We can use pry and pry-byebug gem to help us dive into the source code.

Let’s see an example about to_json vs as_json

I found there’s a to_json and as_json method for active record query result. to_json returns String of json. as_json returns a hash representing the model.

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> Order.where(number: "201504211405490").to_json
# => "[{\"production_status\":\"idle\",\"id\":11,\"number\":\"201504211405490\"}]"
> Order.where(number: "201504211405490").to_json.class
# => String

> Order.where(number: "201504211405490").as_json
# => [{"production_status"=>"idle", "id"=>11, "number"=>"201504211405490"}]
> Order.where(number: "201504211405490").as_json.class
# => Hash

I find that as_json can accepts some optons.

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> Order.where(number: "201504211405490").as_json(only: :number)
# => [{"number"=>"201504211405490"}]

But one day I found somebody used similar options with to_json method. I searched the doc. No doc says to_json can do things like following:

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> Order.where(number: "201504211405490").to_json(only: :number)
# => "[{\"number\":\"201504211405490\"}]"

But the code above actually works. Is to_json uses as_json under the hood?

Dive into the source

Now we can check the source code to find the answer.

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> $ Order.where(number: "201504211405490").to_json

From: /Users/lingceng/.rvm/gems/ruby-2.2.0@baozheng/gems/activesupport-4.2.0/lib/active_support/core_ext/object/json.rb @ line 31:
Owner: Object
Visibility: public
Number of lines: 9

def to_json_with_active_support_encoder(options = nil)
  if options.is_a?(::JSON::State)
    # Called from JSON.{generate,dump}, forward it to JSON gem's to_json
    self.to_json_without_active_support_encoder(options)
  else
    # to_json is being invoked directly, use ActiveSupport's encoder
    ActiveSupport::JSON.encode(self, options)
  end
end

Then we edit the method and add a binding.pry before the if options.is_a?(::JSON::State) line. Remember to recover the change after debug.

> edit Order.where(number: "201504211405490").to_json

Then run code again:

> Order.where(number: "201504211405490").to_json

Now we’ll get into to_json_with_active_support_encoder method and start to debug. After some next and step comand. We can find that to_json uses ActiveSupport::JSON.encode, which uses as_json.

# /lib/active_support/json/encoding.rb#34
# Encode the given object into a JSON string
def encode(value)
  stringify jsonify value.as_json(options.dup)
end

Yes! to_json uses as_json. We find the answer. So we can use to_json(only: :number) with confidence.

Conclusion

Pry and pry-byebug provides many useful commands. Using these commands can help to find the definition of a method, track the call stacks, understand the source structure.

This is beauty of open source. Do not limit yourself. Dive into the code and get your information.