if statement based on form select rails - ruby-on-rails

Question

I have a basic form:
<%=form_for #person do |f| %>
<%= f.label :ThingsWanted, "Stuff I need" %>
<%= f.select :ThingsWanted, ['cat', 'dog', 'purse', 'lipstick', 'fish'], { :include_blank => 'Selector' }, :required => true %>
<%= f.submit 'Done'%>
<%end%>
What I want to do is created an if statement based on what is selected before the form is submitted. Say lipstick is selected
Then I want to create a new form value called explain_yourself
<%if lipstick is selected%>
explain_yourself goes here
<%end%>
I don't know how to create the if-statement or rather access the select value before the form is submitted...

Answer 1

You can use jquery.
Add a class to the form and also to the input tag in question in the HTML
<%=form_for #person do |f| :class => "the_form_class" %>
<%= f.select :ThingsWanted, ['cat', 'dog', 'purse', 'lipstick', 'fish'], { :include_blank => 'Selector' }, :class => "the_input_class" :required => true %>
then in jquery, add an on-submit function to the form
$(".the_form_class").on("submit", some_function)
note: make sure you wrap this on-submit event handler in a document.ready
then define that function (the definition does not need to be wrapped in a document.ready)
function some_function(e){
e.preventDefault();
if( $(".the_input_class").val() == "lipstick"){
do_some_stuff
}
$(".the_form_class").submit();
}

Answer 2

This best workaround I found for this was using parameters and instance variables. Sorry the example is not great, but it works.
Do you have lipstick?
<%= link_to "Yes", onclick: #a=0%></td>
<%= link_to "No", onclick: #a=1%>
<% if params[:onclick].to_i == 1%>
<%= f.label :ThingsWanted, "Stuff I need" %>
<%= f.select :ThingsWanted, ['lipstick'], { :include_blank => 'Selector' }, :class => "the_input_class" :required => true %>
<%else%>
<br/>
<%= f.label :explain_yoself, "Select what you need" %>
<%= f.select :ThingsWanted, ['cat', 'dog', 'purse', 'fish'], { :include_blank => 'Selector' }, :required => true %>
<%= f.text_area :explain_yourself %>
<%end%>