Avoiding fatal errors by chechink precense of reducer function

lib/Cake/ORM/MapReduce.php

@@ -163,6 +163,8 @@ public function emit($value, $key = null) {
  * bucket created during the Map phase call the reduce function.
  *
  * @return void
+ * @throws \LogicException if emitIntermediate was called but no reducer function
+ * was provided
  */
 	protected function _execute() {
 		$mapper = $this->_mapper;
@@ -171,6 +173,10 @@ protected function _execute() {
 		}
 		$this->_data = null;
 
+		if (!empty($this->_intermediate) && empty($this->_reducer)) {
+			throw new \LogicException(__d('cake_dev', 'No reducer function was provided'));
+		}
+
 		$reducer = $this->_reducer;
 		foreach ($this->_intermediate as $key => $list) {
 			$reducer($key, $list, $this);

lib/Cake/Test/TestCase/ORM/MapReduceTest.php

@@ -86,4 +86,21 @@ public function testEmitFinalInMapper() {
 		$this->assertEquals($expected, iterator_to_array($results));
 	}
 
+/**
+ * Tests that a reducer is required when there are intermediate resutls
+ *
+ * @expectedException \LogicException
+ * @return void
+ */
+	public function testReducerRequired() {
+		$data = ['a' => ['one', 'two'], 'b' => ['three', 'four']];
+		$mapper = function ($key, $row, $mr) {
+			foreach ($row as $number) {
+				$mr->emitIntermediate('a', $number);
+			}
+		};
+		$results = new MapReduce(new ArrayIterator($data), $mapper);
+		iterator_to_array($results);
+	}
+
 }