How to use Doctrine Module with Relations?

#### What are you trying to achieve?
i would like to add a product with a related category.
```
$category = [
'id' => 'CATEGORY_ID',
'title' => 'Test Category',
];

$product = [
'id' => 'PRODUCT_ID',
'title' => 'My Test Product',
'category' => 'CATEGORY_ID',
];
$I->haveInRepository('Entity\Category', $category);
$I->haveInRepository('Entity\Product', $product);
```
#### What do you get instead?
setting `category` to a string ` [Doctrine\ORM\ORMInvalidArgumentException] Expected value of type "Entity\Category" for...`
using `categoryID` as key leads to a category set to null (using propel before doctrine, i thought that this might work).

in general it sould be documented that invalid fields are dropped. i think this should throw an exception (should this go into different ticket?).