Get dimension value in expression (ignore field selection)

[#1]

I have a simple table with one dimension (Product_id) and one expression (photo_url)

Product_id | photo_url

In the photo_url expression I need to get the value of the product_id dimension. Im using this:

='$(=minstring(PIC_URL))' & Product_id & '.jpg'

This is the url of the product picture.

PIC_URL is the domain of the site (ex: http://www.mysite.com/)

The problem is when I select some date field in a selection that doesnt contain the product_id. Then I cannot get Product_id value.

How can I get this dimension value ignoring the date field selection?

I tried set analysis but it calculates only once, right?

Thanks in advance