Difference between :class and :id , under what circumstances would we generally use <%= %> and <% %> in Rails

when we make use of rails helpers like form_form, form_tag, many a times especially when we need to use Javascript using the :html option that comes along with these helpers we give :id => "some_value" and :class => "some_value". I just wanted to understand what separates the "id" and "class" from usability perspective. This would help me to better decide when I would need to use either of these options and when would I have to use both of them.

Also,

I just wanted to know exactly understand under what circumstances do we use <%= %> and when would we use <% %> wrt Rails. I have seen their use in a variety of circ开发者_JAVA百科umstances so far.

It would be great if you could answer these questions with relevant e.g.'s,

Many thanks for your time...

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Your 'class' vs 'id' question is really referring to CSS best practices. Here's some info on that: http://css-tricks.com/the-difference-between-id-and-class/

The <%= %> ERB tag outputs the result of the expression in the tag, for example...

<%= ['hello', 'world'].join(' ') %>

Would be replaced with the string "hello world"

Lets say you want to set a variable for later use, for that you would use the <% %> tags because you don't want to output the result yet. Eg:

<% my_var = "test" %>

This outputs nothing, but does set the my_var variable for later use. If you used the <%= by mistake...

<%= my_var = "test" %>

That tag would be replaced with "test" in the resulting rendered page, which probably isnt what you wanted to do in that case. Another common use for <% %> tags are loops.

<% ['item1','item2','item3'].each do |item| %>
  <li><%= item %></li>
<% end %>

Which would result in:

<li>item1</li>
<li>item2</li>
<li>item3</li>

I hope this helps clear up some of your questions!

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Another thing in Rails is that after 3.0 release helpers like form_for etc uses <%= %>, because it add tags for you in the background. (So since .each only prints the text/code in the block, it doesn't require the =)

So <%= %> evaluates the code in between, and then prints the result to your page, and <% %> just runs the code.

If you really need to print something within <% %>, use #concat