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copied!<p>The first chunk is for Rails 3.1 (older versions will be pretty much the same); the second chunk is for the standard non-Rails JSON. Skip to the end if tl;dr.</p> <hr> <p>Your problem is that Rails does this:</p> <pre><code>[Object, Array, FalseClass, Float, Hash, Integer, NilClass, String, TrueClass].each do |klass| klass.class_eval <<-RUBY, __FILE__, __LINE__ # Dumps object in JSON (JavaScript Object Notation). See www.json.org for more info. def to_json(options = nil) ActiveSupport::JSON.encode(self, options) end RUBY end </code></pre> <p>in <code>active_support/core_ext/object/to_json.rb</code>. In particular, that changes Hash's <code>to_json</code> method into just an <code>ActiveSupport::JSON.encode</code> call.</p> <p>Then, looking at <code>ActiveSupport::JSON::Encoding::Encoder</code>, we see this:</p> <pre><code>def encode(value, use_options = true) check_for_circular_references(value) do jsonified = use_options ? value.as_json(options_for(value)) : value.as_json jsonified.encode_json(self) end end </code></pre> <p>So all the Rails JSON encoding goes through <code>as_json</code>. But, you're not defining your own <code>as_json</code> for Set, you're just setting up <code>to_json</code> and getting confused when Rails ignores something that it doesn't use.</p> <p>If you set up your own <code>Set#as_json</code>:</p> <pre><code>class Set def as_json(options = { }) { "json_class" => self.class.name, "data" => { "elements" => self.to_a } } end end </code></pre> <p>then you'll get what you're after in the Rails console and Rails in general:</p> <pre><code>> require 'set' > s = Set.new([1,2,3]) > s.to_json => "{\"json_class\":\"Set\",\"data\":{\"elements\":[1,2,3]}}" > h = { :set => s } > h.to_json => "{\"set\":{\"json_class\":\"Set\",\"data\":{\"elements\":[1,2,3]}}}" </code></pre> <p>Keep in mind that <code>as_json</code> is used to prepare an object for JSON serialization and then <code>to_json</code> produces the actual JSON string. The <code>as_json</code> methods generally return simple serializable data structures, such as Hash and Array, and have direct analogues in JSON; then, once you have something that is structured like JSON, <code>to_json</code> is used to serialize it into a linear JSON string.</p> <hr> <p>When we look at the standard non-Rails JSON library, we see things like this:</p> <pre><code>def to_json(*a) as_json.to_json(*a) end </code></pre> <p>monkey patched into the basic classes (Symbol, Time, Date, ...). So once again, <code>to_json</code> is generally implemented in terms of <code>as_json</code>. In this environment, we need to include the standard <code>to_json</code> as well as the above <code>as_json</code> for Set:</p> <pre><code>class Set def as_json(options = { }) { "json_class" => self.class.name, "data" => { "elements" => self.to_a } } end def to_json(*a) as_json.to_json(*a) end def self.json_create(o) new o["data"]["elements"] end end </code></pre> <p>And we include your <code>json_create</code> class method for the decoder. Once that's all properly set up, we get things like this in <code>irb</code>:</p> <pre><code>>> s = Set.new([1,2,3]) >> s.as_json => {"json_class"=>"Set", "data"=>{"elements"=>[1, 2, 3]}} >> h = { :set => s } >> h.to_json => "{"set":{"json_class":"Set","data":{"elements":[1,2,3]}}}" </code></pre> <hr> <p><strong>Executive Summary</strong>: If you're in Rails, don't worry about doing anything with <code>to_json</code>, <code>as_json</code> is what you want to play with. If you're not in Rails, implement most of your logic in <code>as_json</code> (despite what the documentation says) and add the standard <code>to_json</code> implementation (<code>def to_json(*a);as_json.to_json(*a);end</code>) as well.</p>

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