Composing json from cached strings in ruby

Question

Consider the following scenario, I have quite a few big hashes that I need to put in an array and then convert it to json:

hash1 = { ... big hash ... }
hash2 = { ... big hash ... }
hash3 = { ... big hash ... }
array = [hash1, hash2, hash3]

json = JSON.dump(array)

The problem is that generating json from those hashes takes a long time, so I would like to cache it. However, I can't cache the whole array, only separate items. Obviously putting cached json string in array gives bad results:

hash1 = {:a => 1}
hash1json = JSON.dump(hash1)
array = [hash1json]
JSON.generate(array)
==> ["{\"a\":1}"]

while I need

==> [{"a":1}]

The only way I can think of is doing something like this:

"[#{[hash1json].join(",")}]"
==> [{"a":1}]

which might be enough for this specific case, but it will be much harder if one wanted to cache some deep structure instead of simple array.

Answer 1

Turns out this is actually dead simple:

class CachedJson
  def initialize(str)
    @str = str
  end

  def to_json
    @str
  end
end

puts Yajl::Encoder.encode(:data => [{:a => 1}, '{"b":2}'])
# => {"data":[{"a":1},"{\"b\":2}"]}

puts Yajl::Encoder.encode(:data => [{:a => 1}, CachedJson.new('{"b":2}')])
# => {"data":[{"a":1},{"b":2}]}

Under the hood yajl calls to_json on every object, and this method must return string, so it's just a matter of wrapping cached json string with CachedJson object