added unit test and notes about dbms specific settings

docs/guide/db-dao.md

@@ -323,10 +323,17 @@ You may use the constants named above but you can also use a string that represe
 used in your DBMS following `SET TRANSACTION ISOLATION LEVEL`. For postgres this could be for example
 `SERIALIZABLE READ ONLY DEFERRABLE`.
 
+Note that some DBMS allow setting of the isolation level only for the whole connection so subsequent transactions
+may get the same isolation level even if you did not specify any. When using this feature
+you may need to set the isolation level for all transactions explicitly to avoid conflicting settings.
+At the time of this writing affected DBMS are MSSQL and SQLite.
+
 > Note: SQLite only supports two isolation levels, so you can only use `READ UNCOMMITTED` and `SERIALIZABLE`.
-  Usage of other levels will result in an exception to be thrown.
+Usage of other levels will result in an exception to be thrown.
 
-> Note:
+> Note: PostgreSQL does not allow settin the isolation level before the transaction starts so you can not
+specify the isolation level directly when starting the transaction.
+You have to call [[yii\db\Transaction::setIsolationLevel()]] in this case after the transaction has started.
 
 [isolation levels]: http://en.wikipedia.org/wiki/Isolation_%28database_systems%29#Isolation_levels
 

framework/db/Transaction.php

@@ -86,6 +86,14 @@ class Transaction extends \yii\base\Object
      * also a string containing DBMS specific syntax to be used after `SET TRANSACTION ISOLATION LEVEL`.
      * If not specified (`null`) the isolation level will not be set explicitly and the DBMS default will be used.
      *
+     * > Note: This setting does not work for PostgreSQL, where setting the isolation level before the transaction
+     * has no effect. You have to call [[setIsolationLevel()]] in this case after the transaction has started.
+     *
+     * > Note: Some DBMS allow setting of the isolation level only for the whole connection so subsequent transactions
+     * may get the same isolation level even if you did not specify any. When using this feature
+     * you may need to set the isolation level for all transactions explicitly to avoid conflicting settings.
+     * At the time of this writing affected DBMS are MSSQL and SQLite.
+     *
      * [isolation level]: http://en.wikipedia.org/wiki/Isolation_%28database_systems%29#Isolation_levels
      * @throws InvalidConfigException if [[db]] is `null`.
      */

framework/db/cubrid/Schema.php

@@ -10,6 +10,7 @@ namespace yii\db\cubrid;
 use yii\db\Expression;
 use yii\db\TableSchema;
 use yii\db\ColumnSchema;
+use yii\db\Transaction;
 
 /**
  * Schema is the class for retrieving metadata from a CUBRID database (version 9.1.x and higher).
@@ -284,4 +285,28 @@ class Schema extends \yii\db\Schema
 
         return isset($typeMap[$type]) ? $typeMap[$type] : \PDO::PARAM_STR;
     }
+
+	/**
+	 * @inheritdoc
+	 * @see http://www.cubrid.org/manual/91/en/sql/transaction.html#database-concurrency
+	 */
+	public function setTransactionIsolationLevel($level)
+	{
+		// translate SQL92 levels to CUBRID levels:
+		switch ($level) {
+			case Transaction::SERIALIZABLE:
+				$level = '6'; // SERIALIZABLE
+				break;
+			case Transaction::REPEATABLE_READ:
+				$level = '5'; // REPEATABLE READ CLASS with REPEATABLE READ INSTANCES
+				break;
+			case Transaction::READ_COMMITTED:
+				$level = '4'; // REPEATABLE READ CLASS with READ COMMITTED INSTANCES
+				break;
+			case Transaction::READ_UNCOMMITTED:
+				$level = '3'; // REPEATABLE READ CLASS with READ UNCOMMITTED INSTANCES
+				break;
+		}
+		return parent::setTransactionIsolationLevel($level);
+	}
 }

tests/unit/framework/db/ConnectionTest.php

@@ -3,6 +3,7 @@
 namespace yiiunit\framework\db;
 
 use yii\db\Connection;
+use yii\db\Transaction;
 
 /**
  * @group db
@@ -77,4 +78,47 @@ class ConnectionTest extends DatabaseTestCase
         $this->assertEquals('{{column}}', $connection->quoteColumnName('{{column}}'));
         $this->assertEquals('(column)', $connection->quoteColumnName('(column)'));
     }
+
+	public function testTransaction()
+	{
+		$connection = $this->getConnection(false);
+		$this->assertNull($connection->transaction);
+		$transaction = $connection->beginTransaction();
+		$this->assertNotNull($connection->transaction);
+		$this->assertTrue($transaction->isActive);
+
+		$connection->createCommand()->insert('profile', ['description' => 'test transaction'])->execute();
+
+		$transaction->rollBack();
+		$this->assertFalse($transaction->isActive);
+		$this->assertNull($connection->transaction);
+
+		$this->assertEquals(0, $connection->createCommand("SELECT COUNT(*) FROM profile WHERE description = 'test transaction';")->queryScalar());
+
+		$transaction = $connection->beginTransaction();
+		$connection->createCommand()->insert('profile', ['description' => 'test transaction'])->execute();
+		$transaction->commit();
+		$this->assertFalse($transaction->isActive);
+		$this->assertNull($connection->transaction);
+
+		$this->assertEquals(1, $connection->createCommand("SELECT COUNT(*) FROM profile WHERE description = 'test transaction';")->queryScalar());
+	}
+
+	public function testTransactionIsolation()
+	{
+		$connection = $this->getConnection(true);
+
+		$transaction = $connection->beginTransaction(Transaction::READ_UNCOMMITTED);
+		$transaction->commit();
+
+		$transaction = $connection->beginTransaction(Transaction::READ_COMMITTED);
+		$transaction->commit();
+
+		$transaction = $connection->beginTransaction(Transaction::REPEATABLE_READ);
+		$transaction->commit();
+
+		$transaction = $connection->beginTransaction(Transaction::SERIALIZABLE);
+		$transaction->commit();
+	}
+
 }

tests/unit/framework/db/sqlite/SqliteConnectionTest.php

 <?php
 namespace yiiunit\framework\db\sqlite;
 
+use yii\db\Transaction;
 use yiiunit\framework\db\ConnectionTest;
 
 /**
@@ -45,4 +46,15 @@ class SqliteConnectionTest extends ConnectionTest
         $this->assertEquals('{{column}}', $connection->quoteColumnName('{{column}}'));
         $this->assertEquals('(column)', $connection->quoteColumnName('(column)'));
     }
+
+	public function testTransactionIsolation()
+	{
+		$connection = $this->getConnection(true);
+
+		$transaction = $connection->beginTransaction(Transaction::READ_UNCOMMITTED);
+		$transaction->rollBack();
+
+		$transaction = $connection->beginTransaction(Transaction::SERIALIZABLE);
+		$transaction->rollBack();
+	}
 }