[Python] Django: How to Represent and Query Symmetrical Relationships for a Family Tree?

I am building a family tree application in Django where I need to represent and query marriages symmetrically. Each marriage should have only one record, and the relationship should include both partners without duplicating data. Here's the relevant model structure:

class Person(models.Model):
first_name = models.CharField(max_length=100)
last_name = models.CharField(max_length=100)
spouses = models.ManyToManyField(
'self', through="Marriage", symmetrical=True, related_name="partners"
)


class Marriage(models.Model):
person1 = models.ForeignKey(Person, on_delete=models.CASCADE, related_name="marriages_as_person1")
person2 = models.ForeignKey(Person, on_delete=models.CASCADE, related_name="marriages_as_person2")
start_date = models.DateField(null=True, blank=True)
end_date = models.DateField(null=True, blank=True)


I want to:

1. Ensure both partners appear as spouses for each other symmetrically.
2. Avoid duplicate entries for the same marriage.
3. Efficiently query all spouses of a person.

Here’s the code I’m using to query spouses:

# Query spouses for a person
p1 = Person.objects.create()
p2 = Person.objects.create()
Marriage.objects.create(person1=p1, person2=p2)

p1.spouses.all() # Returns list containing p2
p2.spouses.all() # Returns empty list


However, I’m facing challenges:

1. If p1 spouses are queried, it should contain p2 and if p2 spouses are queried, it should contain p1
2. Both queries are not symmetrical

Questions:

1. Is my model structure correct for representing marriages symmetrically? If not, what improvements should I make?
2. How can I efficiently query all spouses of a person in a database-optimized way while ensuring symmetry?

My use case is to return a list or Person having pids (partner id) as a list of id like below using DRF

[
{
id: 1,
full_name: ‘John’,
pids: [2]
},
{
id: 2,
full_name: ‘Mary’,
pids: [1]
}
]


Current serializer code is

def get_pids(self, obj):
"""
Returns a list of IDs of all the spouses of the person, ensuring bidirectional relationships.
"""
partner_ids = set(obj.spouses.values_list('id', flat=True))
# Ensure bidirectional relationships
for spouse in obj.spouses.all():
partner_ids.update(spouse.spouses.values_list('id', flat=True))

return list(partner_ids)

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