Ruby JSON Schema Validator

This library is intended to provide Ruby with an interface for validating JSON objects against a JSON schema conforming to JSON Schema Draft 3. Legacy support for JSON Schema Draft 2 and JSON Schema Draft 1 is also included.

Dependencies

The JSON::Schema library has no dependencies if the validation methods are called using Ruby objects. However, either the json or the yajl-ruby gem needs to be installed to validate JSON strings or files containing JSON data.

Installation

From rubygems.org:

gem install json-schema

From the git repo:

$ gem build json-schema.gemspec
$ gem install json-schema-0.9.8.gem

Usage

Two base validation methods exist: validate and validate!. The first returns a boolean on whether a validation attempt passes and the latter will throw a JSON::Schema::ValidationError with an appropriate message/trace on where the validation failed.

Both methods take two arguments, which can be either a JSON string, a file containing JSON, or a Ruby object representing JSON data. The first argument to these methods is always the schema, the second is always the data to validate. An optional third options argument is also accepted; available options are used in the examples below.

By default, the validator uses the JSON Schema Draft 3 specification for validation; however, the user is free to specify additional specifications or extend existing ones. Legacy support for Draft 1 and Draft 2 is included by either passing an optional :version parameter to the validate method (set either as :draft1 or draft2), or by declaring the $schema attribute in the schema and referencing the appropriate specification URI. Note that the $schema attribute takes precedence over the :version option during parsing and validation.

Validate Ruby objects against a Ruby schema

require 'rubygems'
require 'json-schema'

schema = {
  "type" => "object",
  "properties" => {
    "a" => {"type" => "integer", "required" => true}
  }
}

data = {
  "a" => 5
}

JSON::Validator.validate(schema, data)

Validate a JSON string against a JSON schema file

require 'rubygems'
require 'json-schema'
  
JSON::Validator.validate('schema.json', '{"a" : 5}')

Validate a list of objects against a schema that represents the individual objects

require 'rubygems'
require 'json-schema'

data = ['user','user','user']
JSON::Validator.validate('user.json', data, :list => true)

Catch a validation error and print it out

require 'rubygems'
require 'json-schema'
  
schema = {
  "type" => "object",
  "properties" => {
    "a" => {"type" => "integer", "required" => true}
  }
}
  
data = {
  "a" => "taco"
}

begin
  JSON::Validator.validate!(schema, data)
rescue JSON::Schema::ValidationError
  puts $!.message
end

Validate an object against a JSON Schema Draft 2 schema

require 'rubygems'
require 'json-schema'

schema = {
  "type" => "object",
  "properties" => {
    "a" => {"type" => "integer", "optional" => true}
  }
}

data = {
  "a" => 5
}

JSON::Validator.validate(schema, data, :version => :draft2)

Extend an existing schema and validate against it

For this example, we are going to extend the JSON Schema Draft 3 specification by adding a ‘bitwise-and’ property for validation.

require 'rubygems'
require 'json-schema'
  
class BitwiseAndAttribute < JSON::Schema::Attribute
  def self.validate(current_schema, data, fragments, validator, options = {})
    if data.is_a?(Integer) && data & current_schema.schema['bitwise-and'].to_i == 0
      message = "The property '#{build_fragment(fragments)}' did not evaluate  to true when bitwise-AND'd with  #{current_schema.schema['bitwise-or']}"
      raise JSON::Schema::ValidationError.new(message, fragments, current_schema)
    end
  end
end

class ExtendedSchema < JSON::Schema::Validator
  def initialize
    super
    extend_schema_definition("http://json-schema.org/draft-03/schema#")
    @attributes["bitwise-and"] = BitwiseAndAttribute
    @uri = URI.parse("http://test.com/test.json")
  end
  
  JSON::Validator.register_validator(self.new)
end

schema = {
  "$schema" => "http://test.com/test.json",
  "properties" => {
    "a" => {
      "bitwise-and" => 1
    },
    "b" => {
      "type" => "string"
    }
  }
}

data = {
  "a" => 0
}

data = {"a" => 1, "b" => "taco"}
JSON::Validator.validate(schema,data) # => true
data = {"a" => 1, "b" => 5}
JSON::Validator.validate(schema,data) # => false
data = {"a" => 0, "b" => "taco"}
JSON::Validator.validate(schema,data) # => false

JSON Backends

The JSON::Schema library currently supports the json and yajl-ruby backend JSON parsers. If either of these libraries are installed, they will be automatically loaded and used to parse any JSON strings supplied by the user.

If more than one of the supported JSON backends are installed, the yajl-ruby parser is used by default. This can be changed by issuing the following before validation:

JSON::Validator.json_backend = :json

Notes

The following core schema attributes are not implemented:

• default – This library aims to solely be a validator and does not modify an object it is validating.

The ‘format’ attribute is only validated for the following values:

• date-time
• date
• time
• ip-address
• ipv6

All other ‘format’ attribute values are simply checked to ensure the instance value is of the correct datatype (e.g., an instance value is validated to be an integer or a float in the case of ‘utc-millisec’).

Additionally, JSON::Validator does not handle any json hyperschema attributes.