JsonValidator is an ActiveModel validator that validates any hash field against JSONSchema, returning errors in the model's own errors attribute.

This gem was originally written to provide deep validation of JSON attributes, which are available alongside primative types in recent versions of PostgreSQL, but it works equally well with ActiveModel objects.

Most of the functionality is dependent on the wonderful json-schema gem.

If you're using Ruby on Rails and ActiveRecord, add a validation to your model like this:

class Foo < ActiveRecord::Base
  validates :bar, json: {
    schema: JSON.parse(File.read('foo_schema.json'))
  }
end

And you have a schema file (ie. foo_schema.json) like this:

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "title": "Universal spoons schema",
    "properties": {
      "handleSize": {
      "type": "integer",
      "minimum": 0
    }
  },
  "required": ["handleSize"]
}

Then whenever an instance of Foo is saved, Foo.bar (assumed to be a hash) will be validated against the JSON schema specified. So, for example:

f = Foo.new(bar: { handleSize: -10 })
f.valid? # false
f.errors.full_messages # ["Bar is invalid (the property '#/handleSize' did not have a minimum value of 0, inclusively)"]

The attribute being validated can be either a hash or a string (which will be parsed as JSON). The schema can be either a hash or a Proc that returns a hash (if you'd like to decide on the schema at runtime), and there's no reason why you could not load your schema from a .json file.