I see the example for defining settings and mapping for an index/model in the README of https://github.com/elastic/elasticsearch-rails/tree/master/elasticsearch-model:

class Article
  settings index: { number_of_shards: 1 } do
    mappings dynamic: 'false' do
      indexes :title, analyzer: 'english', index_options: 'offsets'
    end
  end
end

However, I could not find a

The README needs an overhaul for new releases.

See also: ActiveTriples/ActiveTriples#209

• It will be available when the active model validation feature is enabled.
• attribute :foo, validates: {}
• attributes :foo, :bla, bar: { validates: {} }
• Update README examples.

I got this error message when one of items in array is not in the subset passed to validates.
It seems that I have to add some translations to i18n files, but there is no documentation of what have to be added.