"ObjectId' object is not iterable" error, while fetching data from MongoDB Atlas

Exclude the "_id" from the output.

result = collection.find_one({'OpportunityID': oppid}, {'_id': 0})

I was having a similar problem to this myself. Not having seen your code I am guessing the traceback similarly traces the error to FastAPI/Starlette not being able to process the "_id" field - what you will therefore need to do is change the "_id" field in the results from an ObjectId to a string type and rename the field to "id" (without the underscore) on return to avoid incurring issues with Pydantic.

First of all, if we had some examples of your code, this would be much easier. I can only assume that you are not mapping your MongoDb collection data to your Pydantic BaseModel correctly.

Read this: MongoDB stores data as BSON. FastAPI encodes and decodes data as JSON strings. BSON has support for additional non-JSON-native data types, including ObjectId which can't be directly encoded as JSON. Because of this, we convert ObjectIds to strings before storing them as the _id. I want to draw attention to the id field on this model. MongoDB uses _id, but in Python, underscores at the start of attributes have special meaning. If you have an attribute on your model that starts with an underscore, pydantic—the data validation framework used by FastAPI—will assume that it is a private variable, meaning you will not be able to assign it a value! To get around this, we name the field id but give it an alias of _id. You also need to set allow_population_by_field_name to True in the model's Config class.

Here is a working example:

First create the BaseModel:

class PyObjectId(ObjectId):
    """ Custom Type for reading MongoDB IDs """
    @classmethod
    def __get_validators__(cls):
        yield cls.validate

    @classmethod
    def validate(cls, v):
        if not ObjectId.is_valid(v):
            raise ValueError("Invalid object_id")
        return ObjectId(v)

    @classmethod
    def __modify_schema__(cls, field_schema):
        field_schema.update(type="string")

class Student(BaseModel):
    id: PyObjectId = Field(default_factory=PyObjectId, alias="_id")
    first_name: str
    last_name: str

    class Config:
        allow_population_by_field_name = True
        arbitrary_types_allowed = True
        json_encoders = {ObjectId: str}

Now just unpack everything:

async def get_student(student_id) -> Student:
    data = await collection.find_one({'_id': student_id})
    if data is None:
        raise HTTPException(status_code=404, detail='Student not found.')
    student: Student = Student(**data)
    return student

use db.collection.find(ObjectId:"12348901384918")
here db.collection is database name and use double quotes for the string .

I was trying to iterate through all the documents and what worked for me was this solution https://github.com/tiangolo/fastapi/issues/1515#issuecomment-782835977

These lines just needed to be added after the child of ObjectID class. An example is given in the following link. https://github.com/tiangolo/fastapi/issues/1515#issuecomment-782838556

I had this issue until I upgraded from mongodb version 5.0.9 to version 6.0.0 so mongodb made some changes on their end to handle this if you have the ability to upgrade! I ran into this issue when creating a test server and when I created a new test server that was 6.0.0, it fixed the error.