How can I serialize a MongoDB ObjectId with Marshmallow?

Question

I\'m building and API on top of Flask using marshmallow and mongoengine. When I make a call and an ID is supposed to be serialized I receive the following error:

Answer 1

Similar to @dcollien above, I extended fields.Field and created my own custom field with helpers, similar to how Marshmallow handles field types internally:

from marshmallow import fields, missing
from marshmallow.exceptions import ValidationError
from bson.objectid import ObjectId
from bson.errors import InvalidId
import json

def oid_isval(val: Any) -> bool:
    """
    oid_isval [summary]

    Parameters
    ----------
    val : {Any}
        Value to be assessed if its an ObjectId

    Returns
    ----------
    val : bool
        True if val is an ObjectId, otherwise false
    """    
    if ObjectId.is_valid(val):
        return val

def ensure_objid_type(val: Union[bytes, str, ObjectId]) -> ObjectId:
    """
    Ensures that the value being passed is return as an ObjectId and is a valid ObjectId

    Parameters
    ----------
    val : Union[bytes, str, ObjectId]
        The value to be ensured or converted into an ObjectId and is a valid ObjectId

    Returns
    ----------
    val : ObjectId
        Value of type ObjectId
        
    Raises
    ----------
    ValidationError: Exception
        If it's not an ObjectId or can't be converted into an ObjectId, raise an error.
            
    """
    try:
        # If it's already an ObjectId and it's a valid ObjectId, return it
        if isinstance(val, ObjectId) and oid_isval(val):
            logger.info(f"It's an ObjectId and it's valid! = {val}")
            return val
        
        # Otherwise, if it's a bytes object, decode it and turn it into a string
        elif isinstance(val, bytes):
            val = ObjectId(str(val.decode("utf-8")))
            logger.info(f"Decoded and converted bytes object to ObjectId! = {val}")

        # Otherwise, if it's a string, turn it into an ObjectId and check that it's valid 
        elif isinstance(val, str):
            val = ObjectId(val)
            logger.info(f"Converted str to ObjectId! = {val}")
        
        # Check to see if the converted value is a valid objectId
        if oid_isval(val):
            logger.info(f"It's a valid ObjectId! = {val}")
            return val
    except InvalidId as error:
        logger.error(f"Not a valid ObjectId = {val} | error = {error}")
        raise ValidationError(json.loads(json.dumps(f"{error}")))

class ObjectIdField(fields.Field):
    """Custom field for ObjectIds."""
    # Default error messages
    default_error_messages = {
        "invalid_ObjectId": "Not a valid ObjectId."
    }

    def _serialize(self, value, attr, obj, **kwargs) -> Optional[ObjectId]:
        if value is None:
            return None
        return ensure_objid_type(value)

    def _deserialize(self, value, attr, data, **kwargs):
        if value is None:
            return missing
        if not isinstance(value, (ObjectId, str, bytes)):
            raise self.make_error("_deserialize: Not a invalid ObjectId")
        try:
            return ensure_objid_type(value)
        except UnicodeDecodeError as error:
            raise self.make_error("invalid_utf8") from error
        except (ValueError, AttributeError, TypeError) as error:
            raise ValidationError("ObjectIds must be a 12-byte input or a 24-character hex string") from error