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/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.

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).

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.

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.

I recommended using the size function.

class Customer < ActiveRecord::Base
  has_many :customer_activities
end

class CustomerActivity < ActiveRecord::Base
  belongs_to :customer, counter_cache: true
end

Consider these two models. The customer has many customer activities.

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.

Consider one example: in my database, one customer has 20,000 customer activities and I try to count the number of records of customer activities of that customer with each of count, length and size method. here below the benchmark report of all these methods.

            user     system      total        real
Count:     0.000000   0.000000   0.000000 (  0.006105)
Size:      0.010000   0.000000   0.010000 (  0.003797)
Length:    0.030000   0.000000   0.030000 (  0.026481)

so I found that using :counter_cache Size is the best option to calculate the number of records.