Before the new semester begins, I read a lot of Rails best practices, one book is really helpful: Rails Antipattern. I read about a little trick that reminds me of the days working in Fishtrip.cn.

When I was developing the order statistic product, I need to count the number of total orders. So I do this: Order.succeed.all.length. However, on the code review, our tech lead Medal Huang told me that this would result in bad performence and I should use count or size instead. I was not sure about the difference between those three method calls. And until now when I was reading Rails Antipattern, in chapter 8 Scaling and Deploying there is a comparison between those three methods and explanation.

• @article.comments.count
• @article.comments.length
• @article.comments.size

length will load all your objects just to count them; something like:

select * from comments...

and then return the results count. As you can imagine - it's gonna be slow and inefficient.

count will just issue

select count(*) from comments...

which is better, because we are not loading all comments just to count them

size is smarter - it'll check if the association is already loaded and if true then return the length (without issuing a call to the database).

source:

def size
 loaded? ? @records.length : count
end

size also checks for counter_cache if you have a field named comment_count in your article model, then size will use this field for the count, so there is no need to issue a count on the comments table.

if all fails, size will issue a select count(*) on the database