having trouble understanding :multiple => true in a checkbox - ruby-on-rails

Question

I am very new to Ruby on Rails, I have inherited control of a Ruby/Rails web based database(created by another) for generating and tracking engineering functions. I use Aptana IDE.
My issue is that I am trying to use a form helper to select multiple values for our "product lines". I use 10 check boxes, and I get the correct output of one value if I don't use ":multiple => true".
See output image:
works as intended
<%= f.check_box :product_line, {:class => "field"}, "A9", false %> A9
(10 times with different values where "A9" is, so there are ten checkbox's total, image shows "A7" check box returned)
By simply adding the :multiple => true, the output changes as seen in the image below:
see output image:
returns too much
<%= f.check_box :product_line, {:class => "field", :multiple => true}, "A9", false %> A9
Why are all of those "-" added before the output only if the multiple selection is made? I want it to return only comma separated values of the checkbox, i.e. " A9, A7"
PRODUCT_LINES = [ "A9", "A7", "AG", "AF", "S3", "Legacy", "K/Kpro", "EMW", "HD", "Non-Metallic" ]
other ideas:
Can I loop through an array of :product_line to get what im looking for instead?
I was also playing with the multiple selection in a drop down menu but could never select a second option before the drop down returned and only selected my first selection.

Answer 1

Why are all of those "-" added before the output only if multiple
selection is made?
From the docs (read the Gotcha) when multiple is set to true all selections will be stored in an array of product_lines but un-selected check_boxes will also be reserve as an empty strings in the sent pararms so for the result you are seeing product_lines would be something like:
product_lines = ["", "", "", "A9", "A7"]
Can I loop through an array of :product_line to get what im looking
for instead?
Well, actually this takes us right to the problem in using multiple: true in your code.
The process explained in (1) above is how the check_box_tag was designed, the problem you are seeing is in presentation probably in your ECN#show. Without reviewing code there i am only guessing but usually it's something like:
<%= #ecn.product_lines.join('-') %>
which should be
<%= #ecn.product_lines.reject(&:blank?).join(',') %>
where: reject(&:blank?) to get rid of empty strings then joining with commas as you need
Hope this helps!