Package Foreign Key on uninstall/install/update

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Hi,

I have used on db.xml foreign keys like this

<field name="user_id" type="I">
<unsigned />
<constraint>
, Foreign Key (user_id) references User(id)
</constraint>
</field>

Now when I uninstall and reinstall package I get errors like
mysql error: [1553: Cannot drop index...

My question is: Is there any work around for this. Can I use Foreign Keys with db.xml or do I need to handle those on models?

Thank you for your answers