Symfony Generate fixtures YML from exising database data

Export:
php symfony doctrine:data-dump

Import: (all old data will be removed)
php symfony doctrine:data-load data/fixtures/data.yml
>> doctrine Loading data fixtures from "data/fixtures/data.yml"
>> doctrine Data was successfully loaded

Append:
php symfony doctrine:data-load --append data/fixtures/data.yml
(maybe got error: constraint violation)