How can I prevent n+1 issue from array that is generating additional conditions?

Question

I'm facing n+1 issue when using conditions on array that comes from includes query.

Here are the following tables:

rems
|id| |name|
 1  aaa
 2  bbb

rem_correlatives
|id| |rem_id| |name|  |deleted|  |pending_tr|
 1     1       qqqq1      0          0
 2     1       qqqq2      0          1 
 3     1       qqqq1      1          0
 4     1       qqqq2      1          1
 5     1       qqqq1      0          0
 6     1       qqqq2      0          1 
 7     1       qqqq1      1          0
 8     1       qqqq2      1          1
 9     2       qqqq1      0          0
 10    2       qqqq2      0          1 
 11    2       qqqq1      1          0
 12    2       qqqq2      1          1
 13    2       qqqq1      0          0
 14    2       qqqq2      0          1 
 15    2       qqqq1      1          0
 16    2       qqqq2      1          1

Here are the models:

 class Rem < ApplicationRecord
    has_many :rem_correlatives
 end

 class RemCorrelative < ApplicationRecord
    belongs_to :rem
 end

Here is the controller called rems_controller.rb from the location /app/controllers/rems_controller.rb

 def index
    @list_array = Rem.includes(:rem_correlatives).all
 end

Here is the index view from the location /app/views/rems/index.html.erb

 <% @list_array.each do |array| %>
      <% array.name %>
      <% @check_result = array.rem_correlatives.where("deleted=0 AND pending_tr= 1)%>
        <% if @check_result.present? %>
           No
        <% else %>
           Yes
        <% end %>
      <% end %>
   
 <% end%>

I tried this workaround code that display data array using columns pending_tr=1 and deleted=0 but seems not to be a good practice.

 <% @list_array.each do |array| %>
      <% array.name %>
      <% array.rem_correlatives.each do |rem_correlative|%>
        <!--  Create condition pending_tr=1 AND deleted=0  -->
        <% if rem_correlative.pending_tr == 1 && rem_correlative.deleted == 0%>
           <% @check_condition = "No" %>
        <% else %>
           <% @check_condition = "Yes"%>
        <% end %>
      <% end %>
      <!--  Check results from array if the word yes exists --> 
      <% if @check_condition.include?("Yes") %>
         si 
      <% else %>
        no
      <% end %>          
 <% end%>

Here is the backend result when using the workaround code, its working and not displaying n+1 issue.

How can I prevent n+1 issue from array that is generating additional conditions as good code?

Answer 1

I would create a dedicated association with a scope in the model.

# in app/models/rem.rb
has_many :pending_rem_correlatives,
  class_name: 'RemCorrelative', -> { where(deleted: false, pending_tr: true) }

And would then use this association in the controller and view.

# in the controller
@rems = Rem.includes(:pending_rem_correlatives).all

# in the view
<% @rems.each do |rem| %>
  <%= rem.name %>

  <% if rem.pending_rem_correlatives.any? %>
    No such rem correlatives
  <% else %>
    There are matching rem correlatives
  <% end %>
<% end %>

Answer 2

Currently, in your controller, you are using Rem.includes(:rem_correlatives).all to load the Rem records with their associated rem_correlatives. However, when you later iterate over @list_array and access array.rem_correlatives, it still generates additional queries for each Rem record, causing the N+1 query problem.

To address this, you can preload the necessary associations using the includes method and also leverage a query condition to filter the associated records. Here's how you can modify your controller code:

def index
  @list_array = Rem.includes(:rem_correlatives)
                  .where("rem_correlatives.deleted = 0 AND rem_correlatives.pending_tr = 1")
                  .references(:rem_correlatives)
                  .all
end

With this change, in your view, you can iterate over @list_array without causing additional queries for each Rem record:

<% @list_array.each do |array| %>
  <%= array.name %>
  <% if array.rem_correlatives.present? %>
    No
  <% else %>
    Yes
  <% end %>
<% end %>