Raise an exception on invalid belongs to many data.

If an association uses an incorrect find type (like list), the results
cannot be joined together so we should raise an exception.

Refs #5282

src/ORM/Association/BelongsToMany.php

@@ -277,6 +277,7 @@ class BelongsToMany extends Association {
  * @param \Cake\ORM\Query $fetchQuery The query to get results from
  * @param array $options The options passed to the eager loader
  * @return array
+ * @throws \RuntimeException when the association property is not part of the results set.
  */
 	protected function _buildResultMap($fetchQuery, $options) {
 		$resultMap = [];
@@ -285,6 +286,12 @@ class BelongsToMany extends Association {
 		$hydrated = $fetchQuery->hydrate();
 
 		foreach ($fetchQuery->all() as $result) {
+			if (!isset($result[$property])) {
+				throw new \RuntimeException(sprintf(
+					'"%s" is missing from the belongsToMany results. Results cannot be created.',
+					$property
+				));
+			}
 			$result[$this->_junctionProperty] = $result[$property];
 			unset($result[$property]);
 

tests/TestCase/ORM/QueryRegressionTest.php

@@ -80,6 +80,20 @@ class QueryRegressionTest extends TestCase {
 	}
 
 /**
+ * Tests that eagerloading belongsToMany with find list fails with a helpful message.
+ *
+ * @expectedException \RuntimeException
+ * @return void
+ */
+	public function testEagerLoadingBelongsToManyList() {
+		$table = TableRegistry::get('Articles');
+		$table->belongsToMany('Tags', [
+			'finder' => 'list'
+		]);
+		$table->find()->contain('Tags')->toArray();
+	}
+
+/**
  * Tests that duplicate aliases in contain() can be used, even when they would
  * naturally be attached to the query instead of eagerly loaded. What should
  * happen here is that One of the duplicates will be changed to be loaded using