Question

I have a Rails app running Mongoid on Heroku and I need to set up some indexes to make the database faster. I’ve tried to read about it several places and I know Mongoid has some built-in functions for indexing, but I’m not sure on what to apply them and how often to index.

It is mostly my Design-model I want to index:

scope :full_member_and_show, where(full_member: true).and(show: true).desc(:created_at)
scope :not_full_member_and_show, where(full_member: false).and(show: true).desc(:created_at)

embeds_many :comments
belongs_to :designer

search_in :tags_array

attr_accessible :image,  :tags, :description, :title, :featured, :project_number, :show



field :width
field :height
field :description
field :title
field :tags, type: Array
field :featured, :type => Boolean, :default => false
field :project_number, :type => Integer, :default => 0
field :show, :type => Boolean, :default => true
field :full_member, :type => Boolean, :default => false
field :first_design, :type => Boolean, :default => false

What do I need to index, how exactly do I do it with Mongoid and how often should I do it?

ERROR UPDATE

If try to index the below:

index({ full_member: 1, show: 1 }, { unique: true })

It throws me this error:

Invalid index specification {:full_member=>1, :show=>1}; should be either a string, symbol, or an array of arrays.

Answer 1

You don’t need to index periodically: once you’ve added an index, mongo keeps that index up to date as the collection changes. This is the same as an index in MySQL or Postgres (you may have been thinking of something like solr)

What to index depends on what queries you’ll be making against your dataset. Indexes do carry some overhead when you do updates and consume disk space so you don’t want to add them when you don’t need them.

You tell mongoid what indexes you want by index, for example

class Person
  include Mongoid::Document
  index :city
end

There are loads of examples in the mongoid docs for the various kinds of indexes mongo supports.

Then you run

rake db:mongoid:create_indexes

This determines what indexes you want (based in the calls to index in your model) and then ensures that they exist in the db, creating them if necessary. In development you’d run this after adding indexes to your models. In production it makes sense to run this as part of your deploy (you only need to if you’ve added indexes since the last deploy but it’s way easier to just do it systematically)

There’s a lot of information about how mongo uses indexes in the documentation