mongo-cursor-pagination-alt

Cursor-based pagination for MongoDB.

Based on mongo-cursor-pagination but written from scratch in TypeScript with an API that resembles the cursor connection spec from Relay.

• The problem
• The solution
• Installation
• Usage
• API
  • findPaginated
  • aggregatePaginated
• License

The problem

By default, MongoDB allows us to paginate results with limit and skip. That's known as offset-based pagination. It's definitely the simplest way to paginate, but in some situations it's not very stable.

Imagine you have a restaurant software showing a list of orders paginated with limit: 3 and skip: 0:

• { _id: ObjectId, number: 53 }
• { _id: ObjectId, number: 52 }
• { _id: ObjectId, number: 51 }

Nice, so far so good. Now, when you click the button to see the next page, here's what you'd expect to see (limit: 3, skip: 3):

• { _id: ObjectId, number: 50 }
• { _id: ObjectId, number: 49 }
• { _id: ObjectId, number: 48 }

Pretty reasonable, right? By using skip: 3 we're basically telling MongoDB hey, I want to skip orders #53, #52 and #51.

But imagine a new order (#54) arrived while we were staring at the 1st page. In that case, querying with skip: 3 would actually give us:

• { _id: ObjectId, number: 51 }
• { _id: ObjectId, number: 50 }
• { _id: ObjectId, number: 49 }

That's because in this new scenario we're actually telling MongoDB hey, I want to skip orders #54, #53 and #52.

If you have any trouble to fully comprehend what's happening here, try to imagine the following scenario yourself:

• You've queried the 1st page with limit: 3 and skip: 0.
• While you are looking at the 1st page, 200 new orders arrive.
• What will you see when you try to query the 2nd page with limit: 3 and skip: 3?

The solution

So how can we make sure that the 2nd page always start from the order #50 no matter how many orders have been received in the meantime? Well, a naive approach would be like:

collection
  .find({ number: { $lt: 51 } }
  .sort({ number: -1 })
  .toArray()

And that's somewhat what this lib does but in a much more robust way.

For a more thorough explanation I highly recommend reading the excellent blog post from Mixmax. You can also read the source code — I tried to add helpful comments.

Installation

npm i mongo-cursor-pagination-alt

yarn add mongo-cursor-pagination-alt

Usage

import { MongoClient } from 'mongodb'
import { findPaginated } from 'mongo-cursor-pagination-alt'

const client = await MongoClient.connect(uri)
const db = client.db()
const collection = db.collection('users')

let result

// Get first page
result = await findPaginated(collection, {
  first: 10,
})

// Go to second page
result = await findPaginated(collection, {
  first: 10,
  after: result.pageInfo.endCursor,
})

// Back to first page
result = await findPaginated(collection, {
  last: 10,
  before: result.pageInfo.startCursor,
})

API

findPaginated

Runs a paginated query on top of Collection#find.

Arguments

• collection: The MongoDB collection.
• params:
  • Properties:
    • first: How many documents to get (forwards pagination).
    • after: The cursor (forwards pagination).
    • last: How many documents to get (backwards pagination).
    • before: The cursor (backwards pagination).
    • query: The query to be passed to Collection#find.
    • sort: The sort to be passed to Collection#find.
    • projection: The projection to be passed to Collection#find.

Returns

• edges: An array with the contents of the page.
  • Properties:
    • node: The document.
    • cursor: An opaque string pointing to this document in the context of this query. It can be passed to parameters after or before to get a page starting after or before this document.
• pageInfo:
  • Properties
    • startCursor: The cursor of the first document in this page.
    • endCursor: The cursor of the last document in this page.
    • hasPreviouPage: Whether there's another page before this one.
    • hasNextPage: Whether there's another page after this one.

aggregatePaginated

Runs a paginated aggregation on top of Collection#aggregate.

Arguments

• collection: The MongoDB collection.
• params:
  • Properties:
    • first: How many documents to get (forwards pagination).
    • after: The cursor (forwards pagination).
    • last: How many documents to get (backwards pagination).
    • before: The cursor (backwards pagination).
    • pipeline: The pipeline array to be passed to Collection#aggregate.
    • sort: The sort to be passed to Collection#aggregate.

Returns

• edges: An array with the contents of the page.
  • Properties:
    • node: The document.
    • cursor: An opaque string pointing to this document in the context of this query. It can be passed to parameters after or before to get a page starting after or before this document.
• pageInfo:
  • Properties
    • startCursor: The cursor of the first document in this page.
    • endCursor: The cursor of the last document in this page.
    • hasPreviouPage: Whether there's another page before this one.
    • hasNextPage: Whether there's another page after this one.

License

Released under the MIT License.