PyMongo error – Upsert failed: E11000 duplicate key error collection

The Problem

While working with MongoDB through PyMongo, encountering an E11000 duplicate key error during an upsert operation is a common issue that perplexes many developers. This error arises when an operation attempts to insert or update a document in a collection using unique index fields with values that are already present in other documents within the collection, violating the uniqueness constraint.

The Reason

The error occurs primarily due to the unique indexes specified in a collection. When performing an upsert operation, MongoDB attempts to insert a new document if one doesn’t exist or update an existing one based on the filter criteria. If the operation inadvertently tries to insert a document with a value already present in a unique index, MongoDB halts the operation and returns the E11000 error.

Solution 1: Ensure Unique Values

Before executing an upsert, ensure that the data you aim to insert or update does not violate any unique index constraints by checking its uniqueness within the application logic.

Steps

1. Query the collection to check if a document with the unique field already exists.
2. If the document exists and needs an update, use the _id of the existing document in your upsert filter criteria.
3. If the document does not exist, proceed with the upsert operation with a unique value.

Code Example:

from pymongo import MongoClient
client = MongoClient('your_mongodb_uri')
collection = client.your_database.your_collection

# Check unique value existence
document = collection.find_one({'unique_field': 'unique_value'})
if document:
    # Update existing document
    collection.update_one({'_id': document['_id']}, {'$set': {'field_to_update': 'new_value'}}, upsert=True)
else:
    # Insert as new document
    collection.update_one({'unique_field': 'unique_value'}, {'$set': {'field_to_update': 'new_value'}}, upsert=True)

This approach directly addresses the cause of the error by ensuring no duplicate values are inserted. However, it requires additional queries and may lead to performance implications if not handled efficiently. Additionally, this method may not suit situations where simultaneous writes are possible, as race conditions can arise.

Solution 2: Handle Exceptions Gracefully

Since some scenarios might inevitably lead to violation of the unique constraint, handling the exception gracefully can be an alternative approach. This involves catching the E11000 error and performing a necessary action based on the application’s logic.

Implementation Steps:

1. Wrap the upsert operation in a try-except block, specifically catching the E11000 error.
2. In the catch block, decide on the course of action – log the error, update the document causing the conflict, or notify the user.

Code Example:

from pymongo import MongoClient
from pymongo.errors import DuplicateKeyError
client = MongoClient('your_mongodb_uri')
collection = client.your_database.your_collection

try:
    collection.update_one({'unique_field': 'unique_value'}, {'$set': {'field_to_update': 'new_value'}}, upsert=True)
except DuplicateKeyError:
    # Handle the error, e.g., logging or notifying
    print('Duplicate key error occurred')

This solution provides a way to manage the conflict without altering the database’s integrity. However, it is reactive rather than proactive, meaning the error will still occur, and the handling is after the fact. This may be suitable in cases where conflicts are rare or the potential duplicate entries are difficult to check in advance.

Conclusion

Choosing the right approach to handle the E11000 duplicate key error in PyMongo upserts depends on your application’s requirements, the specific use-case, and how often duplicates might occur. Proactively ensuring unique values can prevent the error from occurring but may require additional logic to check for duplicates. Conversely, gracefully handling the exception allows the application to continue running but does not prevent the error itself. Both methods have their uses and can be beneficial depending on the scenario.