Resolving Duplicate Key Issues in React Infinite Scroll with MongoDB Sort Behavior

Introduction

While implementing infinite scroll functionality using React and MongoDB, I encountered an issue where data was duplicating at pagination boundaries. This article explores the cause of this problem, which stems from MongoDB's sorting behavior, and presents a concrete solution using the _id field.

Target Audience

• Developers working with React and MongoDB applications
• Those implementing or planning to implement infinite scroll/pagination
• Anyone interested in understanding database sorting behaviors

TL;DR

MongoDB's sorting behavior can lead to inconsistent ordering of documents with identical sort values. This causes key duplication warnings in React infinite scroll implementations. The solution involves adding the _id field as a secondary sort key.

Development Environment

- React 18.x
- MongoDB 8.0.0
- MongoDB Node.js Driver 6.9.0
- TypeScript 5.x
- Node.js 23.5.0

Problem Details

When querying a MongoDB Collection with a large dataset using filters, duplicate data appeared in the infinite scroll display. Specifically, the last item from the first fetch overlapped with the first item from the second fetch.

Warning Message

Warning: Encountered two children with the same key, `.$xxxxxxxxxxxx`. 
Keys should be unique so that components maintain their identity across updates. 
Non-unique keys may cause children to be duplicated and/or omitted — the behavior 
is unsupported and could change in a future version.

Reproduction Conditions

The issue occurs when:

• MongoDB query includes filter conditions
• Sorting is in ascending order
• Retrieving data beyond the first page

Problematic Code

// Backend: MongoDB query
const sortObject = {
  [sortBy]: order === "asc" ? 1 : -1
};
const items = await Collection.find(query)
  .sort(sortObject)
  .skip(skip)
  .limit(limit)
  .lean()
  .exec();

// Frontend: React component
const loadMore = async (page: number) => {
  const response = await fetchItems(page);
  setState((prev) => ({
    items: [...prev.items, ...response.items],
    page,
  }));
};

Root Cause Analysis

MongoDB Sorting Behavior Characteristics

1. Documents with identical sort values don't maintain consistent ordering
2. When combined with pagination (skip and limit), the same document might appear on different pages
3. This results in React displaying items with duplicate keys

Solution

Add _id as a secondary sort key.
Reference: MongoDB Documentation: sort()

Modified MongoDB Query

const sortObject = {
  [sortBy]: order === "asc" ? 1 : -1,
  _id: 1  // Add _id as a secondary sort key
};
const items = await Collection.find(query)
  .sort(sortObject)
  .skip(skip)
  .limit(limit)
  .lean()
  .exec();

Why This Works

1. The _id field in MongoDB guarantees uniqueness
2. When primary sort keys have equal values, _id ordering ensures consistent results
3. This prevents pagination duplication

Technical Analysis

MongoDB Sort Behavior

• By default, the order of documents with identical sort values is indeterminate
• This behavior becomes more pronounced in distributed systems and sharding scenarios
• Stable sorting requires including a unique value in the sort keys

Summary

When implementing infinite scroll with MongoDB, sorting behavior can cause issues in these scenarios:

1. Sort fields contain duplicate values
2. Pagination boundaries in large datasets
3. Multiple filter condition combinations

Utilizing the _id field as a secondary sort key enables stable sort results and clean infinite scroll implementation.

References

• MongoDB Documentation: sort()
• React Documentation: Lists and Keys