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Effective caching for nested JSON models.
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Typical as_json definitions may involve lots of database point queries and method calls. When returning collections of objects, a single call may yield hundreds of database queries that can take seconds. This library mitigates the problem by implementing a module called CachedJson.

CachedJson enables returning multiple JSON formats and versions from a single class and provides some rules for yielding embedded or referenced data. It then uses a scheme where fragments of JSON are cached for a particular (class, id) pair containing only the data that doesn't involve references/embedded documents. To get the full JSON for an instance, CachedJson will combine fragments of JSON from the instance with fragments representing the JSON for its references. In the best case, when all of these fragments are cached, this falls through to a few cache lookups followed by a couple Ruby hash merges to create the JSON.

Using Mongoid::CachedJson we were able to cut our JSON API average response time by about a factor of 10.


This gem is compatible with Mongoid 3, 4 and 5.



Add mongoid-cached-json to your Gemfile.

gem 'mongoid-cached-json'

Include Mongoid::CachedJson in your models.

class Gadget
  include Mongoid::CachedJson

  field :name
  field :extras

  belongs_to :widget

  json_fields \
    name: {},
    extras: { properties: :public }


class Widget
  include Mongoid::CachedJson

  field :name
  has_many :gadgets

  json_fields \
    name: {},
    gadgets: { type: :reference, properties: :public }


Invoke as_json.

widget = Widget.first

# the `:short` version of the JSON, `gadgets` not included

# equivalent to the above
widget.as_json(properties: :short)

# `:public` version of the JSON, `gadgets` returned with `:short` JSON, no `:extras`
widget.as_json(properties: :public)

# `:all` version of the JSON, `gadgets` returned with `:all` JSON, including `:extras`
widget.as_json(properties: :all)


By default Mongoid::CachedJson will use an instance of ActiveSupport::Cache::MemoryStore in a non-Rails and Rails.cache in a Rails environment. You can configure it to use any other cache store.

Mongoid::CachedJson.configure do |config|
  config.cache =

The default JSON version returned from as_json is :unspecified. If you wish to redefine this, set Mongoid::CachedJson.config.default_version.

Mongoid::CachedJson.configure do |config|
  config.default_version = :v2

Defining Fields

Mongoid::CachedJson supports the following options:

  • :hide_as_child_json_when is an optional function that hides the child JSON from as_json parent objects, eg. cached_json hide_as_child_json_when: lambda { |instance| ! instance.secret? }

Mongoid::CachedJson field definitions support the following options:

  • :definition can be a symbol or an anonymous function, eg. description: { definition: :name } or description: { definition: lambda { |instance| } }
  • :type can be :reference, required for referenced objects
  • :properties can be one of :short, :public, :all, in this order
  • :version can be a single version for this field to appear in
  • :versions can be an array of versions for this field to appear in
  • :reference_properties can be one of :short, :public, :all, default will select the reference properties format dynamically (see below)

Reference Properties

When calling as_json on a model that contains references to other models the value of the :properties option passed into the as_json call will be chosen as follows:

  • Use the value of the :reference_properties option, if specified.
  • For :short JSON, use :short.
  • For :public JSON, use :short.
  • For :all JSON, use :all.

The dynamic selection where :public generates :short references allows to return smaller embedded collections, while :all allows to fetch deep data. Another way of looking at this is to say that a field in a :short JSON appears in collections, a field declared in the :public JSON appears for all users and the field declared in the :all JSON appears for object owners only.

To override this behavior and always return the :short JSON for a child reference, use :reference_properties. In the following example we would want Person.as_json(properties: :all) to return the social security number for that person, but not for all their friends.

class Person
  include Mongoid::Document
  include Mongoid::CachedJson

  field :name
  field :ssn
  has_and_belongs_to_many :friends, class_name: 'Person'

  json_fields \
    name: {},
    ssn: { properties: :all },
    friends: { properties: :public, reference_properties: :short }



You can set an optional version or versions attribute on JSON fields. Consider the following definition where the first version defined :name, then split it into :first, :middle and :last in version :v2 and introduced a date of birth in :v3.

class Person
  include Mongoid::Document
  include Mongoid::CachedJson

  field :first
  field :last

  def name
    [ first, middle, last ].compact.join(' ')

  json_fields \
    first: { versions: [ :v2, :v3 ] },
    last: { versions: [ :v2, :v3 ] },
    middle: { versions: [ :v2, :v3 ] },
    born: { versions: :v3 },
    name: { definition: :name }

person = Person.create(first: 'John', middle: 'F.', last: 'Kennedy', born: 'May 29, 1917')
person.as_json # { name: 'John F. Kennedy' }
person.as_json(version: :v2) # { first: 'John', middle: 'F.', last: 'Kennedy', name: 'John F. Kennedy' }
person.as_json(version: :v3) # { first: 'John', middle: 'F.', last: 'Kennedy', name: 'John F. Kennedy', born: 'May 29, 1917' }


You can define global transformations on all JSON values with Mongoid::CachedJson.config.transform. Each transformation must return a value. In the following example we extend the JSON definition with an application-specific :trusted field and encode any content that is not trusted.

class Widget
  include Mongoid::Document
  include Mongoid::CachedJson

  field :name
  field :description

  json_fields \
    name: { trusted: true },
    description: {}
  Mongoid::CachedJson.config.transform do |field, definition, value|
    trusted = !!definition[:trusted]
    trusted ? value : CGI.escapeHTML(value)

Mixing with Standard as_json

Taking part in the Mongoid::CachedJson json_fields scheme is optional: you can still write as_json methods where it makes sense.

Turning Caching Off

You can set Mongoid::CachedJson.config.disable_caching = true. It may be a good idea to set it to ENV['DISABLE_JSON_CACHING'], in case this turns out not to be The Solution To All Of Your Performance Problems (TM).

Testing JSON

This library overrides as_json, hence testing JSON results can be done at model level.

describe 'as_json' do
  before :each do
    @person = Person.create!(first: 'John', last: 'Doe')
  it 'returns name' do
    expect(@person.as_json(properties: :public)[:name]).to eql 'John Doe'

It's also common to test the results of the API using the Pathy library.

describe 'as_json' do
  before :each do
    person = Person.create!(first: 'John', last: 'Doe')
  it 'returns name' do
    get "/api/person/#{}"
    expect(response.body.at_json_path('name')).to eql 'John Doe'

Testing Cache Invalidation

Cache is invalidated by calling :expire_cached_json on an instance.

describe 'updating a person' do
  before :each
    @person = Person.create!(name: 'John Doe')
  it 'invalidates cache' do
    expect(@person).to receive(:expire_cached_json)
    @person.update_attributes!(name: 'updated')

You may also want to use this RSpec matcher.

describe 'updating a person' do
  it 'invalidates cache' do
    expect do
      @person.update_attributes!(name: 'updated') invalidate @person


This gem implements two interesting optimizations.

Bulk Reference Resolving w/ Local Store

Consider an array of Mongoid instances, each with numerous references to other objects. It's typical to see such instances reference the same object. Mongoid::CachedJson first collects all JSON references, then resolves them after suppressing duplicates. This significantly reduces the number of cache queries.

Fetching Cache Data in Bulk

Various cache stores, including Memcached, support bulk read operations. The Dalli gem exposes this via the read_multi method. Mongoid::CachedJson will always invoke read_multi where available, which significantly reduces the number of network roundtrips to the cache servers.



Copyright and License

MIT License, see LICENSE for details.

(c) 2012-2014 Artsy and Contributors

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