Added migrations model for interacting with data in migrations table

Can currently

* Get all migrations
* Fetch the current version for each "module"
* Get a list of migrations that need to be run to bring the db up to date

classes/model/minion/migration.php

@@ -0,0 +1,164 @@
+<?php
+
+/**
+ * Model for managing migrations
+ **/
+class Model_Minion_Migration extends Model
+{
+	/**
+	 * Database connection to use
+	 * @var Kohana_Database
+	 */
+	protected $_db = NULL;
+
+	/**
+	 * The table that's used to store the migrations
+	 * @var string
+	 */
+	protected $_table = 'migrations';
+
+	/**
+	 * Constructs the model, taking a Database connection as the first and only 
+	 * parameter
+	 *
+	 * @param Kohana_Database Database connection to use
+	 */
+	public function __construct(Kohana_Database $db)
+	{
+		$this->_db = $db;
+	}
+
+	/**
+	 * Creates a new select query which includes all fields in the migrations 
+	 * table plus a `id` field which is a combination of the timestamp and the 
+	 * description
+	 *
+	 * @return Database_Query_Builder_Select
+	 */
+	protected function _select()
+	{
+		return DB::select('*', DB::expr('CONCAT(CAST(`timestamp` AS CHAR), "_", `description`) AS `id`'))->from($this->_table);
+	}
+
+	/**
+	 * Selects all migrations from the migratinos table
+	 *
+	 * @return Kohana_Database_Result
+	 */
+	public function fetch_all()
+	{
+		return $this->_select()
+			->execute($this->_db);
+	}
+
+	/**
+	 * Fetches the latest version for all installed modules
+	 *
+	 * If a module does not have any applied migrations then no result will be 
+	 * returned for it
+	 *
+	 * @return Kohana_Database_Result
+	 */
+	public function fetch_current_versions()
+	{
+		return $this->_select()
+			->where('applied', '>', 0)
+			->group_by('module')
+			->execute();
+	}
+
+	/**
+	 * Fetch a list of migrations that need to be applied in order to reach the 
+	 * required version
+	 *
+	 * @param string Migration's Module
+	 * @param string Target migration id
+	 */
+	public function fetch_required_migrations($modules = NULL, $target = NULL)
+	{
+		if( ! empty($modules) AND ! is_array($modules))
+		{
+			$modules = array($modules => $target);
+		}
+
+		// Get an array of the latest migrations, with the module name as the 
+		// array key
+		$migrations = $this->fetch_current_versions()->as_array('module');
+
+		if(empty($modules))
+		{
+			$keys = array_keys($migrations);
+
+			$modules = array_combine($keys, $keys);
+		}
+
+		$migrations_to_apply = array();
+
+		// What follows is a bit of icky code, but there aren't many "nice" ways around it
+		//
+		// Basically we need to get a list of migrations that need to be performed, but 
+		// the ordering of the migrations varies depending on whether we're wanting to 
+		// migrate up or migrate down.  As such, we can't just apply a generic "order by x"
+		// condition
+		//
+		// If you have a better way of doing this, please let me know :)
+
+		foreach($modules as $module => $target)
+		{
+			$query = $this->_select()->or_where('module', '=', $module);
+
+			// one of these conditions occurs if 
+			// a) the user specified they want to bring this module up to date
+			// or
+			// b) if they just want to bring all modules up to date
+			//
+			// Basically this checks that the user hasn't explicitly specified a version
+			// to migrate to
+			if($target !== NULL AND $target !== $module)
+			{
+				list($timestamp, $description) = explode('_', $target, 2);
+
+				$current_timestamp = isset($migrations[$module]) ? $migrations[$module]['timestamp'] : NULL;
+
+				// If the current version is the requested version then nothing needs to be done
+				if($current_timestamp === $timestamp)
+				{
+					continue;
+				}
+
+				$query->and_where('module', '=', $module);
+
+				// If they haven't applied any migrations for this module
+				// yet and are just wanting to apply all migrations (i.e. roll forward)
+				if($current_timestamp === NULL)
+				{
+					$query
+						->and_where('timestamp', '<=', $timestamp);
+				}
+				// If we need to move forward
+				elseif($timestamp > $current_timestamp)
+				{
+					$query
+						->and_where('timestamp', '>',  $current_timestamp)
+						->and_where('timestamp', '<=', $timestamp);
+				}
+				// If we want to roll back
+				elseif($timestamp < $current_timestamp)
+				{
+					$query
+						->and_where('timestamp', '<', $current_timestamp)
+						->and_where('timestamp', '>=', $timestamp);
+				}
+				
+				foreach($query->execute($this->_db) as $row)
+				{
+					$migrations_to_apply[$module][] = $row;
+				}
+			}
+
+			unset($query);
+		}
+
+		return $migrations_to_apply;
+	}
+}