Apigility + Doctrine - ManyToMany fetch Eager not working from either side - data present

I have a setup with Country and Currency objects. A Country may have 0 or more Currency objects and vice versa there's the Currency that may be used by 0 or more Country objects. The problem is the data is not returned to the front-end (api requests).

In Country Entity

/**
 * @var Collection|ArrayCollection|Currency[]
 * @ORM\ManyToMany(targetEntity="Currency", inversedBy="countries", fetch="EAGER")
 * @ORM\JoinTable(
 *     name="country_country_currencies",
 *     joinColumns={@ORM\JoinColumn(name="country_id", referencedColumnName="id")},
 *     inverseJoinColumns={@ORM\JoinColumn(name="currency_id", referencedColumnName="id")}
 * )
 */
protected $currencies;

In Currency Entity

/**
 * @var Collection|ArrayCollection|Currency[]
 * @ORM\ManyToMany(targetEntity="Country", mappedBy="currencies", fetch="EAGER")
 */
protected $countries;

Both sides have a __construct() function setting the initial values to new ArrayCollection(). Both have their own get*(), add*() and remove*() functions.

When debugging the DoctrineResource::fetch() function on a single object (e.g. /currencies/45) the $entity object does contain the data when executing return $entity (last line of fethc()), so I know the data is there.

However, when it finally returns to the "front-end", I'm missing the data of the ManyToMany relation:

As you can see above: countries is empty.

When requesting from the other side (from Country), the currencies is empty.

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Additional info

Other similar questions had suggestions in comments or answers I'll put here straight away as I already checked those.

Doctrine caching - I ran the following commands:

• ./vendor/bin/doctrine-module orm:clear-cache:query
• ./vendor/bin/doctrine-module orm:clear-cache:result
• ./vendor/bin/doctrine-module orm:clear-cache:metadata

I've made sure that Zend Framework caching is disabled (also, development mode is enabled via composer development-enable).

The Entity proxies generated have also been deleted multiple times to make sure they aren't the issue. I've tried also without fetch="EAGER" (without is the original actually), but that yields the same result.

Using wrong hydrator for Entities: both are configured to use the Doctrine EntityManager (same for both):

'zf-hal' => [
    'metadata_map' => [
        \Country::class => [
            'route_identifier_name' => 'id',
            'entity_identifier_name' => 'id',
            'route_name' => 'country.rest.doctrine.country',
            'hydrator' => 'Country\\V1\\Rest\\Country\\CountryHydrator',
            'max_depth' => 3,
        ],
        \Country\V1\Rest\Country\CountryCollection::class => [
            'entity_identifier_name' => 'id',
            'route_name' => 'country.rest.doctrine.country',
            'is_collection' => true,
        ],
        // other entity
    ],
],
// some more config
'zf-apigility' => [
    'doctrine-connected' => [
        \Country\V1\Rest\Country\CountryResource::class => [
            'object_manager' => 'doctrine.entitymanager.orm_default',
            'hydrator' => 'Country\\V1\\Rest\\Country\\CountryHydrator',
        ],
        // other entity
    ],
],
'doctrine-hydrator' => [
    'Country\\V1\\Rest\\Country\\CountryHydrator' => [
        'entity_class' => \Salesupply\Core\Country\Entity\Country::class,
        'object_manager' => 'doctrine.entitymanager.orm_default',
        'by_value' => true,
        'strategies' => [],
        'use_generated_hydrator' => true,
    ],
    // other entity
],

Another suggested the content negotiation type might be set to json instead of HalJson: both (everything really) is set to HalJson:

'zf-content-negotiation' => [
    'controllers' => [
        'Country\\V1\\Rest\\Country\\Controller' => 'HalJson',
        'Country\\V1\\Rest\\Currency\\Controller' => 'HalJson',
    ],
],

The ApiSkeletons vendor package has this by design. I've opened an issue on Github some months back.

To ensure you receive back collections:

• Create a strategy class and extend the AllowRemoveByValue strategy of Doctrine.
• Overwrite the extract function to return a Collection, either filled or empty

That's it.

Full class:

namespace Application\Strategy;

use DoctrineModule\Stdlib\Hydrator\Strategy\AllowRemoveByValue;
use ZF\Hal\Collection;

class UniDirectionalToManyStrategy extends AllowRemoveByValue
{
    public function extract($value)
    {
        return new Collection($value ?: []);
    }
}

Apply this strategy where you need it. E.g. your Advert as many Applications, so the config should be modified like so:

'doctrine-hydrator' => [
    'Application\\V1\\Entity\\ApplicationHydrator' => [
        // other config
        'strategies' => [
            'relatedToManyPropertyName' => \Application\Strategy\UniDirectionalToManyStrategy::class,
        ],
    ],
],

Now collections should be returned.

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Quick note: this will only work as a strategy for the *ToMany side.