query_type.rb

module Types
  class QueryType < GraphQL::Schema::Object
    # Add root-level fields here.
    # They will be entry points for queries on your schema.

    # queries are represented as fields, use [Types::XXX] for the list
    field :product, Types::ProductType,
          null: true,
          resolve: ->(_obj, args, _ctx) {
            Product.find(args.id)
          } do
            argument :id, String, required: true
            argument :days, Int, required: false
          end
  end
end