JSI: JSON Schema Instantiation

JSI offers an Object-Oriented representation for JSON data using JSON Schemas. Given your JSON Schemas, JSI constructs Ruby classes which are used to instantiate your JSON data. These classes let you use JSON with all the niceties of OOP such as property accessors and application-defined instance methods.

To learn more about JSON Schema see https://json-schema.org/.

A JSI class aims to be a fairly unobtrusive wrapper around its instance - "instance" here meaning the JSON data which instantiate the JSON Schema. The instance is usually a Hash or an Array but may be basic types, or in fact any object. A JSI class adds accessors for property names described by its schema, schema validation, and other nice things. Mostly though, you use a JSI as you would use its underlying data, calling the same methods (e.g. #[], #map, #repeated_permutation) and passing it to anything that duck-types expecting #to_ary or #to_hash.

Example

Words are boring, let's code. Here's a schema in yaml:

description: "A Contact"
type: "object"
properties:
  name: {type: "string"}
  phone:
    type: "array"
    items:
      type: "object"
      properties:
        location: {type: "string"}
        number: {type: "string"}

And here's how you'd normally instantiate the class for that schema using JSI:

# this would usually use a YAML.load/JSON.parse/whatever; it's inlined for copypastability.
Contact = JSI.class_for_schema({"description" => "A Contact", "type" => "object", "properties" => {"name" => {"type" => "string"}, "phone" => {"type" => "array", "items" => {"type" => "object", "properties" => {"location" => {"type" => "string"}, "number" => {"type" => "string"}}}}}})

This definition gives you not just the Contact class, but classes for the whole nested structure. To instantiate it, we need some JSON data (expressed here as YAML)

name: bill
phone:
- location: home
  number: "555"
nickname: big b

So, if we construct an instance like:

# this would usually use a YAML.load/JSON.parse/whatever; it's inlined for copypastability.
bill = Contact.new({"name" => "bill", "phone" => [{"location" => "home", "number" => "555"}], "nickname" => "big b"})
# => #{<Contact Hash>
#   "name" => "bill",
#   "phone" => #[<JSI::SchemaClasses["23d8#/properties/phone"] Array>
#     #{<JSI::SchemaClasses["23d8#/properties/phone/items"] Hash> "location" => "home", "number" => "555"}
#   ],
#   "nickname" => "big b"
# }

Note that the keys are strings. JSI, being designed with JSON in mind, is geared toward string keys. Symbol keys will not match to schema properties, and so act the same as any other key not recognized from the schema.

The nested classes can be seen in the #inspect output as JSI::SchemaClasses[schema_id] where schema_id is a generated value.

We get accessors for the Contact:

bill.name
# => "bill"

but also nested accessors - #phone is an instance of its array-type schema, and each phone item is an instance of another object-type schema with location and number accessors:

bill.phone.map(&:location)
# => ["home"]

We also get validations, as you'd expect given that's largely what json-schema exists to do:

bill.validate
# => true

... and validations on the nested schema instances (#phone here), showing in this example validation failure:

bad = Contact.new('phone' => [{'number' => [5, 5, 5]}])
# => #{<Contact Hash>
#   "phone" => #[<JSI::SchemaClasses["23d8#/properties/phone"] Array>
#     #{<JSI::SchemaClasses["23d8#/properties/phone/items"] Hash>
#       "number" => #[<JSI::SchemaClasses["23d8#/properties/phone/items/properties/number"] Array> 5, 5, 5]
#     }
#   ]
# }
bad.phone.fully_validate
# => ["The property '#/0/number' of type array did not match the following type: string in schema 23d8"]

These validations are done by the json-schema gem - JSI does not do validations on its own.

Since the underlying instance is a ruby hash (json object), we can use it like a hash with #[] or, say, #transform_values:

bill.transform_values(&:size)
# => {"name" => 4, "phone" => 1, "nickname" => 5}
bill['nickname']
# => "big b"

There's plenty more JSI has to offer, but this should give you a pretty good idea of basic usage.

Terminology and Concepts

JSI::Base is the base class for each JSI class representing a JSON Schema.
a "JSI class" is a subclass of JSI::Base representing a JSON schema.
"instance" is a term that is significantly overloaded in this space, so documentation will attempt to be clear what kind of instance is meant:
- a schema instance refers broadly to a data structure that is described by a JSON schema.
- a JSI instance (or just "a JSI") is a ruby object instantiating a JSI class. it has a method #instance which contains the underlying data.
a schema refers to a JSON schema. a JSI::Schema represents such a schema. a JSI class allows instantiation of a schema as a JSI instance.

JSI classes

A JSI class (that is, subclass of JSI::Base) is a starting point but obviously you want your own methods, so you reopen the class as you would any other. referring back to the Example section above, we reopen the Contact class:

class Contact
  def full_address
    address.values.join(", ")
  end
  def name
    super + ' esq.'
  end
  def name=(name)
    super(name.chomp(' esq.'))
  end
end

bill.name
# => "bill esq."
bill.name = 'rob esq.'
# => "rob esq."
bill['name']
# => "rob"

Note the use of super - you can call to accessors defined by JSI and make your accessors act as wrappers (these accessor methods are defined on an included module instead of the JSI class for this reason). You can also use [] and []=, of course, with the same effect.

If you want to add methods to a subschema, get the class_for_schema for that schema and open up that class. You can leave the class anonymous, as in this example:

phone_schema = Contact.schema['properties']['phone']['items']
JSI.class_for_schema(phone_schema).class_eval do
  def number_with_dashes
    number.split(//).join('-')
  end
end
bill.phone.first.number_with_dashes
# => "5-5-5"

If you want to name the class, this works:

Phone = JSI.class_for_schema(Contact.schema['properties']['phone']['items'])
class Phone
  def number_with_dashes
    number.split(//).join('-')
  end
end

Either syntax is slightly cumbersome and a better syntax is in the works.

ActiveRecord serialization

A really excellent place to use JSI is when dealing with serialized columns in ActiveRecord.

Let's say you're sticking to json types in the database - you have to do so if you're using json columns, or json serialization, and if you have dealt with arbitrary yaml- or marshal-serialized objects in ruby, you have probably found that approach has its shortcomings when the implementation of your classes changes.

But if your database contains json, then your deserialized objects in ruby are likewise Hash / Array / basic types. You have to use subscripts instead of accessors, and you don't have any way to add methods to your data types.

JSI gives you the best of both with JSICoder. This coder dumps objects which are simple json types, and loads instances of a specified JSI class. Here's an example:

class User < ActiveRecord::Base
  serialize :contact_info, JSI::JSICoder.new(Contact)
end

Now user.contacts will return an array of Contact instances, from the json type in the database, with Contact's accessors, validations, and user-defined instance methods.

See the gem arms if you wish to serialize the dumped JSON-compatible objects further as text.

Keying Hashes (JSON Objects)

Unlike Ruby, JSON only supports string keys. It is recommended to use strings as hash keys for all JSI instances, but JSI does not enforce this, nor does it do any key conversion. It should be possible to use ActiveSupport::HashWithIndifferentAccess as the instance of a JSI in order to gain the benefits that offers over a plain hash. This is not tested behavior, but JSI should behave correctly with any instance that responds to #to_hash.

Contributing

Issues and pull requests are welcome on GitHub at https://github.com/notEthan/jsi.

License

JSI is open source software available under the terms of the MIT License.