I am new to beanie, trying to model nested categories as a part of a FastAPI app, so I modeled the category collection as follows

class CategoryBase(Document):
    name: Annotated[str, Indexed(unique=True)] = Field(max_length=100)
    description: Optional[str] = Field(max_length=800, default=None)
    options: Optional[list[str]] = Field(default_factory=list)


class CategoryIn(CategoryBase):
    parent: Optional[PydanticObjectId] = Field(default=None)
    children: Optional[list[PydanticObjectId]] = Field(default_factory=list)


class Category(CategoryIn):
    parent: Optional[Link["Category"]] = Field(default=None)
    children: Optional[list[BackLink["Category"]]] = Field(default_factory=list, 
                                                        original_field="parent")
    date_created: datetime = datetime.now(timezone.utc)
    date_modified: datetime = datetime.now(timezone.utc)
    
    class Settings:
        name = "categories"

I built an end point that creates a “Category”. When I send a payload to it, a new document is created but I get the error in the title of the question.

The internal code that interact with the database looks as follows

class CategoryOps:
    def __init__(self, _id: PydanticObjectId):
        self.id = _id
    
    async def get_category(self):
        cat = await Category.get(self.id)
        return cat

    async def get_parent(self):
        cat = await self.get_category()
        parent = (await Category.get(cat.parent) if cat else None)
        return parent

    async def get_children(self):
        cat = await self.get_category()
        children = ( [await Category.get(child_id) for child_id in cat.children]
                    if self.category.children 
                    else [] )
        return children


class CategoryRepo:

    async def create(cat: CategoryIn) -> Category:
        parent, children = None, []
        
        if cat.parent:
            parent = await CategoryOps(cat.parent).get_category()
            cat.parent = parent
            
        if cat.children:
            children = [await CategoryOps(c).get_category() for c in cat.children]
            cat.children = children
            
        new_category = Category(**cat.model_dump())
        await new_category.save(link_rule=WriteRules.WRITE)       
        await new_category.sync()
        
        return new_category

I expect when I pass a payload that includes a parent id, the children’s field in the parents object automatically gets populated with the link of the newly created category. The same if the payload includes children ids, I expect each of the children objects has its parent field populated with the id of the newly created category.