bookshelf-cursor-pagination

Bookshelf plugin that implements cursor based pagination (also known as keyset pagination).

Install

npm install bookshelf-cursor-pagination

Usage

fetchCursorPage is the same as fetchPage but with cursors instead. A cursor is a series of column values that uniquely identify the position of a row in a result set. If only the primary ID is sorted a cursor is simply the primary ID of a row. Arguments:

limit: size of page (defaults to 10)
before: array of values that correspond to sorted columns
after: array of values that correspond to sorted columns

If there is no sorting and the cursor (before or after) has one element, we implicitly sort by the id attribute.

before and after are mutually exclusive. before means we fetch the page of results before the row represented by the cursor. after means we fetch the page of results before the row represented by the cursor.

import cursorPagination from 'bookshelf-cursor-pagination'

// ...

bookshelf.plugin(cursorPagination)

// ...
class Car extends Bookshelf.Model {
  get tableName() { return 'cars' }
}

const result = await Car.collection()
  .orderBy('manufacturer_id')
  .orderBy('description')
  .fetchCursorPage({
    after: [/* manufacturer_id */ '8', /* description */ 'Cruze'],
  })

console.log(result.models)

// ...

console.log(result.pagination)

/*
{ limit: 10,
  rowCount: 27,
  hasMore: true,
  cursors: { after: [ '17', 'Impreza' ], before: [ '8', 'Impala' ] },
  orderedBy:
   [ { name: 'manufacturer_id', direction: 'asc', tableName: 'cars' },
     { name: 'description', direction: 'asc', tableName: 'cars' } ] }
*/

// A next() method is also available on the collection to fetch the next
// set of result

Example of stable iteration with cursors:

// will iterate by batches of 5 until the end
const iter = async (doSomething, after) => {
  const coll = await Car.collection()
    .orderBy('id')
    .fetchCursorPage({ after, limit: 5 })
  await doSomething(coll)
  if (coll.pagination.hasMore) {
    return iter(doSomething, coll.pagination.cursors.after)
  }
}

iter((collection) => {
  console.log(collection.models.length)
  // 5
})

This plugin also adds a forEach method that takes the same arguments as fethPage and a callback which is called for every result set.

For example:

const main = async () => {
  await Car
    .collection()
    .orderBy('id')
    .forEach({ limit: 5 }, async (coll) => {
      // do something with collection
    })
  console.log('iterated over all rows!')
}

Joins and/or .format

fetchCursorPage will break if one of the sorted columns is not accessible via model.get(colName) (either because the column is not returned by the select or because the bookshelf object implements a .format() method).

In order to avoid this issue, you can implement a toCursorValue on your model that will handle those edge cases. For example:

Car.prototype.toCursorValue = function ({ name, tableName }) {
  if (tableName === this.tableName) return this.get(name)
  if (tableName === 'engines' && name === 'name') {
    return this.get('engine_name')
  }
  throw new Error(`cannot extract cursor for ${tableName}.${name}`)
}

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