TLDR: Blank lines in a .csv.erb template render as whitespace in the final CSV output.

Using the method I described for exporting CSV, I created a new template and was testing it. When I downloaded the CSV file and opened it, it appeared that the file was empty. What was going on? I noticed that the scroll bar was rather long, so I started scrolling… and scrolling… and scrolling, down 1369 lines. There’s the data. Then, I looked at the total lines in the file: 2738, which is double 1369. It appears that, somewhere, I was inserting 1368 blank lines into the file prior to the data being written.

I started debugging my .csv.erb template to see where this whitespace was coming from. First, I set a breakpoint in the template right before rendering the CSV. Ruby’s CSV library wraps a string in a CSV object, and I could verify that the string had no whitespace.

Next, I started removing elements from the CSV template until I found the culprit. What I found was that an entire section of the CSV document was where the problem was happening. No matter what subsection of that section I removed, the issue would still persist. If I removed the entire thing, the output would begin at line one.

Finally, I decided to try something that I thought was crazy: I had defined a few variables in ERB tags earlier in the document, and I had inserted a blank line between each of these for readability. What if I removed those? The output began at line one. Bingo! Not so crazy after all!

If you have more whitespace than you bargained for in CSV output and are using a .csv.erb view template, check for blank lines. These could be the culprit.