Only allow self reference to a model that has a reference to the same owner

I have the following data model:

public class Bubble
{
    public int Id { get; set; }
    public string Name { get; set; }

    public ICollection <Level> Children { get; set; }
}

public class Level
{
    public int Id { get; set; }
    public int BubbleId { get; set; }
    public int? ParentId { get; set; }

    public string Name { get; set; }

    public Bubble Bubble { get; set; }

    public Level Parent { get; set; }
    public virtual ICollection<Level> Children { get; set; }
}

I want to store multiple sets of hierarchical data. Each set is owned by a bubble that contains levels (nodes). A level can either have it's self reference (parent) set to null (head of tree) or another level (sub node of parent) .

Now my problem is, that technically a level can have a parent (self reference) that does belong to another bubble. That should be impossible.

How can i make sure, that only levels with the same BubbleId FK can be assigned as a parent?