PyMongo error: Upsert must be an instance of dict, not str

Overview

The ‘PyMongo error: Upsert must be an instance of dict, not str’ is a common issue faced by developers when using the PyMongo library. This error typically arises when attempting to update or insert data into a MongoDB collection using an incorrect format. Understanding the error’s root causes and knowing how to fix it is crucial for maintaining the integrity of your database operations. Below, we explore the causes of this error and provide practical solutions to resolve it.

Understanding the Error

The error message clearly indicates that the upsert operation expected a dictionary object, but instead received a string. In MongoDB, an upsert operation is a combination of update and insert. This operation requires specific arguments, including the query condition and the new data to be upserted. Both of these should be dictionary objects in Python when using PyMongo. Should you pass a string instead of a dictionary for the new data or query condition, this error will occur.

Solution 1: Convert String to Dictionary

If your data is mistakenly formatted as a string instead of a dictionary, converting it to the correct format will resolve the error.

1. If you have serialized data (e.g., a JSON string), use the json.loads() method to convert it to a dictionary.
2. Ensure that your data follows the correct structure for an upsert operation in Mongo, which includes specifying fields to be updated or inserted.

Code Example:

import json
from pymongo import MongoClient

def db_connection():
    client = MongoClient('mongodb://localhost:27017/')
    return client['your_database_name']

def upsert_document(collection, query_condition, new_data):
    collection.update_one(query_condition, {'$set': new_data}, upsert=True)

coll = db_connection().your_collection_name
new_data = '{"field1": "value1", "field2": "value2"}' # This is the problematic string
new_data_dict = json.loads(new_data) # Converting to dictionary
query_condition = {'field1': 'value1'}

upsert_document(coll, query_condition, new_data_dict)

Notes: This solution is straightforward but requires that your data is in a serializeable format. If your data is already a dictionary or not in a JSON format, this method may not be applicable.

Solution 2: Verify Data Type Before Upsert

Proactively checking the data type before performing the upsert operation can prevent errors. This method involves programmatically verifying that the data variable is a dictionary before attempting the upsert.

1. Perform a type check on the data variable.
2. If the data is not a dictionary, convert it to the correct format or raise an error.

Example:

from pymongo import MongoClient

def db_connection():
    client = MongoClient('mongodb://localhost:27017/')
    return client['your_database_name']

def upsert_document(collection, query_condition, data):
    if not isinstance(data, dict):
        raise ValueError('Upsert data must be a dictionary')
    collection.update_one(query_condition, {'$set': data}, upsert=True)

coll = db_connection().your_collection_name

data = '{"key": "value"}' # this would cause an error

try:
    upsert_document(coll, {'key': 'value'}, data)
except ValueError as e:
    print(f'Error: {e}')

Notes: This method effectively prevents the error but requires additional code to handle data validation. It’s an effective preventive measure but increases code complexity.

Understanding that the upsert operation in PyMongo requires dictionary objects can help prevent this common error. Ensuring your data is in the correct format before attempting an upsert operation will save time and avoid unnecessary debugging. Both suggested solutions have their benefits and limitations, so choosing the right one depends on your specific situation.