case 'dblib':
            case 'mssql':
            case 'sybase':
                return '"';
            case 'mysql':
            case 'sqlite':
            case 'sqlite2':
            default:
                return '`';
        }
    }

    /**
     * Returns the PDO instance used by the the ORM to communicate with
     * the database. This can be called if any low-level DB access is
     * required outside the class.
     */
    public static function get_db()
    {
        self::_setup_db(); // required in case this is called before Idiorm is instantiated

        return self::$_db;
    }

    /**
     * Add a query to the internal query log. Only works if the
     * 'logging' config option is set to true.
     *
     * This works by manually binding the parameters to the query - the
     * query isn't executed like this (PDO normally passes the query and
     * parameters to the database which takes care of the binding) but
     * doing it this way makes the logged queries more readable.
     */
    protected static function _log_query($query, $parameters)
    {
        // If logging is not enabled, do nothing
        if (!self::$_config['logging']) {
            return false;
        }

        if (count($parameters) > 0) {
            // Escape the parameters
            $parameters = array_map(array(self::$_db, 'quote'), $parameters);

            // Replace placeholders in the query for vsprintf
            $query = str_replace("?", "%s", $query);

            // Replace the question marks in the query with the parameters
            $bound_query = vsprintf($query, $parameters);
        } else {
            $bound_query = $query;
        }

        self::$_last_query = $bound_query;
        self::$_query_log[] = $bound_query;

        return true;
    }

    /**
     * Get the last query executed. Only works if the
     * 'logging' config option is set to true. Otherwise
     * this will return null.
     */
    public static function get_last_query()
    {
        return self::$_last_query;
    }

    /**
     * Get an array containing all the queries run up to
     * now. Only works if the 'logging' config option is
     * set to true. Otherwise returned array will be empty.
     */
    public static function get_query_log()
    {
        return self::$_query_log;
    }

    // ------------------------ //
    // --- INSTANCE METHODS --- //
    // ------------------------ //

    /**
     * "Private" constructor; shouldn't be called directly.
     * Use the ORM::for_table factory method instead.
     */
    protected function __construct($table_name, $data=array())
    {
        $this->_table_name = $table_name;
        $this->_data = $data;
    }

    /**
     * Create a new, empty instance of the class. Used
     * to add a new row to your database. May optionally
     * be passed an associative array of data to populate
     * the instance. If so, all fields will be flagged as
     * dirty so all will be saved to the database when
     * save() is called.
     */
    public function create($data=null)
    {
        $this->_is_new = true;
        if (!is_null($data)) {
            return $this->hydrate($data)->force_all_dirty();
        }

        return $this;
    }

    /**
     * Specify the ID column to use for this instance or array of instances only.
     * This overrides the id_column and id_column_overrides settings.
     *
     * This is mostly useful for libraries built on top of Idiorm, and will
     * not normally be used in manually built queries. If you don't know why
     * you would want to use this, you should probably just ignore it.
     */
    public function use_id_column($id_column)
    {
        $this->_instance_id_column = $id_column;

        return $this;
    }

    /**
     * Create an ORM instance from the given row (an associative
     * array of data fetched from the database)
     */
    protected function _create_instance_from_row($row)
    {
        $instance = self::for_table($this->_table_name);
        $instance->use_id_column($this->_instance_id_column);
        $instance->hydrate($row);

        return $instance;
    }

    /**
     * Tell the ORM that you are expecting a single result
     * back from your query, and execute it. Will return
     * a single instance of the ORM class, or false if no
     * rows were returned.
     * As a shortcut, you may supply an ID as a parameter
     * to this method. This will perform a primary key
     * lookup on the table.
     */
    public function find_one($id=null)
    {
        if (!is_null($id)) {
            $this->where_id_is($id);
        }
        $this->limit(1);
        $rows = $this->_run();

        if (empty($rows)) {
            return false;
        }

        return $this->_create_instance_from_row($rows[0]);
    }

    /**
     * Tell the ORM that you are expecting multiple results
     * from your query, and execute it. Will return an array
     * of instances of the ORM class, or an empty array if
     * no rows were returned.
     */
    public function find_many()
    {
        $rows = $this->_run();

        return array_map(array($this, '_create_instance_from_row'), $rows);
    }

    /**
     * Tell the ORM that you wish to execute a COUNT query.
     * Will return an integer representing the number of
     * rows returned.
     */
    public function count()
    {
        $this->select_expr('COUNT(*)', 'count');
        $result = $this->find_one();

        return ($result !== false && isset($result->count)) ? (int) $result->count : 0;
    }

     /**
     * This method can be called to hydrate (populate) this
     * instance of the class from an associative array of data.
     * This will usually be called only from inside the class,
     * but it's public in case you need to call it directly.
     */
    public function hydrate($data=array())
    {
        $this->_data = $data;

        return $this;
    }

    /**
     * Force the ORM to flag all the fields in the $data array
     * as "dirty" and therefore update them when save() is called.
     */
    public function force_all_dirty()
    {
        $this->_dirty_fields = $this->_data;

        return $this;
    }

    /**
     * Perform a raw query. The query should contain placeholders,
     * in either named or question mark style, and the parameters
     * should be an array of values which will be bound to the
     * placeholders in the query. If this method is called, all
     * other query building methods will be ignored.
     */
    public function raw_query($query, $parameters)
    {
        $this->_is_raw_query = true;
        $this->_raw_query = $query;
        $this->_raw_parameters = $parameters;

        return $this;
    }

    /**
     * Add an alias for the main table to be used in SELECT queries
     */
    public function table_alias($alias)
    {
        $this->_table_alias = $alias;

        return $this;
    }

    /**
     * Internal method to add an unquoted expression to the set
     * of columns returned by the SELECT query. The second optional
     * argument is the alias to return the expression as.
     */
    protected function _add_result_column($expr, $alias=null)
    {
        if (!is_null($alias)) {
            $expr .= " AS " . $this->_quote_identifier($alias);
        }

        if ($this->_using_default_result_columns) {
            $this->_result_columns = array($expr);
            $this->_using_default_result_columns = false;
        } else {
            $this->_result_columns[] = $expr;
        }

        return $this;
    }

    /**
     * Add a column to the list of columns returned by the SELECT
     * query. This defaults to '*'. The second optional argument is
     * the alias to return the column as.
     */
    public function select($column, $alias=null)
    {
        $column = $this->_quote_identifier($column);

        return $this->_add_result_column($column, $alias);
    }

    /**
     * Add an unquoted expression to the list of columns returned
     * by the SELECT query. The second optional argument is
     * the alias to return the column as.
     */
    public function select_expr($expr, $alias=null)
    {
        return $this->_add_result_column($expr, $alias);
    }

    /**
     * Add a DISTINCT keyword before the list of columns in the SELECT query
     */
    public function distinct()
    {
        $this->_distinct = true;

        return $this;
    }

    /**
     * Internal method to add a JOIN source to the query.
     *
     * The join_operator should be one of INNER, LEFT OUTER, CROSS etc - this
     * will be prepended to JOIN.
     *
     * The table should be the name of the table to join to.
     *
     * The constraint may be either a string or an array with three elements. If it
     * is a string, it will be compiled into the query as-is, with no escaping. The
     * recommended way to supply the constraint is as an array with three elements:
     *
     * first_column, operator, second_column
     *
     * Example: array('user.id', '=', 'profile.user_id')
     *
     * will compile to
     *
     * ON `user`.`id` = `profile`.`user_id`
     *
     * The final (optional) argument specifies an alias for the joined table.
     */
    protected function _add_join_source($join_operator, $table, $constraint, $table_alias=null)
    {
        $join_operator = trim("{$join_operator} JOIN");

        $table = $this->_quote_identifier($table);

        // Add table alias if present
        if (!is_null($table_alias)) {
            $table_alias = $this->_quote_identifier($table_alias);
            $table .= " {$table_alias}";
        }

        // Build the constraint
        if (is_array($constraint)) {
            list($first_column, $operator, $second_column) = $constraint;
            $first_column = $this->_quote_identifier($first_column);
            $second_column = $this->_quote_identifier($second_column);
            $constraint = "{$first_column} {$operator} {$second_column}";
        }

        $this->_join_sources[] = "{$join_operator} {$table} ON {$constraint}";

        return $this;
    }

    /**
     * Add a simple JOIN source to the query
     */
    public function join($table, $constraint, $table_alias=null)
    {
        return $this->_add_join_source("", $table, $constraint, $table_alias);
    }

    /**
     * Add an INNER JOIN souce to the query
     */
    public function inner_join($table, $constraint, $table_alias=null)
    {
        return $this->_add_join_source("INNER", $table, $constraint, $table_alias);
    }

    /**
     * Add a LEFT OUTER JOIN souce to the query
     */
    public function left_outer_join($table, $constraint, $table_alias=null)
    {
        return $this->_add_join_source("LEFT OUTER", $table, $constraint, $table_alias);
    }

    /**
     * Add an RIGHT OUTER JOIN souce to the query
     */
    public function right_outer_join($table, $constraint, $table_alias=null)
    {
        return $this->_add_join_source("RIGHT OUTER", $table, $constraint, $table_alias);
    }

    /**
     * Add an FULL OUTER JOIN souce to the query
     */
    public function full_outer_join($table, $constraint, $table_alias=null)
    {
        return $this->_add_join_source("FULL OUTER", $table, $constraint, $table_alias);
    }

    /**
     * Internal method to add a WHERE condition to the query
     */
    protected function _add_where($fragment, $values=array())
    {
        if (!is_array($values)) {
            $values = array($values);
        }
        $this->_where_conditions[] = array(
            self::WHERE_FRAGMENT => $fragment,
            self::WHERE_VALUES => $values,
        );

        return $this;
    }

    /**
     * Helper method to compile a simple COLUMN SEPARATOR VALUE
     * style WHERE condition into a string and value ready to
     * be passed to the _add_where method. Avoids duplication
     * of the call to _quote_identifier
     */
    protected function _add_simple_where($column_name, $separator, $value)
    {
        $column_name = $this->_quote_identifier($column_name);

        return $this->_add_where("{$column_name} {$separator} ?", $value);
    }

    /**
     * Return a string containing the given number of question marks,
     * separated by commas. Eg "?, ?, ?"
     */
    protected function _create_placeholders($number_of_placeholders)
    {
        return join(", ", array_fill(0, $number_of_placeholders, "?"));
    }

    /**
     * Add a WHERE column = value clause to your query. Each time
     * this is called in the chain, an additional WHERE will be
     * added, and these will be ANDed together when the final query
     * is built.
     */
    public function where($column_name, $value)
    {
        return $this->where_equal($column_name, $value);
    }

    /**
     * More explicitly named version of for the where() method.
     * Can be used if preferred.
     */
    public function where_equal($column_name, $value)
    {
        return $this->_add_simple_where($column_name, '=', $value);
    }

    /**
     * Add a WHERE column != value clause to your query.
     */
    public function where_not_equal($column_name, $value)
    {
        return $this->_add_simple_where($column_name, '!=', $value);
    }

    /**
     * Special method to query the table by its primary key
     */
    public function where_id_is($id)
    {
        return $this->where($this->_get_id_column_name(), $id);
    }

    /**
     * Add a WHERE ... LIKE clause to your query.
     */
    public function where_like($column_name, $value)
    {
        return $this->_add_simple_where($column_name, 'LIKE', $value);
    }

    /**
     * Add where WHERE ... NOT LIKE clause to your query.
     */
    public function where_not_like($column_name, $value)
    {
        return $this->_add_simple_where($column_name, 'NOT LIKE', $value);
    }

    /**
     * Add a WHERE ... > clause to your query
     */
    public function where_gt($column_name, $value)
    {
        return $this->_add_simple_where($column_name, '>', $value);
    }

    /**
     * Add a WHERE ... < clause to your query
     */
    public function where_lt($column_name, $value)
    {
        return $this->_add_simple_where($column_name, '<', $value);
    }

    /**
     * Add a WHERE ... >= clause to your query
     */
    public function where_gte($column_name, $value)
    {
        return $this->_add_simple_where($column_name, '>=', $value);
    }

    /**
     * Add a WHERE ... <= clause to your query
     */
    public function where_lte($column_name, $value)
    {
        return $this->_add_simple_where($column_name, '<=', $value);
    }

    /**
     * Add a WHERE ... IN clause to your query
     */
    public function where_in($column_name, $values)
    {
        $column_name = $this->_quote_identifier($column_name);
        $placeholders = $this->_create_placeholders(count($values));

        return $this->_add_where("{$column_name} IN ({$placeholders})", $values);
    }

    /**
     * Add a WHERE ... NOT IN clause to your query
     */
    public function where_not_in($column_name, $values)
    {
        $column_name = $this->_quote_identifier($column_name);
        $placeholders = $this->_create_placeholders(count($values));

        return $this->_add_where("{$column_name} NOT IN ({$placeholders})", $values);
    }

    /**
     * Add a WHERE column IS NULL clause to your query
     */
    public function where_null($column_name)
    {
        $column_name = $this->_quote_identifier($column_name);

        return $this->_add_where("{$column_name} IS NULL");
    }

    /**
     * Add a WHERE column IS NOT NULL clause to your query
     */
    public function where_not_null($column_name)
    {
        $column_name = $this->_quote_identifier($column_name);

        return $this->_add_where("{$column_name} IS NOT NULL");
    }

    /**
     * Add a raw WHERE clause to the query. The clause should
     * contain question mark placeholders, which will be bound
     * to the parameters supplied in the second argument.
     */
    public function where_raw($clause, $parameters=array())
    {
        return $this->_add_where($clause, $parameters);
    }

    /**
     * Add a LIMIT to the query
     */
    public function limit($limit)
    {
        $this->_limit = $limit;

        return $this;
    }

    /**
     * Add an OFFSET to the query
     */
    public function offset($offset)
    {
        $this->_offset = $offset;

        return $this;
    }

    /**
     * Add an ORDER BY clause to the query
     */
    protected function _add_order_by($column_name, $ordering)
    {
        $column_name = $this->_quote_identifier($column_name);
        $this->_order_by[] = "{$column_name} {$ordering}";

        return $this;
    }

    /**
     * Add an ORDER BY column DESC clause
     */
    public function order_by_desc($column_name)
    {
        return $this->_add_order_by($column_name, 'DESC');
    }

    /**
     * Add an ORDER BY column ASC clause
     */
    public function order_by_asc($column_name)
    {
        return $this->_add_order_by($column_name, 'ASC');
    }

    /**
     * Add a column to the list of columns to GROUP BY
     */
    public function group_by($column_name)
    {
        $column_name = $this->_quote_identifier($column_name);
        $this->_group_by[] = $column_name;

        return $this;
    }

    /**
     * Build a SELECT statement based on the clauses that have
     * been passed to this instance by chaining method calls.
     */
    protected function _build_select()
    {
        // If the query is raw, just set the $this->_values to be
        // the raw query parameters and return the raw query
        if ($this->_is_raw_query) {
            $this->_values = $this->_raw_parameters;

            return $this->_raw_query;
        }

        // Build and return the full SELECT statement by concatenating
        // the results of calling each separate builder method.
        return $this->_join_if_not_empty(" ", array(
            $this->_build_select_start(),
            $this->_build_join(),
            $this->_build_where(),
            $this->_build_group_by(),
            $this->_build_order_by(),
            $this->_build_limit(),
            $this->_build_offset(),
        ));
    }

    /**
     * Build the start of the SELECT statement
     */
    protected function _build_select_start()
    {
        $result_columns = join(', ', $this->_result_columns);

        if ($this->_distinct) {
            $result_columns = 'DISTINCT ' . $result_columns;
        }

        $fragment = "SELECT {$result_columns} FROM " . $this->_quote_identifier($this->_table_name);

        if (!is_null($this->_table_alias)) {
            $fragment .= " " . $this->_quote_identifier($this->_table_alias);
        }

        return $fragment;
    }

    /**
     * Build the JOIN sources
     */
    protected function _build_join()
    {
        if (count($this->_join_sources) === 0) {
            return '';
        }

        return join(" ", $this->_join_sources);
    }

    /**
     * Build the WHERE clause(s)
     */
    protected function _build_where()
    {
        // If there are no WHERE clauses, return empty string
        if (count($this->_where_conditions) === 0) {
            return '';
        }

        $where_conditions = array();
        foreach ($this->_where_conditions as $condition) {
            $where_conditions[] = $condition[self::WHERE_FRAGMENT];
            $this->_values = array_merge($this->_values, $condition[self::WHERE_VALUES]);
        }

        return "WHERE " . join(" AND ", $where_conditions);
    }

    /**
     * Build GROUP BY
     */
    protected function _build_group_by()
    {
        if (count($this->_group_by) === 0) {
            return '';
        }

        return "GROUP BY " . join(", ", $this->_group_by);
    }

    /**
     * Build ORDER BY
     */
    protected function _build_order_by()
    {
        if (count($this->_order_by) === 0) {
            return '';
        }

        return "ORDER BY " . join(", ", $this->_order_by);
    }

    /**
     * Build LIMIT
     */
    protected function _build_limit()
    {
        if (!is_null($this->_limit)) {
            return "LIMIT " . $this->_limit;
        }

        return '';
    }

    /**
     * Build OFFSET
     */
    protected function _build_offset()
    {
        if (!is_null($this->_offset)) {
            return "OFFSET " . $this->_offset;
        }

        return '';
    }

    /**
     * Wrapper around PHP's join function which
     * only adds the pieces if they are not empty.
     */
    protected function _join_if_not_empty($glue, $pieces)
    {
        $filtered_pieces = array();
        foreach ($pieces as $piece) {
            if (is_string($piece)) {
                $piece = trim($piece);
            }
            if (!empty($piece)) {
                $filtered_pieces[] = $piece;
            }
        }

        return join($glue, $filtered_pieces);
    }

    /**
     * Quote a string that is used as an identifier
     * (table names, column names etc). This method can
     * also deal with dot-separated identifiers eg table.column
     */
    protected function _quote_identifier($identifier)
    {
        $parts = explode('.', $identifier);
        $parts = array_map(array($this, '_quote_identifier_part'), $parts);

        return join('.', $parts);
    }

    /**
     * This method performs the actual quoting of a single
     * part of an identifier, using the identifier quote
     * character specified in the config (or autodetected).
     */
    protected function _quote_identifier_part($part)
    {
        if ($part === '*') {
            return $part;
        }
        $quote_character = self::$_config['identifier_quote_character'];

        return $quote_character . $part . $quote_character;
    }

    /**
     * Create a cache key for the given query and parameters.
     */
    protected static function _create_cache_key($query, $parameters)
    {
        $parameter_string = join(',', $parameters);
        $key = $query . ':' . $parameter_string;

        return sha1($key);
    }

    /**
     * Check the query cache for the given cache key. If a value
     * is cached for the key, return the value. Otherwise, return false.
     */
    protected static function _check_query_cache($cache_key)
    {
        if (isset(self::$_query_cache[$cache_key])) {
            return self::$_query_cache[$cache_key];
        }

        return false;
    }

    /**
     * Clear the query cache
     */
    public static function clear_cache()
    {
        self::$_query_cache = array();
    }

    /**
     * Add the given value to the query cache.
     */
    protected static function _cache_query_result($cache_key, $value)
    {
        self::$_query_cache[$cache_key] = $value;
    }

    /**
     * Execute the SELECT query that has been built up by chaining methods
     * on this class. Return an array of rows as associative arrays.
     */
    protected function _run()
    {
        $query = $this->_build_select();
        $caching_enabled = self::$_config['caching'];

        if ($caching_enabled) {
            $cache_key = self::_create_cache_key($query, $this->_values);
            $cached_result = self::_check_query_cache($cache_key);

            if ($cached_result !== false) {
                return $cached_result;
            }
        }

        self::_log_query($query, $this->_values);
        $statement = self::$_db->prepare($query);
        $statement->execute($this->_values);

        $rows = array();
        while ($row = $statement->fetch(PDO::FETCH_ASSOC)) {
            $rows[] = $row;
        }

        if ($caching_enabled) {
            self::_cache_query_result($cache_key, $rows);
        }

        return $rows;
    }

    /**
     * Return the raw data wrapped by this ORM
     * instance as an associative array. Column
     * names may optionally be supplied as arguments,
     * if so, only those keys will be returned.
     */
    public function as_array()
    {
        if (func_num_args() === 0) {
            return $this->_data;
        }
        $args = func_get_args();

        return array_intersect_key($this->_data, array_flip($args));
    }

    /**
     * Return the value of a property of this object (database row)
     * or null if not present.
     */
    public function get($key)
    {
        return isset($this->_data[$key]) ? $this->_data[$key] : null;
    }

    /**
     * Return the name of the column in the database table which contains
     * the primary key ID of the row.
     */
    protected function _get_id_column_name()
    {
        if (!is_null($this->_instance_id_column)) {
            return $this->_instance_id_column;
        }
        if (isset(self::$_config['id_column_overrides'][$this->_table_name])) {
            return self::$_config['id_column_overrides'][$this->_table_name];
        } else {
            return self::$_config['id_column'];
        }
    }

    /**
     * Get the primary key ID of this object.
     */
    public function id()
    {
        return $this->get($this->_get_id_column_name());
    }

    /**
     * Set a property to a particular value on this object.
     * Flags that property as 'dirty' so it will be saved to the
     * database when save() is called.
     */
    public function set($key, $value)
    {
        $this->_data[$key] = $value;
        $this->_dirty_fields[$key] = $value;
    }

    /**
     * Check whether the given field has been changed since this
     * object was saved.
     */
    public function is_dirty($key)
    {
        return isset($this->_dirty_fields[$key]);
    }

    /**
     * Save any fields which have been modified on this object
     * to the database.
     */
    public function save()
    {
        $query = array();
        $values = array_values($this->_dirty_fields);

        if (!$this->_is_new) { // UPDATE
            // If there are no dirty values, do nothing
            if (count($values) == 0) {
                return true;
            }
            $query = $this->_build_update();
            $values[] = $this->id();
        } else { // INSERT
            $query = $this->_build_insert();
        }

        self::_log_query($query, $values);
        $statement = self::$_db->prepare($query);
        $success = $statement->execute($values);

        // If we've just inserted a new record, set the ID of this object
        if ($this->_is_new) {
            $this->_is_new = false;
            if (is_null($this->id())) {
                $this->_data[$this->_get_id_column_name()] = self::$_db->lastInsertId();
            }
        }

        $this->_dirty_fields = array();

        return $success;
    }

    /**
     * Build an UPDATE query
     */
    protected function _build_update()
    {
        $query = array();
        $query[] = "UPDATE {$this->_quote_identifier($this->_table_name)} SET";

        $field_list = array();
        foreach ($this->_dirty_fields as $key => $value) {
            $field_list[] = "{$this->_quote_identifier($key)} = ?";
        }
        $query[] = join(", ", $field_list);
        $query[] = "WHERE";
        $query[] = $this->_quote_identifier($this->_get_id_column_name());
        $query[] = "= ?";

        return join(" ", $query);
    }

    /**
     * Build an INSERT query
     */
    protected function _build_insert()
    {
        $query[] = "INSERT INTO";
        $query[] = $this->_quote_identifier($this->_table_name);
        $field_list = array_map(array($this, '_quote_identifier'), array_keys($this->_dirty_fields));
        $query[] = "(" . join(", ", $field_list) . ")";
        $query[] = "VALUES";

        $placeholders = $this->_create_placeholders(count($this->_dirty_fields));
        $query[] = "({$placeholders})";

        return join(" ", $query);
    }

    /**
     * Delete this record from the database
     */
    public function delete()
    {
        $query = join(" ", array(
            "DELETE FROM",
            $this->_quote_identifier($this->_table_name),
            "WHERE",
            $this->_quote_identifier($this->_get_id_column_name()),
            "= ?",
        ));
        $params = array($this->id());
        self::_log_query($query, $params);
        $statement = self::$_db->prepare($query);

        return $statement->execute($params);
    }

    // --------------------- //
    // --- MAGIC METHODS --- //
    // --------------------- //
    public function __get($key)
    {
        return $this->get($key);
    }

    public function __set($key, $value)
    {
        $this->set($key, $value);
    }

    public function __isset($key)
    {
        return isset($this->_data[$key]);
    }
}