How to override to_json in Rails?

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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]) }

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