render magic could use more detail...

In chapter 11 page 131

“The render() method will iterate over any collection that
is passed to it.”

This could use more information on what is going on. I think what is happening is the render command is replaces this:

<% @cart.line_items.each do |item| %>

This is an iteration over a the line items and when you call
<% render @cart.line_items %> it is somehow replacing the .each do and iterating over the line_items to populate this:

<= line_item.quantity %>×
<= line_item.product.title >
<= number_to_currency(line_item.total_price) %>

But notice the change which is not pointed out in the book :

Before you the ”.each do |item|” set up the variable item to be passed to the following:

<= item.quantity %>×
<= item.product.title >
<= number_to_currency(item.total_price) %>

now item.quantity is replaced by line_item.quantity and so on so that the iterating of line_items called by the magic render method can find the quantity and title in the line_item database.

if this is the wrong way of thinking of render then please correct me.

I am having trouble understanding render as well.
In the rails doc, render does not take any object as parameter
http://guides.rubyonrails.org/layouts_and_rende…

Basically I want to render the object in different ways, and I am trying to figure out how to tell render which erb to use.

The way to tell render to use a different template is via:

render template: 'name'

The way to pass objects to a template is to set instance variables (staring with an ”@” sign) and reference them within the template.