Find out why many to many relationships information does not work

[todo]

https://github.com/Gewaer/gewaer-cms-api/blob/e254695226a5735d46b3826a7ef57ba3c0c570f6/api-publisher/controllers/PostsController.php#L130-L135

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This issue was generated by todo based on a @todo comment in e254695 when #48 was merged. cc @gewaer.