Contain the junction table when proxying finders.

When proxying finder methods from a belongstomany association, if that
association has conditions we should contain the junction table as the
conditions very likely use columns on that table.

Refs #7707

src/ORM/Association/BelongsToMany.php

@@ -771,6 +771,29 @@ function () use ($sourceEntity, $targetEntities, $options) {
         $sourceEntity->dirty($property, false);
     }
 
+    /**
+     * Proxies the finding operation to the target table's find method
+     * and modifies the query accordingly based of this association
+     * configuration.
+     *
+     * If your association includes conditions, the junction table will be
+     * included in the query's contained associations.
+     *
+     * @param string|array $type the type of query to perform, if an array is passed,
+     *   it will be interpreted as the `$options` parameter
+     * @param array $options The options to for the find
+     * @see \Cake\ORM\Table::find()
+     * @return \Cake\ORM\Query
+     */
+    public function find($type = null, array $options = [])
+    {
+        $query = parent::find($type, $options);
+        if ($this->conditions()) {
+            $query->contain([$this->junction()->alias()]);
+        }
+        return $query;
+    }
+
     /**
      * Replaces existing association links between the source entity and the target
      * with the ones passed. This method does a smart cleanup, links that are already

tests/TestCase/ORM/Association/BelongsToManyTest.php

@@ -854,6 +854,35 @@ public function testReplaceLinkSuccess()
         $this->assertFalse($entity->dirty('tags'));
     }
 
+    /**
+     * Test that find() applies a contain() when the association uses conditions.
+     *
+     * @return void
+     */
+    public function testFindAppliesContainWithConditions()
+    {
+        $connection = ConnectionManager::get('test');
+        $this->article->belongsToMany('Tags', [
+            'targetTable' => $this->tag,
+            'conditions' => ['ArticlesTags.favorite' => true]
+        ]);
+        $query = $this->getMock(
+            '\Cake\ORM\Query',
+            [],
+            [$connection, $this->article]
+        );
+        $query->expects($this->once())
+            ->method('where')
+            ->with(['ArticlesTags.favorite' => true])
+            ->will($this->returnSelf());
+        $query->expects($this->once())
+            ->method('contain')
+            ->with(['ArticlesTags'])
+            ->will($this->returnSelf());
+        $this->tag->method('find')->will($this->returnValue($query));
+
+        $this->article->Tags->find();
+    }
 
     /**
      * Tests that replaceLinks() will contain() the target table when