Adding fix and test for #3626

The code was assuming that if oyu passed _joinData to the save method
that belonged to something already in the database, which was not all of
the time. Removing the assumption, but also making saving a long list of
associaitions a bit slower as it has to check the existance of the row
first.

src/ORM/Association/BelongsToMany.php

@@ -822,7 +822,6 @@ protected function _collectJointEntities($sourceEntity, $targetEntities) {
 				continue;
 			}
 
-			$joint->isNew(false);
 			$result[] = $joint;
 		}
 

tests/Fixture/SpecialTagFixture.php

@@ -0,0 +1,52 @@
+<?php
+/**
+ * CakePHP(tm) Tests <http://book.cakephp.org/2.0/en/development/testing.html>
+ * Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ *
+ * Licensed under The MIT License
+ * For full copyright and license information, please see the LICENSE.txt
+ * Redistributions of files must retain the above copyright notice
+ *
+ * @copyright     Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ * @link          http://book.cakephp.org/2.0/en/development/testing.html CakePHP(tm) Tests
+ * @since         3.0.0
+ * @license       http://www.opensource.org/licenses/mit-license.php MIT License
+ */
+namespace Cake\Test\Fixture;
+
+use Cake\TestSuite\Fixture\TestFixture;
+
+/**
+ * A fixture for a join table containing additional data
+ *
+ */
+class SpecialTagFixture extends TestFixture {
+
+/**
+ * fields property
+ *
+ * @var array
+ */
+	public $fields = array(
+		'id' => ['type' => 'integer'],
+		'article_id' => ['type' => 'integer', 'null' => false],
+		'tag_id' => ['type' => 'integer', 'null' => false],
+		'highlighted' => ['type' => 'boolean', 'null' => true],
+		'highlighted_time' => ['type' => 'timestamp', 'null' => true],
+		'_constraints' => [
+			'primary' => ['type' => 'primary', 'columns' => ['id']],
+			'UNIQUE_TAG2' => ['type' => 'unique', 'columns' => ['article_id', 'tag_id']]
+		]
+	);
+
+/**
+ * records property
+ *
+ * @var array
+ */
+	public $records = array(
+		array('article_id' => 1, 'tag_id' => 3, 'highlighted' => false, 'highlighted_time' => null),
+		array('article_id' => 2, 'tag_id' => 1, 'highlighted' => true, 'highlighted_time' => '2014-06-01 10:10:00')
+	);
+}
+

tests/TestCase/ORM/QueryRegressionTest.php

@@ -31,7 +31,14 @@ class QueryRegressionTest extends TestCase {
  *
  * @var array
  */
-	public $fixtures = ['core.user', 'core.article', 'core.tag', 'core.articles_tag', 'core.author'];
+	public $fixtures = [
+		'core.user',
+		'core.article',
+		'core.tag',
+		'core.articles_tag',
+		'core.author',
+		'core.special_tag'
+	];
 
 /**
  * Tear down
@@ -119,4 +126,36 @@ public function testNullableTimeColumn() {
 		$this->assertNull($entity->created);
 	}
 
+
+/**
+ * Test for https://github.com/cakephp/cakephp/issues/3626
+ *
+ * Checks that join data is actually created and not tried to be updated every time
+ * @return void
+ */
+	public function testCreateJointData() {
+		$articles = TableRegistry::get('Articles');
+		$articles->belongsToMany('Highligths', [
+			'className' => 'TestApp\Model\Table\TagsTable',
+			'foreignKey' => 'article_id',
+			'targetForeignKey' => 'tag_id',
+			'through' => 'SpecialTags'
+		]);
+		$entity = $articles->get(2);
+		$data = [
+			'id' => 2,
+			'highligths' => [
+				[
+					'name' => 'New Special Tag',
+					'_joinData' => ['highlighted' => true, 'highlighted_time' => '2014-06-01 10:10:00']
+				]
+			]
+		];
+		$entity = $articles->patchEntity($entity, $data, ['Highligths' => ['associated' => ['_joinData']]]);
+		$articles->save($entity);
+		$entity = $articles->get(2, ['contain' => ['Highligths']]);
+		$this->assertEquals(4, $entity->highligths[0]->_joinData->tag_id);
+		$this->assertEquals('2014-06-01', $entity->highligths[0]->_joinData->highlighted_time->format('Y-m-d'));
+	}
+
 }