How to update 1:1 relations in Extbase - typo3

Question

I have 2 models as below
class Communication extends \TYPO3\CMS\Extbase\DomainObject\AbstractEntity {
/**
* #var Filter
*/
protected $filter;
}
class Filter extends \TYPO3\CMS\Extbase\DomainObject\AbstractEntity {
/**
* #var \TYPO3\CMS\Extbase\Persistence\ObjectStorage<\TYPO3\CMS\Extbase\Domain\Model\FrontendUserGroup>
*/
protected $usergroup;
}
Template as below
<f:form class="form-horizontal" name="communication" action="update" object="{communication}">
<div class="form-group">
<label class="col-xs-3 col-md-2 control-label" for="bodytext">{f:translate(key: 'text.usergroup')}</label>
<div class="col-xs-9 col-md-10">
<f:for each="{usergroups}" as="group">
<label class="checkbox-inline">
<f:form.checkbox property="filter.usergroup" value="{group.uid}" /> {group.title}
</label>
</f:for>
</div>
</div>
<div class="form-group">
<div class="col-xs-9 col-md-10 col-xs-offset-3 col-md-offset-2">
<button type="submit" class="btn btn-primary">{f:translate(key: 'button.update')}</button>
</div>
</div>
</f:form>
When I try to update a Communication with $this->communicationRepository->update($communication), it always creates a new Filter rather than updating its own. Why this?

Answer 1

This is because there is no relation to the filter that is already there. For keeping the filter you need to say what id it has. I found out it works with an __identity field:
EDIT: it is probably better to wrap the field with an if condition in case there is no filter set
<f:if condition="{communication.filter.uid}">
<f:form.hidden name="communication[filter][__identity]" value="{communication.filter.uid}" />
</f:if>
Not sure, but it is possible that is also works with an uid property field:
<f:form.hidden property="filter.uid" />