ActiveRecord: size vs count

Question

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!

Solution 1

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

Solution 2

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/lengthaltogether, 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.

Solution 3

tl;dr

• If you know you won't be needing the data use count.
• If you know you will use or have used the data use length.
• If you don't know where it is used or the speed difference is neglectable, use size...

count

Resolves to sending a Select count(*)... query to the DB. The way to go if you don't need the data, but just the count.

Example: count of new messages, total elements when only a page is going to be displayed, etc.

length

Loads the required data, i.e. the query as required, and then just counts it. The way to go if you are using the data.

Example: Summary of a fully loaded table, titles of displayed data, etc.

size

It checks if the data was loaded (i.e. already in rails) if so, then just count it, otherwise it calls count. (plus the pitfalls, already mentioned in other entries).

def size
  loaded? ? @records.length : count(:all)
end

What's the problem?

That you might be hitting the DB twice if you don't do it in the right order (e.g. if you render the number of elements in a table on top of the rendered table, there will be effectively 2 calls sent to the DB).

Solution 4

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.

Solution 5

The following strategies all make a call to the database to perform a COUNT(*) query.

Model.count

Model.all.size

records = Model.all
records.count

The following is not as efficient as it will load all records from the database into Ruby, which then counts the size of the collection.

records = Model.all
records.size

If your models have associations and you want to find the number of belonging objects (e.g. @customer.orders.size), you can avoid database queries (disk reads). Use a counter cache and Rails will keep the cache value up to date, and return that value in response to the size method.

View more solutions

Comments

[sscirrus]

If size adapts to the situation anyway, then what need is there for length and count at all?

[Batkins]

@sscirus - So that size can make a call to them when you make the call to size (after it determines which one to call).

[mrbrdo]

Be careful with just defaulting to size, however. For example if you create a new record without going through the relation, i.e. Comment.create(post_id: post.id), your post.comments.size will not be up to date, while post.comments.count will. So just be careful.

[Shawn J. Goff]

Also, if you build several objects through a relation: company.devices.build(:name => "device1"); company.devices.build(:name => "device2"), then company.devices.size and .length will include the number of objects you've built but haven't saved, .count will report only the count from the database.

[fengd]

one thing to mention is that count will issue a db query every time. so avoid using it inside loops

[Alex C]

@sscirrus, size is a dangerous command since it's automated, sometimes you do want to query the db again.

[Damon Aw]

Both Model.all.size and Model.all.count generate a count query in Rails 4 and above. The real advantage of size is that it doesn't generate the count query if the association is already loaded. In Rails 3 and below, I believe Model.all is not a relation, hence all the records are already loaded. This answer might be out of date and I suggest deleting it.

[admazzola]

I used '.size' on a Collection from a has_many instance and even though there was one record in the collection, size was returning a '0'. Using .count returned the correct value of '1'.

[cbliard]

In fact, prior to rails 5.0.0.beta1, this behavior would be triggered even if there is a _count column (without the counter_cache: true directive on the association). This has been fixed in github.com/rails/rails/commit/e0cb21f5f7

[Todd]

This answer may be missing some important information from the latest Rails documentation (currently Rails 5.2). The #length method will perform a SELECT * ... query, where the #size method will perform a SELECT COUNT(*) ... query. Therefore #size is no longer able to be used "when you dont want to bother with these considerations" as it has a different use case than #length.

[A moskal escaping from Russia]

"If you don't know what you are doing"?

[estani]

@SebastianPalma too harsh? ok, rephrase it to be more exact (and polite :)