ActiveRecord: size vs count

 

In Rails, you can find the number of records using both Model.size and Model.count . If you're dealing with more complex queries is there any advantage to using one method over the other? How are they different?

For instance, I have users with photos. If I want to show a table of users and how many photos they have, will running many instances of user.photos.size be faster or slower than user.photos.count ?

Thanks!

You should read that, it's still valid.

You'll adapt the function you use depending on your needs.

Basically:

  • if you already load all entries, say User.all , then you should use length to avoid another db query

  • if you haven't anything loaded, use count to make a count query on your db

  • if you don't want to bother with these considerations, use size which will adapt

    • As the other answers state:

  • count will perform an SQL COUNT query
  • length will calculate the length of the resulting array
  • size will try to pick the most appropriate of the two to avoid excessive queries

    • But there is one more thing. We noticed a case where size acts differently to count / length altogether, and I thought I'd share it since it is rare enough to be overlooked.

  • If you use a :counter_cache on a has_many association, size will use the cached count directly, and not make an extra query at all. 

    class Image < ActiveRecord::Base
  belongs_to :product, counter_cache: true
end

class Product < ActiveRecord::Base
  has_many :images
end

> product = Product.first  # query, load product into memory
> product.images.size      # no query, reads the :images_count column
> product.images.count     # query, SQL COUNT
> product.images.length    # query, loads images into memory

    • This behaviour is documented in the Rails Guides, but I either missed it the first time or forgot about it.

    Sometimes size "picks the wrong one" and returns a hash (which is what count would do)

    In that case, use length to get an integer instead of hash .

    链接地址: http://www.djcxy.com/p/49724.html

    上一篇: Rails单表继承与小写类型名称

    下一篇: ActiveRecord：大小与数量