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

User · Accepted Answer

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

User · Answer

I recommended using the size function   class Customer  lt  ActiveRecord  Base   has many  customer activities end  class CustomerActivity  lt  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

User · Answer

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 what you are doing  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

User · Answer

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

User · Answer

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  lt  ActiveRecord  Base   belongs to  product  counter cache  true end  class Product  lt  ActiveRecord  Base   has many  images end   gt  product   Product first    query  load product into memory  gt  product images size        no query  reads the  images count column  gt  product images count       query  SQL COUNT  gt  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

User · Answer

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

[ruby-on-rails] ActiveRecord: size vs count

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