Are redirect_to and render exchangeable?

For the code below, what happens if replacing redirect_to with render or vise verse?

def create
  @product = Product.new(params[:product])

  respond_to do |format|
    if @product.save
      format.html { redirect_to(@product, :notice => 'Product was successfully created.') }

    else
      format.html { render :action => "new" }
    end
  end
end

It seems OK replacing one with the other in code above. Is there a place where only redirect_to or render has to be used? Render does nothing but rendering a view. Redirect_to sends 302 request to server and current parameters are lost after redirecting.

Thanks.

If you're using render, when the user refreshes the page, it will submit the previous POST request again. This may cause undesired results like duplicate purchase and others.

But if you're using redirect_to, when the user refreshes the page, it will just request that same page again. This is also known as the Post/Redirect/Get (PRG) pattern.

So the place where redirect_to should be used is when you're doing a HTTP POST request and you don't want the user to resubmit the request when it's done (which may cause duplicate items and other problems).

In Rails, when a model fails to be saved, render is used to redisplay the form with the same entries that was filled previously. This is simpler because if you use redirect, you'll have to pass the form entries either using parameters or session. The side effect is that if you refresh the browser, it will try to resubmit the previous form entries. This is acceptable because it will probably fail the same way, or if it's successful now, it was what the user should expect in the first place anyway.

For more in depth explanation about render and redirect, you should read this article.

When you redirect you will generate a new request that hits a controller method, render just renders the associated view. You use render in the create because you want to keep the state of the model object if the save fails so that you can render info about its errors. If you tried to redirect to the new_product path you would create a new model object and loose all the form data the user entered and any errors etc etc

EDIT (with some more info):

An example of a situation where you MUST use redirect_to is if your view template uses instance variables that are not initialized in the controller method you are redirecting from. So you probably could not call render {:action => 'index'} in your create method because the index template probably makes use of a @products variable but your only initialized @product so it would cause an exception

Here is a complete list of what the two methods do that I follow:

1) redirect_to will issue an HTTP 302 status code by default. A 302 redirect is a temporary change and redirects users and search engines to the desired page for a limited amount of time until it is removed. You can optionally specifiy a 301 status code to redirect_to. A 301 status code is used when any page has been permanently moved to another location. Users will now see the new page as it has replaced the old page. This will change the URL of the page when it shows in search engine results.

2) redirect_to will issue a new HTTP request, since it is redirects to a different controller action or URL. You should not make the browser need to make a fresh call unless you really have to, so always question when you are using redirect_to and if it is the right thing, or perhaps a render would be better. - redirect_to will cause any automatic template rendering of the current action to be skipped.

3) render will issue an HTTP 200 status code by default ( but with an invalid ActiveRecord object, you may want to change this to 422 unprocessable entity). The HTTP 200 OK success status response code indicates that the request has succeeded. The 422 (Unprocessable Entity) status code means the server understands the content type of the request entity nd the syntax of the request entity is correct but was unable to process the contained instructions.

4) render will render a template and any instance variables defined in the controller action will be available in the template. Of course, instance variables will not be available if the subsequent action that redirect_to invokes. IMPORTANT POINT: Redirect hits the controller while Render does not, so if you render a different template, it will not hit the action associated with that template and so those instance variables will not be available!

5) With render, use flash.now, instead of the normal flash.

flash.now[:error] = "There was a problem"  
# not 
flash[:error] = "There was a problem"

6) If you don't, then the flash message may not show up on the page that's rendered, and it will show up on the next page that's visited.

7) render will not cause the current action to stop executing! redirect_to will not cause the current action to stop executing! You need to invoke 'return' if you need to bypass further execution of code in the action! In the below code, there is an explicit render at the bottom and so you must do a return to avoid an error of redirect and render both being present:

def update
  @record = Record.new(record_params)
  if @record.save
    flash[:success] = "record was successfully saved"
    redirect_to records_path
    return
  end
  flash.now[:error] = "please fix the problems in the record"
  render :edit
end

Another option:

def update
  @record = Record.new(record_params)
  if @record.save
    flash[:success] = "record was successfully saved"
    redirect_to records_path
  else
    flash.now[:error] = "please fix the problems in the record"
    render :edit
  end  
end

8) The flash message provides a way to pass temporary primitive-types (String, Array, Hash) between actions. Anything you place in the flash will be exposed to the very next action and then cleared out. This is a great way of doing notices and alerts:

class PostsController < ActionController::Base
  def create
    # save post
    flash[:notice] = "Post successfully created"
    redirect_to @post
  end

  def show
    # doesn't need to assign the flash notice to the template, that's done automatically
  end
end

show.html.erb
  <% if flash[:notice] %>
    <div class="notice"><%= flash[:notice] %></div>
  <% end %>

Since you can have both notices and alerts in the flash, you can display both notices and alerts this way:

<% flash.each do |key, value| %>
  <%= content_tag :div, value, class: "flash #{key}" %>
<% end %>