Making sure a variable is array before checking values on it, fixes

Cake/ORM/Query.php

@@ -293,7 +293,8 @@ public function contain($associations = null, $override = false) {
 		}
 
 		$associations = (array)$associations;
-		if (isset(current($associations)['instance'])) {
+		$current = current($associations);
+		if (is_array($current) && isset($current['instance'])) {
 			$this->_containments = $this->_normalizedContainments = $associations;
 			return $this;
 		}

Test/TestCase/ORM/QueryTest.php

@@ -1703,4 +1703,29 @@ public function testCacheWriteIntegration() {
 		$query->all();
 	}
 
+
+/**
+ * Integration test to show filtering associations using contain and a closure
+ *
+ * @return void
+ */
+	public function testContainWithClosure() {
+		$table = TableRegistry::get('authors');
+		$table->hasMany('articles');
+		$query = new Query($this->connection, $table);
+		$query
+			->select()
+			->contain(['articles' => function($q) {
+				return $q->where(['articles.id' => 1]);
+			}]);
+
+		$ids = [];
+		foreach ($query as $entity) {
+			foreach ((array)$entity->articles as $article) {
+				$ids[] = $article->id;
+			}
+		}
+		$this->assertEquals([1], array_unique($ids));
+	}
+
 }