add an sqlite config option to enable foreign key constraints

For Sqlite, foreign key constraints need to be explicitly enabled for
every database connection. So enabling them in the migrations (as suggested
by the documentation) does not work.

This PR enables the contraints for every connection to an sqlite database
that has the config option `foreign_key_constraints` set to `true`.

If the option is not set, nothing changes, so no breaking change is
introduced.

As foreign key constraints are the default behavior for all other databases
I suggest switching this around for the next release and enable foreign
keys unless this is set to `false`.

If this is accepted I'll submit two more PRs for documentation and
laravel/laravel.

src/Illuminate/Database/SQLiteConnection.php

@@ -10,6 +10,23 @@
 
 class SQLiteConnection extends Connection
 {
+    /**
+     * Create a new database connection instance.
+     *
+     * @param  \PDO|\Closure     $pdo
+     * @param  string   $database
+     * @param  string   $tablePrefix
+     * @param  array    $config
+     * @return void
+     */
+    public function __construct($pdo, $database = '', $tablePrefix = '', array $config = [])
+    {
+        parent::__construct($pdo, $database, $tablePrefix, $config);
+
+        if ($this->getForeignKeyConstraintsSetting() == true) {
+            $this->getSchemaBuilder()->enableForeignKeyConstraints();
+        }
+    }
     /**
      * Get the default query grammar instance.
      *
@@ -63,4 +80,14 @@ protected function getDoctrineDriver()
     {
         return new DoctrineDriver;
     }
+
+    /**
+     * Get the database connection foreign_key_constraints setting.
+     *
+     * @return boolean|null
+     */
+    protected function getForeignKeyConstraintsSetting()
+    {
+        return $this->getConfig('foreign_key_constraints');
+    }
 }

tests/Database/DatabaseConnectionFactoryTest.php

@@ -102,4 +102,17 @@ public function testCustomConnectorsCanBeResolvedViaContainer()
 
         $this->assertEquals('connector', $factory->createConnector(['driver' => 'foo']));
     }
+
+    public function testSqliteForeignKeyConstraints()
+    {
+        $this->db->addConnection([
+            'driver' => 'sqlite',
+            'database' => ':memory:',
+            'foreign_key_constraints' => true,
+        ], 'constraints_set');
+
+        $this->assertEquals(0, $this->db->connection()->select('PRAGMA foreign_keys')[0]->foreign_keys);
+
+        $this->assertEquals(1, $this->db->connection('constraints_set')->select('PRAGMA foreign_keys')[0]->foreign_keys);
+    }
 }