Ruby serialize struct with JSON

You can also define a struct with a to_json method. Depends if you're happy with calling to_h.to_json. If it's only called once internally in a class, it may be tolerable and ignore this. But if the struct is used throughout a system below is a convient helper method on the struct.

require 'struct'
require 'json'

MyStruct = Struct.new(:foo, :bar) do
  def to_json
    to_h.to_json
  end
end

simple_struct = MyStruct.new("hello", "world")
simple_struct.to_json

# => "{\"foo\":\"hello\",\"bar\":\"world\"}"

In this case, person.to_json is not doing what you expect.

When you require 'json', the JSON library inserts a #to_json method on Object that is a fallback if there's no specialized #to_json method provided elsewhere. This inserted method is basically the same as calling #to_s#to_json on an object.

In the case of your Person class here, #to_s outputs the standard Object#to_s, which, by default, doesn't provide a string parseable by the JSON library.

However, Struct does provide a #to_h method that can be used to convert that struct to a Hash, and Hash is (upon requiring the JSON library) aware of how to generate a JSON parseable output.

So simply changing:

json = Person.new('Adam', 19).to_json
puts json

to:

person = Person.new('Adam', 19)
puts person.to_h.to_json

will do what you expect.

(An aside, I would actually recommend implementing #to_json on the Person class directly as calling #to_h#to_json violates the Law of Demeter.)