How to put a constraint using foreign key fields in Django?

Question

Consider following models:

class Model1:
    x = an integer
    y = an integer

class Model2:
    f1 = models.ForeignKey(Model1)
    f2 = models.ForeignKey(Model1)

Now, I want to put a constraint on model 2 such that no duplicate pairs of f1.x and f2.x get inserted in model 2.

How should I achieve it? I will be populating model 2 using bulk_create so maybe customising clean() or save() won't work.

I'm new to Django and not able to figure it out. Any help will be appreciated.

There another similar question here: How to define two fields "unique" as couple

But it's not helpful in my case.