Django One to Many Loose Relationship - django

Question

Backstory
Data is being pulled from an accounting system that can have department, market, and client relationship data associated with it. The relationship data are all TEXT/CHAR fields. They are not integer columns. There are over 2 million rows.
The problem
The problem I've been running into is how to add the lines without relationship validation that could fail because the related table (like Market) is missing the value or has been changed (because we are looking far back in the past). The naming of the columns in the Django database (more detail below), and querying Django models with a join that don't have a relationship attribute on the class.
models.py
from typing import Optional
from django.db import models
class Line(models.Model):
entry_number: int = models.IntegerField(db_index=True)
posting_date: date = models.DateField()
document_number: Optional[str] = models.CharField(max_length=150, null=True, default=None)
description: Optional[str] = models.CharField(max_length=150, null=True, default=None)
department: Optional[str] = models.CharField(max_length=150, null=True, default=None)
market: Optional[str] = models.CharField(max_length=150, null=True, default=None)
amount: Decimal = models.DecimalField(max_digits=18, decimal_places=2)
client: Optional[str] = models.CharField(max_length=150, null=True, default=None)
# This relationship works
account = models.ForeignKey(Account, on_delete=models.DO_NOTHING, related_name='lines')
class Department(models.Model):
code: str = models.CharField(max_length=10, db_index=True, unique=True, primary_key=True)
name: str = models.CharField(max_length=100, null=True)
class Market(models.Model):
code: str = models.CharField(max_length=10, db_index=True, unique=True, primary_key=True)
name: str = models.CharField(max_length=100, null=True)
The data for the line is filled in the a sql statement grabbing the data from the accounting system.
In SqlAlchemy
What I am looking for is something like this which can be represented in sqlalchemy.
class Line(...):
# snipped
client_rel: Client = relationship("Client", primaryjoin=client == foreign(Client.code), viewonly=True)
department_rel: Department = relationship("Department", primaryjoin=department == foreign(Department.code), viewonly=True)
market_rel: Market = relationship("Market", primaryjoin=market == foreign(Market.code), viewonly=True)
# snipped the other related classes
The SQLAlchemy code allows for primaryjoins which are relationships only enforced at the python level not in the database. It also allows for the relatanship's loose foreign key to be named almost anything. As far as I can tell Django's foreign key column has to be "name"_id and can't be changed.