How do I rescue an exception from inside an Enumerator? - ruby-on-rails

Question

I'm writing an application in Rails and I want to get a huge amount of information from an API – which I'm streaming through an Enumerator object as a CSV export. I want to rescue an error that is called within the Enumerator.
CONTROLLER: Enumerator
def csv_lines( url )
Enumerator.new do |y|
per_page = 200
# Parse parameters and get shelf information
_params = BrowseScraper.get_params(url)
shelf = BrowseScraper.get_preso( _params, 0 )
total_items = shelf['response']['total_results']['all'].to_i
total_pages = ( total_items / per_page.to_f ).ceil
shelf_info = BrowseScraper.crawl_ids( shelf['response']['query']['category'] )
y << BrowseScraper.csv_header(url, shelf_info, total_items, ["Tool ID", "Name", "Price", "URL"])
total_pages.times { |i| y << BrowseScraper.csv_body( _params, per_page, i+1) }
end
end
The following functions are raising errors, but I can't catch them outside of the Enumerator:
MODEL: methods
def self.get_params
response = open(url)
raise if response.code != 200
end
CONTROLLER: Display
def export
url = params[:url]
raise StandardError, "Please enter a Browse URL below" if !url || url.empty?
respond_to do |format|
format.csv do
render_csv(url)
end
format.html { render_csv(url) }
end
rescue => e
flash[:error] = e.message
redirect_to scraper_path
end
private
def render_csv( url )
set_file_headers
set_streaming_headers
response.status = 200
# Rails should iterate this enumerator
self.response_body = csv_lines(url)
end
def set_file_headers( name = "browse_export" )
headers["Content-Type"] ||= 'text/csv'
headers["Content-Disposition"] = "attachment; filename=\"#{name}.csv\""
headers["Content-Transfer-Encoding"] = "binary"
headers["Last-Modified"] = Time.now.ctime.to_s
end
def set_streaming_headers
#nginx doc: Setting this to "no" will allow unbuffered responses suitable for Comet and HTTP streaming applications
headers['X-Accel-Buffering'] = 'no'
headers["Cache-Control"] ||= "no-cache"
headers.delete("Content-Length")
end
Rescuing the error raised in export works. Rescuing an error within the Enumerator works (example:
Enumerator do |y|
begin
y << BrowseScraper.get_params(_params)
rescue => e
Rails.logger.error "Failed to get parameters: #{e.message}"
end
end
How can I rescue an exception outside of the Enumerator so I can properly redirect the user with a flash message? How do I pass the exception from within the Enumerator object? What is it about the Enumerator that isn't letting me rescue it with:
def method
Enumerator do |y|
y << BrowseScraper.get_params(_params)
end
rescue => e
Rails.logger.error "Error in Enumerator is #{e.message}"
end

Answer 1

I think I've figured out what's going on here. When you write code in an Enumerator, the block isn't actually executed within the Enumerator. Therefore, if I add a rescue within the Enumerator, it doesn't matter.
This is because the |y| in Enumerator is actually a yielder object which does the yielding (more on that in the Enumerator documentation or the Enumerator::Yielder documentation.
You have to rescue things beforehand.