将外键数据类型从 bigint 更改为字符串

问：

当我尝试保存模型的实例时，我收到错误，因为外键中的数据类型错误，它说它必须是 bigint，但是主键是模型的炭化，它是作为外键的。

我再次检查了主键，我尝试显式链接到该模型的名称字段

这是我 models.py

class Department(models.Model):
    name = models.CharField(max_length=50,primary_key=True)
    building = models.ForeignKey(Building, on_delete=models.PROTECT)

class Purchase(models.Model):
    invoice_no = models.CharField(max_length=30,primary_key=True)
    date = models.DateField()
    department = models.ForeignKey(Department, to_field='name', db_column='department_name', on_delete=models.PROTECT)
    seller = models.CharField(max_length=100)


invendb=# \d inventory_purchase
                     Table "public.inventory_purchase"
     Column      |          Type          | Collation | Nullable | Default 
-----------------+------------------------+-----------+----------+---------
 invoice_no      | character varying(30)  |           | not null | 
 date            | date                   |           | not null | 
 seller          | character varying(100) |           | not null | 
 department_name | bigint                 |           | not null | 
Indexes:
    "inventory_purchase_invoice_no_2b78f964_pk" PRIMARY KEY, btree (invoice_no)
    "inventory_purchase_department_id_28c8ef42" btree (department_name)
    "inventory_purchase_invoice_no_2b78f964_like" btree (invoice_no varchar_pattern_ops)