How to remove N+1 queries

Question

I have a rails API that currently has quite a few N+1 queries that I'd like to reduce.

As you can see it's going through quite a few loops before returning the data.

The relationships are as follows:

Company Model

class Company < ApplicationRecord
  has_many :jobs, dependent: :destroy
  has_many :contacts, dependent: :destroy
  has_many :listings
end

Job Model

class Job < ApplicationRecord
  belongs_to :company
  has_many :listings
  has_and_belongs_to_many :technologies
  has_and_belongs_to_many :tools

  scope :category, -> ( category ) { where category: category }
end

Listing Modal

class Listing < ApplicationRecord
  belongs_to :job, dependent: :destroy
  belongs_to :company, dependent: :destroy

  scope :is_active, -> ( active ) { where is_active: active }
end

Job Serializer

class SimpleJobSerializer < ActiveModel::Serializer
  attributes  :id,
              :title, 
              :company_name,                  

  attribute :technology_list, if: :technologies_exist
  attribute :tool_list, if: :tools_exist

  def technology_list
    custom_technologies = []

    object.technologies.each do |technology|
      custom_technology = { label: technology.label, icon: technology.icon }
      custom_technologies.push(custom_technology)
    end

    return custom_technologies
  end

  def tool_list
    custom_tools = []

    object.tools.each do |tool|
      custom_tool = { label: tool.label, icon: tool.icon }
      custom_tools.push(custom_tool)
    end

    return custom_tools    
  end

  def tools_exist
    return object.tools.any?
  end

  def technologies_exist
    return object.technologies.any?
  end

  def company_name
    object.company.name
  end
end

Current query in controller

Job.eager_load(:listings).order("listings.live_date DESC").where(category: "developer", listings: { is_active: true }).first(90)

I've tried to use eager_load to join the listings to the Jobs to make the request more efficient but i'm unsure how to handle this when some of the n+1 queries are coming from inside the serializer as it tries to look at tools and technologies.

Any help would be much appreciated!

Answer 1

You might was well eager load tools and technologies since you know that the serializer is going to use them:

Job.eager_load(:listings, :tools, :technologies)
   .order("listings.live_date DESC")
   .where(category: "developer", listings: { is_active: true })
   .first(90)

After that you really need to refactor that serializer. #each should only be used when you are only interested in the side effects of the iteration and not the return value. Use #map, #each_with_object, #inject etc. These calls can be optimized. return is implicit in ruby so you only explicitly return if you are bailing early.

class SimpleJobSerializer < ActiveModel::Serializer
  # ...
  def tool_list
    object.tools.map { |t| { label: tool.label, icon: tool.icon } }
  end
  # ...
end