---
title: Redis Partial Word Match -- You (Auto)complete Me
teaser:
tags: web,redis
author: Harlow Ward
published_on: 2013-04-25
---

![Redis autocomplete][5]

We can use partial word matching to rapidly search strings of text such as Names,
Cities, States, etc.

We can do this by indexing strings into Redis [sets][6] based on partial matches of
the string. The indexing process takes a string and breaks it into left-bound
substrings which are placed into the appropriate Redis sets.

## Partial word matching movie titles

In this example we’ll enable partial word matching queries of movie titles.

The movie titles will be stored in plain text. The keys are simple key-value
pairs that are stored with the key structure `movies:id`, where `id` is
incremented:

    $ redis-cli get movies:1
    "Bad Santa"
    $ redis-cli get movies:2
    "Batman"
    $ redis-cli get movies:3
    "Bad Company"

The movie `Batman` would be decomposed into the following sets (assuming that
the indexing started at two characters):

    ba
    bat
    batm
    batma
    batman

We'll index the movie titles into sets based on partial match of their titles.
To prevent any key collisions in Redis we'll create keys with the following
structue `index:abc:movies`.

View the indexed ids using Redis [smembers][7] command:

    $ redis-cli smembers index:ba:movies
    1) "1"
    2) "2"
    3) "3"

    $ redis-cli smembers index:bat:movies
    1) "2"

Note: The keys for the movie titles have been parameterized (lowercase and spaces
replaced with dashes).

We'll use a Ruby class `Autocomplete` to wrap the Redis calls:

    # autocomplete.rb
    require 'redis'

    class Autocomplete
      def initialize(partial_word)
        @partial_word = partial_word
      end

      def movies
        redis.sort "index:#{partial_word}:movies", by: :nosort, get: 'movies:*'
      end

      private

      def redis
        Redis.current
      end

      attr_reader :partial_word
    end

    movies = Autocomplete.new('bat').movies

We'll use the [sort][2] command with the `by: :nosort` parameter to query the
partial word match index and return the matched movie names.

### Related Reading

[Redis Set Intersection - Using sets to filter data][3]

[Redis Pub/Sub…how does it work?][4]

[2]: http://redis.io/commands/sort
[3]: https://thoughtbot.com/blog/post/46335890055/redis-set-intersection-using-sets-to-filter-data
[4]: https://thoughtbot.com/blog/post/6325247416/redis-pub-sub-how-does-it-work
[5]: http://img42.imageshack.us/img42/4776/autocompletef.png
[6]: http://redis.io/topics/data-types
[7]: http://redis.io/commands/smembers