Trouble when "filtering" eager loaded data

Question

I am using Ruby on Rails 4 and I would like to understand why during the eager loading process are run further SQL queries even if data is eager loaded. That is, I have the following code that eager loads :comments the right way:

@articles = @current_user.articles.includes(:comments)

When the above code runs and I "track" what is happening in the logger with the following code:

@articles.each do |article|
  logger.debug article.comments
end

Then the logger says:

Article Load (0.4ms) SELECT ...
Comment Load (0.5ms) SELECT ... WHERE `articles`.`id` IN (...)

#<ActiveRecord::Associations::CollectionProxy [#<Comment id: 1, title: "Hello A">, #<Comment id: 2, title: "Hello B">]>

#<ActiveRecord::Associations::CollectionProxy [#<Comment id: 3, title: "Hello A">, #<Comment id: 4, title: "Hello C">]>

#<ActiveRecord::Associations::CollectionProxy [#<Comment id: 5, title: "Hello D">, #<Comment id: 6, title: "Hello E">]>

...

The above output indicates that the eager loading is working as expected: no N+1 problem since ActiveRecord::Associations::CollectionProxy objects are loaded when running article.comments.

However when I try to run code as like the following (note the find_by clause):

@articles.each do |article|
  logger.debug article.comments.find_by(:title => "Hello A")
end

Then the logger says:

Article Load (0.4ms) SELECT ...
Comment Load (0.5ms) SELECT ... WHERE `articles`.`id` IN (...)

Comment Load (0.4ms) SELECT ... AND `comments`.`title` = 'HELLO A'
#<Comment id: 1, title: "Hello A">

Comment Load (0.4ms) SELECT ... AND `comments`.`title` = 'HELLO A'
#<Comment id: 3, title: "Hello A">

Comment Load (0.4ms) SELECT ... AND `comments`.`title` = 'HELLO A'
nil

...

The above output indicates that the eager loading is not working as expected: a SQL query runs for each comment.

So, my questions/doubts are:

1. Why in the last case the find_by clause makes the eager loading to do not work (note: it happens even in cases when I "filter" article.comments by using a clause other than find_by)?
2. Should Ruby on Rails handle data already loaded in ActiveRecord::Associations::CollectionProxy objects as array so that it avoids to hit the database?!
3. How can I solve the problem in order to avoid the N+1 problem in the last case?

Answer 1

Just to confirm: David Underwood is correct that find_by will make a database call. In fact, find_by is basically just a wrapper for where and take, which will indeed make a DB call.

An alternative way to accomplish what you're looking for is to simply treat the collection proxy as an array, by using the find method, like so:

@articles.each do |article|
    logger.debug article.comments.find {|comment| comment.title == "Hello A"}
end

---

Update:

I have to admit, this one was a bit of a doozy.

The way to accomplish this in the manner you're looking for is to add another has_many relationship that specifically contains the filtering condition you want, like so:

class Article < ActiveRecord::Base

    has_many :hello_A_comments, -> { where(title: "Hello A") }, class_name: "Comment"

    # rest of class
end

And then, you eager load using this new association, like so:

@articles = @current_user.articles.includes(:hello_A_comments)

This part is very important: You now access the association not through the original :comments association method, but through the new hello_A_comments method instead, like so:

@articles.first.hello_a_comments

Unfortunately, as you can see, this method is not very dynamic, and sadly I do not know how to allow for variable conditions in the association in the case of eager loading. This answer might be a good resource to look at, but in the case of eager loading, honestly I don't believe its possible. If that is an issue, you might be stuck with the array method I mentioned earlier.

Answer 2

I suspect that find_by is hard-wired to make a database call.

The objects listed in the first example are of type CollectionProxy, meaning you can still make SQL queries on them. Since find_by is part of ActiveRecord, calling it on a Proxy class should go to the DB.

I suspect that if you alter your code to use an Enumerable method such as find_all on the comments collection then you should be OK, but this isn't very efficient (find_all runs in linear time)

Alternately, roll everything up into a single join query by doing something like this:

Article.joins(:comments).where(comments: {title: "My Title")

Or if you need all Articles whether they have matching comments or not, you can simply add a condition to your original includes:

Article.includes(:comments).where(comments: {title: "My Title")