ManyToManyField.add() doesn't respect a unique constraint in intermediate table

[Yuta Okamoto]

I added the following Company and Member models, and CompanyMember as their intermediate table with a unique constraint including role field in addition to through_fields.

from django.db import models

class Company(models.Model):
    pass

class Member(models.Model):
    companies = models.ManyToManyField(Company, through='CompanyMember', related_name='members')

class CompanyMember:
    company = models.ForeignKey(Company, on_delete=models.CASCADE)
    member = models.ForeignKey(Member, on_delete=models.CASCADE)
    role = models.SmallIntegerField()

    class Meta:
        constraints = [
            models.UniqueConstraint(fields=['company', 'member', 'role'], name='company_member_role'),
        ]

In this situation, company.members.add() silently fails to add existing member with different role specified via through_defaults.

company = Company.objects.create()
member = Member.objects.create()

company.members.add(member, through_defaults={'role': 1})
assert company.members.through.objects.all().count() == 1

company.members.add(member, through_defaults={'role': 2})
assert company.members.through.objects.all().count() == 2  # fails

We need to workaround by adding the relation to the intermediate table directly.

company.members.through.objects.create(
    company.members.through(company=company, member=member, role=2)
)