How to override to_json in Rails?

Question

---

Update:

This issue was not properly explored. The real issue lies within render :json.

The first code paste in the original question will yield the expected result. However, there is still a caveat. See this example:

render :json => current_user

is NOT the same as

render :json => current_user.to_json

That is, render :json will not automatically call the to_json method associated with the User object. In fact, if to_json is being overridden on the User model, render :json => @user will generate the ArgumentError described below.

summary

# works if User#to_json is not overridden
render :json => current_user

# If User#to_json is overridden, User requires explicit call
render :json => current_user.to_json

This all seems silly to me. This seems to be telling me that render is not actually calling Model#to_json when type :json is specified. Can someone explain what's really going on here?

Any genii that can help me with this can likely answer my other question: How to build a JSON response by combining @foo.to_json(options) and @bars.to_json(options) in Rails

---

Original Question:

I've seen some other examples on SO, but I none do what I'm looking for.

I'm trying:

class User < ActiveRecord::Base

  # this actually works! (see update summary above)
  def to_json
    super(:only => :username, :methods => [:foo, :bar])
  end

end

I'm getting ArgumentError: wrong number of arguments (1 for 0) in

/usr/lib/ruby/gems/1.9.1/gems/activesupport-2.3.5/lib/active_support/json/encoders/object.rb:4:in `to_json

Any ideas?

Answer 1

You are getting ArgumentError: wrong number of arguments (1 for 0) because to_json needs to be overridden with one parameter, the options hash.

def to_json(options)
  ...
end

---

Longer explanation of to_json, as_json, and rendering:

In ActiveSupport 2.3.3, as_json was added to address issues like the one you have encountered. The creation of the json should be separate from the rendering of the json.

Now, anytime to_json is called on an object, as_json is invoked to create the data structure, and then that hash is encoded as a JSON string using ActiveSupport::json.encode. This happens for all types: object, numeric, date, string, etc (see the ActiveSupport code).

ActiveRecord objects behave the same way. There is a default as_json implementation that creates a hash that includes all the model's attributes. You should override as_json in your Model to create the JSON structure you want. as_json, just like the old to_json, takes an option hash where you can specify attributes and methods to include declaratively.

def as_json(options)
  # this example ignores the user's options
  super(:only => [:email, :handle])
end

In your controller, render :json => o can accept a string or an object. If it's a string, it's passed through as the response body, if it's an object, to_json is called, which triggers as_json as explained above.

So, as long as your models are properly represented with as_json overrides (or not), your controller code to display one model should look like this:

format.json { render :json => @user }

The moral of the story is: Avoid calling to_json directly, allow render to do that for you. If you need to tweak the JSON output, call as_json.

format.json { render :json => 
    @user.as_json(:only => [:username], :methods => [:avatar]) }