Model1.objects.filter(field_1__field2=some_string).first()

class Model1(models.Model):
    field_1 = models.ForeignKey(Model2)

class Model2(models.Model):
    field2 = models.CharField()

class Model3(models.Model):
    field_3 = models.ForeignKey(Model2, related_name='field4')
    field_5 = models.CharField()