Mongodb Question posted in
The official documentation can be found here.

Mongodb – How can I use decimal in my schema for FastAPI mongo?

I have python fastapi app with mongodb.
Below I provided schema and code to create a document.
I need to use decimal, because there’re problems with floating point in float data type. However, there are problems with using condecimal with mongo.

class FlowerTypeSchema(BaseModel):
    id: UUID = None
    merchant_id: Optional[UUID] = None
    name: LangSchema
    description: LangSchema
    photo: Optional[str] = None
    custom: Optional[bool] = False
    rating: Optional[condecimal(ge=0)] = 0
    tags: List[str]
    consumables: List[ProductsSpentSchema]
    created_at: datetime = None
    updated_at: datetime = None
    deleted_at: datetime = None

async def create_flower_type(flower_type: FlowerTypeSchema) -> FlowerTypeResponse:
    flower_type.id = uuid4()
    flower_type.photo = None
    flower_type.created_at = datetime.now(pytz.timezone(TIMEZONE)).strftime("%Y-%m-%dT%H:%M:%S")
    flower_type.updated_at = datetime.now(pytz.timezone(TIMEZONE)).strftime("%Y-%m-%dT%H:%M:%S")
    await flower_types_collection.insert_one(flower_type.dict())
    return FlowerTypeResponse(**flower_type.dict())

I want to use decimal for rating (and for price later), but I am getting an error:

2024-06-01 14:46:32,147 - uvicorn.error - ERROR - Exception in ASGI application
Traceback (most recent call last):
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesuvicornprotocolshttph11_impl.py", line 408, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesuvicornmiddlewareproxy_headers.py", line 84, in __call__
    return await self.app(scope, receive, send)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesfastapiapplications.py", line 1106, in __call__
    await super().__call__(scope, receive, send)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesstarletteapplications.py", line 122, in __call__
    await self.middleware_stack(scope, receive, send)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesstarlettemiddlewareerrors.py", line 184, in __call__
    raise exc
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesstarlettemiddlewareerrors.py", line 162, in __call__
    await self.app(scope, receive, _send)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesstarlettemiddlewarecors.py", line 83, in __call__
    await self.app(scope, receive, send)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesstarlettemiddlewareexceptions.py", line 79, in __call__
    raise exc
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesstarlettemiddlewareexceptions.py", line 68, in __call__
    await self.app(scope, receive, sender)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesfastapimiddlewareasyncexitstack.py", line 20, in __call__
    raise e
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesfastapimiddlewareasyncexitstack.py", line 17, in __call__
    await self.app(scope, receive, send)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesstarletterouting.py", line 718, in __call__
    await route.handle(scope, receive, send)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesstarletterouting.py", line 276, in handle
    await self.app(scope, receive, send)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesstarletterouting.py", line 66, in app
    response = await func(request)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesfastapirouting.py", line 274, in app
    raw_response = await run_endpoint_function(
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagesfastapirouting.py", line 191, in run_endpoint_function
    return await dependant.call(**values)
  File "c:UsersAkmalITOyGulog-py-merchant-content-serviceroutesflower_types.py", line 31, in create_flower_type_data
    new_flower_type = await create_flower_type(flower_type=data)
  File "c:UsersAkmalITOyGulog-py-merchant-content-servicedatabaseflower_types.py", line 17, in create_flower_type
    await flower_types_collection.insert_one(flower_type.dict())
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libconcurrentfuturesthread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagespymongocollection.py", line 669, in insert_one
    self._insert_one(
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagespymongocollection.py", line 609, in _insert_one
    self.__database.client._retryable_write(acknowledged, _insert_command, session)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagespymongomongo_client.py", line 1523, in _retryable_write
    return self._retry_with_session(retryable, func, s, bulk)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagespymongomongo_client.py", line 1421, in _retry_with_session
    return self._retry_internal(
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagespymongo_csot.py", line 107, in csot_wrapper
    return func(self, *args, **kwargs)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagespymongomongo_client.py", line 1462, in _retry_internal
    ).run()
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagespymongomongo_client.py", line 2315, in run
    return self._read() if self._is_read else self._write()
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagespymongomongo_client.py", line 2422, in _write
    return self._func(self._session, conn, self._retryable)  # type: ignore
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagespymongocollection.py", line 597, in _insert_command
    result = conn.command(
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagespymongohelpers.py", line 322, in inner
    return func(*args, **kwargs)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagespymongopool.py", line 996, in command
    self._raise_connection_failure(error)
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagespymongopool.py", line 968, in command
    return command(
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagespymongonetwork.py", line 151, in command
    request_id, msg, size, max_doc_size = message._op_msg(
  File "C:UsersAkmalAppDataLocalProgramsPythonPython310libsite-packagespymongomessage.py", line 762, in _op_msg
    return _op_msg_uncompressed(flags, command, identifier, docs, opts)
bson.errors.InvalidDocument: cannot encode object: Decimal('4.5'), of type: <class 'decimal.Decimal'>
Tags:

3

Answers


  1. 0

    You are trying to use a Decimal field type which does not have an equivalent in JSON and BSON. So PyMongo won’t automatically convert it to something compatible. Even when using a response_model, FastAPI/Pydantic don’t have default handlers for this, unlike datetime, UUID, etc.

    Part 1: Conversion for MongoDB

    You’ll need to convert that to a string or a Decimal128 which can be used as a MongoDB type.

    Decimal128() values are 128-bit decimal-based floating-point numbers that emulate decimal rounding with exact precision.

    Do that with @field_serializer. Also make use of flower_type.model_dump() instead of flower_type.dict():

    from bson.decimal128 import Decimal128

class FlowerTypeSchema(BaseModel):
    ...  # other fields
    rating: Optional[condecimal(ge=0)] = 0
    
    @field_serializer('rating', when_used='unless-none')
    def bytes_as_base64(r: Union[Decimal, condecimal]):
        return Decimal128(r)

    Part 2: Conversion for FastAPI response

    For the FastAPI side, since you have a FlowerTypeResponse as well as a FlowerTypeSchema, you can also use that class with inheritance and override rating to convert to string. Like in the FastAPI docs example for In/OutUser

    class FlowerTypeResponse(FlowerTypeSchema):  # inherit from FlowerTypeSchema
    rating: Optional[str] = "0"  # note the change in type
    
    # and add a validator which will convert the Decimal to string
    ...
    Login or Signup to reply.
  2. 0

    The problem is that pymongo can’t handle Decimal, but fastapi can’t handle bson.Decimal128. So you need different serialization methods for different contexts. I had to solve the same problem for my application that handles currency values. I did it with a custom type Money.

    ########### Custom Serialization Stuff for Decimal128 Money Type
def validate_money(v: Any) -> Decimal:
    """
    BeforeValidator:
    Convert anything to a Decimal
    """
    match v:
        case Decimal128():
            logger.trace(f"validate_money: input is {type(v)}")
            return v.to_decimal()
        case float():
            return Decimal(str(v))
        case Decimal():
            return v
        case _:
            logger.trace(f"validate_money: input is {type(v)}")
            try:
                return Decimal(v)
            except Exception as e:
                raise ValueError("amount must be a valid number") from e


def serialize_money(v: Decimal, info: SerializationInfo) -> Decimal128 | float:
    """
    PlainSerializer:
    Serialize, depending on Context
    """
    if info.context:
        output_type = info.context.get("money")
        if output_type == "Decimal128":
            logger.trace(f"serialize_money: {type(v)} to Decimal128")
            return Decimal128(str(v))
    logger.trace(f"serialize_money: {type(v)} to float")
    return float(v)


Money = Annotated[
    Decimal,
    BeforeValidator(validate_money),
    PlainSerializer(serialize_money),
]

    Money will take a range of input values and convert them to Decimal. By default it will serialize to a float, so it’s usable in json output.

    But it offers a context switch to serialize it as bson.Decimal128:
    So in your case, when preparing a model to store into database, you would use:

    flower_type_to_mongodb = flower_type.model_dump(context={"money": Decimal128"})
await flower_types_collection.insert_one(flower_type_to_mongodb)

    I’m open for suggestions on how to improve this btw, but the good thing is that it works in a reusable way.

    Login or Signup to reply.
  3. 0

    You can create your custom codec to work with Decimal. Check the link.

    It should looks like:

        from bson.decimal128 import Decimal128
    from bson.codec_options import TypeCodec, TypeRegistry, CodecOptions
    class DecimalCodec(TypeCodec):
        python_type = Decimal 
        bson_type = Decimal128

    def transform_python(self, value):
        return Decimal128(value)

    def transform_bson(self, value):
        return value.to_decimal()

    decimal_codec = DecimalCodec()
    type_registry = TypeRegistry([decimal_codec])
    codec_options = bson.codec_options.CodecOptions(type_registry=type_registry)
    db.get_collection("flower_types_collection", codec_options=codec_options).insert_one(flower_type.dict())
    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search