if i show a field of type \"entity\" in my form, and i want to filter this entity type based on a argument I pass from the controller, how do i do that.. ?
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You can use $plumeOptions to pass everything your argument, but you'll need to add a getDefaultOptions() in PlumeOptionsType to specify the default value for your option.
See for instance https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Form/Extension/Core/Type/CheckboxType.php to see what this method should look like.