Automatic level-agnostic secondary indexing
Last updated 10 months ago by bret .
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$ cnpm install level-auto-index 
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Automatic secondary indexing for leveldb and subleveldown.

npm install level-auto-index

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var level = require('level')
var AutoIndex = require('level-auto-index')
var sub = require('subleveldown')
var keyReducer = AutoIndex.keyReducer

var db = level()

var posts = sub(db, 'posts', {valueEncoding: 'json'})
var idx = {
  title: sub(db, 'title'),
  length: sub(db, 'length'),
  tag: sub(db, 'tag')

// add a title index
posts.byTitle = AutoIndex(posts, idx.title, keyReducer('title'))

// add a length index
// append the post.id for unique indexes with possibly overlapping values
posts.byLength = AutoIndex(posts, idx.length, function (post) {
  return post.body.length + '!' + post.id

// Create multiple index keys on an index
posts.byTag = AutoIndex(posts, idx.tag, function (post) {
    if (!post || !post.tags || !Array.isArray(post.tags)) return
    return post.tags.map(function (tag) {
      return [tag, post.id].join('!')
  }, { multi: true })

posts.put('1337', {
  id: '1337',
  title: 'a title',
  body: 'lorem ipsum',
  tags: [ 'foo', 'bar', 'baz' ]
}, function (err) {
  if (err) throw err

  posts.byTitle.get('a title', function (err, post) {
    if (err) throw err
    console.log('get', post)
    // => get: { id: '1337', title: 'a title', body: 'lorem ipsum' }

    posts.del('1337', function (err) {
      if (err) throw err
      posts.byTitle.get('a title', function (err) {
        // => NotFoundError

    start: 10,
    end: 20
  }).on('data', console.log.bind(console, 'read'))
  // => read { key: '1337', value: { id: '1337', title: 'a title', body: 'lorem ipsum' } }

    start: 10,
    end: 20
  }).on('data', console.log.bind(console, 'key'))
  // => key 1337

    start: 10,
    end: 20
  }).on('data', console.log.bind(console, 'value'))
  // => value { id: '1337', title: 'a title', body: 'lorem ipsum' }


AutoIndex(db, idb, reduce, opts)

Automatically index a db level into the idb level using a reduce function that creates the index key. The db and idb levels should be in isolated key namespaces, either by being two different levels or mafintosh/subleveldown partitions. The db hook is mutated by hypermodules/level-hookdown to set up the prehooks used for indexing. Only db keys are stored as values to save space and reduce data redundancy.

Secondary returns an AutoIndex level that helps prune old index values, and automatically looks up source documents from db as you access keys on the AutoIndex level.

The reduce functions get the value of the put or batch operations. Make sure that this value has everything you need to create your index keys.

function reducer (value) {
  var idxKey = value.foo + '!' + value.bar
  return idxKey

Available opts:

  multi: false // Reducer returns an array of keys to associate with the primary key

Multi-key index's are for when you you want to write multiple index entries into an index. This is useful for 'tag' fields, where a document may have n tags per document, and you would like to index documents by 'tag'. When creating a multi-key index, your reducer must return an array of keys to index by.

AutoIndex#get(key, opts, cb)

Get the value that has been indexed with key.


Create a readable stream that has indexes as keys and indexed data as values.


A level manifest that you can pass to multilevel.


A shortcut reducer for simplistic key indexing. You might need more than this.

function keyReducer (reducerString) {
  function keyRdc (value) {
    return value[reducerString]
  return keyRdc

For a higher level api for creating secondary indexes see hypermodules/level-idx.


A shortcut reducer for simplistic multi-key indexing. You might need more than this.

function multiKeyReducer (multiFieldName, primaryKeyFieldName) {
  return function multiKeyrdc (document) {
    if (!document || !document[multiFieldName] || !Array.isArray(document[multiFieldName])) return
    return document[multiFieldName].map(function (tag) {
      return [tag, document[primaryKeyFieldName]].join('!')


The level instance that we are indexing.


The level instance that we are using for the index.

See Also

This module is a variant of

but aimed at decoupling the index storage fromt the indexd db and also being compatable with subleveldown. It came out of the work trying to make level-secondary compatable with subleveldown and level-sublevel. That work lives here: github.com/bcomnes/level-secondary/commit/9b2f914e53.

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