Performance - Multiple association call on object

Question

I have a method called on user object which has many documents (associated).

Inside method I had to call documents many places where caller is self by default.

So I was wondering whether it will call documents for user so many times, and thought that I will call once and refer by docs, docs = self.documents or docs = documents and I will use this reference wherever user's documents are needed & thus we can avoid calling association method documents on user object

But does it really call again and again or just cache it for first time when it gets called?

I checked in console, When I call user.documents, it loaded documents (db call) but later for same call It was not loading.

Suggest how it works. Is it good to use reference variable for first call and use it further ?

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

Rails automatically caches the result of database calls. From the Rails Guides:

Query caching is a Rails feature that caches the result set returned by each query. If Rails encounters the same query again for that request, it will use the cached result set as opposed to running the query against the database again.

For example:

class ProductsController < ApplicationController

  def index
    # Run a find query
    @products = Product.all

    ...

    # Run the same query again
    @products = Product.all
  end

end

The second time the same query is run against the database, it's not actually going to hit the database. The first time the result is returned from the query it is stored in the query cache (in memory) and the second time it's pulled from memory.

However, it's important to note that query caches are created at the start of an action and destroyed at the end of that action and thus persist only for the duration of the action. If you'd like to store query results in a more persistent fashion, you can with low level caching.

My recommendation is not to assign it to a variable because it does nothing to improve the readability of the code and the performance difference is negligible. It could introduce confusion; if I were reading the code and saw someone replaced all calls to documents with docs I would wonder why and would have to take time to understand why.

Ultimately, setting docs = self.documents just tells Ruby "docs should point at the same memory location as self.documents", and regardless of which one you call Ruby will return the same data from the same memory location. There is a performance difference between calling a method and calling a variable, but that performance difference is so minor in comparison to something like the speed of a database call that it can be ignored; there are much better ways to improve the performance of an app than switching method calls to variable calls.

If your concern is that you don't want to type out documents over and over again when you could just type docs, then use alias_method:

class User < ApplicationRecord
  has_many :documents
  alias_method :docs, :documents
end

Then there is no difference between calling user.documents and user.docs -- they call the same method. But again, does it do anything to improve readability of the code? In my opinion, no.

Stick with calling documents.

Performance - Multiple association call on object

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