Add stub methods for describing foreign keys to all schema classes.

Describing foreign keys often requires separate logic from indexes. Add
skeleton methods to do that work.

lib/Cake/Database/Schema/Collection.php

@@ -79,11 +79,7 @@ public function describe($name) {
 			$name,
 			$this->_connection->config()
 		);
-		try {
-			$statement = $this->_connection->execute($sql, $params);
-		} catch (\PDOException $e) {
-			throw new Exception($e->getMessage(), 500, $e);
-		}
+		$statement = $this->_executeSql($sql, $params);
 		if (count($statement) === 0) {
 			throw new Exception(__d('cake_dev', 'Cannot describe %s. It has 0 columns.', $name));
 		}
@@ -97,15 +93,37 @@ public function describe($name) {
 			$name,
 			$this->_connection->config()
 		);
+		$statement = $this->_executeSql($sql, $params);
+		foreach ($statement->fetchAll('assoc') as $row) {
+			$this->_dialect->convertIndexDescription($table, $row);
+		}
+
+		list($sql, $params) = $this->_dialect->describeForeignKeySql(
+			$name,
+			$this->_connection->config()
+		);
+		$statement = $this->_executeSql($sql, $params);
+		foreach ($statement->fetchAll('assoc') as $row) {
+			$this->_dialect->convertForeignKeyDescription($table, $row);
+		}
+		return $table;
+	}
+
+/**
+ * Helper method to run queries and convert Exceptions to the correct types.
+ *
+ * @param string $sql The sql to run.
+ * @param array $params Parameters for the statement.
+ * @return Cake\Database\Statement Prepared statement
+ * @throws Cake\Database\Exception on query failure.
+ */
+	protected function _executeSql($sql, $params) {
 		try {
 			$statement = $this->_connection->execute($sql, $params);
+			return $statement;
 		} catch (\PDOException $e) {
 			throw new Exception($e->getMessage(), 500, $e);
 		}
-		foreach ($statement->fetchAll('assoc') as $row) {
-			$this->_dialect->convertIndexDescription($table, $row);
-		}
-		return $table;
 	}
 
 }

lib/Cake/Database/Schema/MysqlSchema.php

@@ -211,6 +211,24 @@ public function convertIndexDescription(Table $table, $row) {
 		}
 	}
 
+/**
+ * Generate the SQL to describe the foreign keys on a table.
+ *
+ * @return array List of sql, params
+ */
+	public function describeForeignKeySql($table) {
+		return ['', []];
+	}
+
+/**
+ * Convert a foreign key description into constraints on the Table object.
+ *
+ * @param Cake\Database\Table $table The table instance to populate.
+ * @param array $row The row of data.
+ * @return void
+ */
+	public function convertForeignKey(Table $table, $row) {
+	}
 /**
  * Generate the SQL to truncate a table.
  *

lib/Cake/Database/Schema/PostgresSchema.php

@@ -239,6 +239,25 @@ public function convertIndexDescription(Table $table, $row) {
 		]);
 	}
 
+/**
+ * Generate the SQL to describe the foreign keys on a table.
+ *
+ * @return array List of sql, params
+ */
+	public function describeForeignKeySql($table) {
+		return ['', []];
+	}
+
+/**
+ * Convert a foreign key description into constraints on the Table object.
+ *
+ * @param Cake\Database\Table $table The table instance to populate.
+ * @param array $row The row of data.
+ * @return void
+ */
+	public function convertForeignKey(Table $table, $row) {
+	}
+
 /**
  * Generate the SQL fragment for a single column.
  *

lib/Cake/Database/Schema/SqliteSchema.php

@@ -189,6 +189,25 @@ public function convertIndexDescription(Table $table, $row) {
 		}
 	}
 
+/**
+ * Generate the SQL to describe the foreign keys on a table.
+ *
+ * @return array List of sql, params
+ */
+	public function describeForeignKeySql($table) {
+		return ['', []];
+	}
+
+/**
+ * Convert a foreign key description into constraints on the Table object.
+ *
+ * @param Cake\Database\Table $table The table instance to populate.
+ * @param array $row The row of data.
+ * @return void
+ */
+	public function convertForeignKey(Table $table, $row) {
+	}
+
 /**
  * Generate the SQL fragment for a single column in Sqlite
  *