```ruby
DiscussionPolicy = Struct.new(:user, :discussion) do

  def update?
    user.admin? or discussion.user == user
  end

  def destroy?
    user.admin?
  end
end```

Fix bug with nil user.
user
```ruby
DiscussionPolicy = Struct.new(:user, :discussion) do

  def update?
    return false unless user
    user.admin? or discussion.user == user
  end

  def destroy?
    return false unless user
    user.admin?
  end
end
```
assistant