Rendering JSON in controller

Question

I was reading a book and in a chapter about Controllers when it talks about rendering stuff  for JSON it has an example like this but doesn t go in to details so I couldn t figure out the bigger picture that this example fits in   render  json   gt   projects   include   gt  tasks   And also some example with JSONP using it with callback functions   render  json   gt   record   callback   gt   updateRecordDisplay    Can someone explain these

User · Answer

For the instance of   render  json   gt   projects   include   gt   tasks   You are stating that you want to render  projects as JSON  and include the association tasks on the Project model in the exported data   For the instance of   render  json   gt   projects   callback   gt   updateRecordDisplay    You are stating that you want to render  projects as JSON  and wrap that data in a javascript call that will render somewhat like   updateRecordDisplay   projects    gt         This allows the data to be sent to the parent window and bypass cross-site forgery issues

User · Answer

You ll normally be returning JSON either because   A  You are building part   all of your application as a Single Page Application  SPA  and you need your client-side JavaScript to be able to pull in additional data without fully reloading the page   or  B  You are building an API that third parties will be consuming and you have decided to use JSON to serialize your data   Or  possibly  you are eating your own dogfood and doing both  In both cases render  json   gt  some data will JSON-ify the provided data   The  callback key in the second example needs a bit more explaining  see below   but it is another variation on the same idea  returning data in a way that JavaScript can easily handle    Why  callback   JSONP  the second example  is a way of getting around the Same Origin Policy that is part of every browser s built-in security   If you have your API at api yoursite com and you will be serving your application off of services yoursite com your JavaScript will not  by default  be able to make XMLHttpRequest  XHR - aka ajax  requests from services to api   The way people have been sneaking around that limitation  before the Cross-Origin Resource Sharing spec was finalized  is by sending the JSON data over from the server as if it was JavaScript instead of JSON    Thus  rather than sending back     name    John    age   45    the server instead would send back   valueOfCallbackHere   name    John    age   45     Thus  a client-side JS application could create a script tag pointing at api yoursite com your endpoint name John and have the valueOfCallbackHere function  which would have to be defined in the client-side JS  called with the data from this other origin

User · Answer

What exactly do you want to know  ActiveRecord has methods that serialize records into JSON  For instance  open up your rails console and enter ModelName all to json and you will see JSON output  render  json essentially calls to json and returns the result to the browser with the correct headers  This is useful for AJAX calls in JavaScript where you want to return JavaScript objects to use  Additionally  you can use the callback option to specify the name of the callback you would like to call via JSONP   For instance  lets say we have a User model that looks like this   name   Max   email   m m com    We also have a controller that looks like this   class UsersController  lt  ApplicationController     def show          user   User find params  id           render json   user     end end   Now  if we do an AJAX call using jQuery like this     ajax       type   GET       url    users 5       dataType   json       success  function data           alert data name     Will alert Max                     As you can see  we managed to get the User with id 5 from our rails app and use it in our JavaScript code because it was returned as a JSON object  The callback option just calls a JavaScript function of the named passed with the JSON object as the first and only argument    To give an example of the callback option  take a look at the following   class UsersController  lt  ApplicationController     def show          user   User find params  id           render json   user  callback   testFunction      end end   Now we can crate a JSONP request as follows   function testFunction data        alert data name      Will alert Max     var script   document createElement  script    script src     users 5    document getElementsByTagName  head   0  appendChild script     The motivation for using such a callback is typically to circumvent the browser protections that limit cross origin resource sharing  CORS   JSONP isn t used that much anymore  however  because other techniques exist for circumventing CORS that are safer and easier

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