The foreign key constraints on metrics.file_download are all wrong.

If a user_agent is deleted the file_download will be deleted (cascaded). We want to keep the download even if the user_agent is deleted. Change the constraint to set_null.

If a user is deleted the file_download will be deleted (cascaded). If a User has downloaded a file we do not want to removed them from the system. Change the constraint to no_action (deferred).