cron couldn't run Scrapy
Problem solved. Rather than running the crawl as root, use crontab -u user -e to create a crontab for user, and run as user.
The code in crontab 0 * * * * cd /home/scrapy/foo/ && scrapy crawl foo >> /var/log/foo.log
It failed to run the crawl, as there was no log in my log file.
I tested using 0 * * * * cd /home/scrapy/foo/ && pwd >> /var/log/foo.log, it echoed '/home/scrapy/foo' in log. 
I also tried PATH=/usr/local/bin and PATH=/usr/bin, but no success.
I'm able to run it manually by typing cd /home/scrapy/foo/ && scrapy crawl foo in command line.
Any thoughts? Thanks.