Programming language: Crystal
Tags: Data Formats    
Latest version: v0.1

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json_on_steroids provides powerful JSON document transformation and edition with some advanced features to navigate through the keys.

Basically, it can be seen as a JSON::Any object on steroid.

The current Crystal stdlib JSON implementation is made of immutable structures. While being performant, it turns out dealing with JSON blob with a fully typed language can be really painful.

Transforming JSON to hash or array is often not enough and lead to obscure type errors and ninja type castings.

JSON::OnSteroids trades some performance to offers functionalities:

  • Mutate or construct from zero a JSON schema easily
  • Pass Hash, Array, NamedTuple etc... as parameters of keys without hassle
  • Check whether a schema is dirty (has changed) or not
  • Errors telling you what key/sub-schema is wrong at runtime
  • Out of the box support of Time, which happens to be common in JSON
  • Interface well with PostgreSQL's JSONB column and Clear ORM


Build a JSON::OnSteroids structure:

  # build from scratch (empty object `{}`)
  json = JSON::OnSteroids.new

  # build from initial json
  json = JSON.parse(%<{"a": 1, "b": 2}>).on_steroids!
  json = JSON::OnSteroids.new(JSON.parse(%<{"a": 1, "b": 2}>))

  # build from initial hash and named tuples:
  json = JSON::OnSteroids.new a: 1, b: 2
  json = JSON::OnSteroids.new({"a" => 1, "b" => 2})

  # build from empty object json
  json = JSON::OnSteroids.new

  # build array
  json = JSON::OnSteroids.new [1,2,3]

  # build a value (is it useful?)
  json = JSON::OnSteroids.new "a value"

Exporting to json

  json.to_json #obvious enough

Getter / setters

The API of JSON::Any for getters are supported. So, as with JSON::Any, you can access to a specific field by using [] then casting using as_xxx where xxxx can be i, s, b etc...

Basically, a JSON::OnSteroids can be passed wherever a JSON::Any object is required, while it's not true in the other way.

JSON::OnSteroids add setters:

json = JSON.parse(%<{"type": "event", "type": "favorites_numbers","data": [1,2,3,4] }>).on_steroids!

json["data"][0] = "Hello"

json.to_json # => {"type": "event", "type": "favorites_numbers","data": ["Hello",2,3,4] }

# Automatically import from named tuples, arrays and hashes

from_tuple = {
  type: "collection",
  pages: {
    count: 3,
    current: 2,
    next_page: "http://myservice/api/collection?page=3"
  data: [
    { type: "_user", id: 1 },
    { type: "_user", id: 2 }

json = JSON::OnSteroids.new(from_tuple)


Digging allows you to fetch a key in your JSON schema:

  json = JSON.parse(%<{"type": "event", "is": "favorites_numbers","data": [1,2,3,4] }>).on_steroids!
  puts json.dig("data.1").as_i #=> 2

Two flavors of dig method exists: dig(string) which throw an error if the key is not found / schema doesn't match and dig?(string) which return nil in case it can't dig (the key doesn't match).

Set / remove in place

Digging can be combined with set in place and remove feature:

  json = JSON.parse(%<{"other": {"counter": 123}}>).on_steroids!

  json.dig("other.counter").set(&.as_i.+(1)) # Add 1 to the counter
  puts json.to_json # => {"other": {"counter": 124}}

  puts json.to_json # => {"other": {}}


JSON::OnSteroids objects are aware of few states:

  • You can reverse traversing by calling json.parent
  • How deep they are in the document by calling json.depth
  • If they are dirty (e.g. they mutate) by calling json.dirty?
  • In case they belongs to a map or an array, which key they are mapped to by calling json.key
  • The full path of an element can be found by calling json.path

JSON::OnSteroids objects can introspect about their mutation state. It also can return the schema with the only mutated elements:

  json = JSON.parse(%<{"key": 1, "other": {"counter": 123}}>).on_steroids!

  json["other"]["counter"] = 543

  json.dirty? #=> true

  puts json.dirty_only.to_json # => {"other": {"counter": 543}}

This is useful for:

  • Snapshot between your JSON document
  • Merge-able noSQL databases will crunch this in no time \o/

You can clean a dirty object by calling clean! on it:

json.dirty? # => true
json.clean! #
json.dirty? # => false


Performance tradeoff

JSON::OnSteroids works by encapsulating all values in a wrapper class. Performance wise, it creates new object everytime you mutate a value. Moreover, each key keep a reference to the parent and a dirty boolean field.

In term of CPU, it has a small overhead, usually negligible. In terms of memory, the overhead of the wrapper is 17 bytes per fields, array items comprised. It can then become voluminous in case of processing large JSON of few megabytes.

In case you dealing with very-large JSON or are in a memory-constrainted environnement, I would recommend you to use data mapping or serialization strategies.

Interfacing with libraries


Currently, json_on_steroids interface with Clear, as that's how I use it.

Add this optional requirement to use it into clear

require "json_on_steroids/ext/clear"

Now your jsonb columns are mapped :).

  column jsonb : JSON::OnSteroids

Usage of dirty? allows the edition in place of your json:

  mdl = Model.query.first!
  mdl.jsonb["my"]["content"] = "is awesome"
  mdl.changed? # => true
  mdl.update_h #=> {"jsonb" => JSON::OnSteroids}

If you want to interface in another ORM or libraries, pull request are welcome !

Future on this shard

Currently not implemented but planned:


WIP: Not implemented yet.

You can search through the document a key responding to a specific rule:

puts "Events from facebook or google:"

# search every elements which contains the keys `type` and `provider`:
json.search(type: "event", provider: /^(facebook|google)$/){ |evt|
  puts evt["url"]