Rails 5 - n+1 query issue inside simple_form gem - ruby-on-rails

Question

I'm trying to resolve every n+1 issue with Bullet gem in my Rails, but I can't figure out this particular problem:
_form.html.erb
<%= simple_form_for([:admin, #course, #lesson]) do |f| %>
<div class="form-inputs">
<div class="col-md-6">
<%= f.input :start_at, minute_step: 5, start_year: Date.today.year %>
</div>
<div class="col-md-6">
<%= f.input :end_at, minute_step: 5, start_year: Date.today.year %>
</div>
<div class="col-md-6">
<!--Here is the problem-->
<%= f.association :room, :label_method => lambda { |room| "#{room.title} (#{room.building.title})"}%>
<!--Here is the problem-->
</div>
<div class="col-md-6">
<%= f.association :teacher, :label_method => lambda { |teacher| "#{teacher.first_name} #{teacher.last_name}"},
collection: #course.teachers%>
</div>
</div>
<div class="col-md-12">
<div class="form-actions text-center">
<%= f.submit class: "btn btn-primary btn-lg" %>
</div>
</div>
<% end %>
Form is rendered to new.html.erb and edit.html.erb. Problem occurs on line with room association where I'm trying to get room.building.title.
Here's the screenshoot:
As you can see, I only want to display room name with its centre name, but Bullet throws an error. Problem is that I'm using that lambda function so creating a variable in a controller doesn't help (or at least I don't know how to create it properly) since problem occurs inside select box.
Here I'm including my controller:
lessons_controller.rb
module Admin
class LessonsController < Admin::AdminController
helper_method :convert_time, :convert_day
before_action :set_course
before_action :set_lesson, only: [:show, :edit, :update, :destroy]
def index
#lessons = Lesson.includes(:teacher,:course, :room => :building).where(course_id: #course).paginate(page: params[:page], per_page: 10)
# #course_lessons = #course.lessons.paginate(page: params[:page], per_page: 10)
end
def new
##room = Room.all.includes(:building)
#lesson = Lesson.new
end
def create
#lesson = Lesson.new(lesson_params)
#lesson.course = #course
if #lesson.save
flash[:success] = "Lesson was created"
redirect_to admin_course_lessons_path(#course)
else
render 'new'
end
end
def edit
end
def update
if #lesson.update(lesson_params)
flash[:success] = "Lesson was updated"
redirect_to admin_course_lessons_path(#course)
else
render 'edit'
end
end
def show
end
def destroy
#lesson.destroy
flash[:danger] = "Lesson was deleted"
redirect_to admin_course_lessons_path(#course)
end
private
def set_course
#course = Course.find(params[:course_id])
end
def set_lesson
#lesson = Lesson.find(params[:id])
end
def lesson_params
params.require(:lesson).permit(:start_at,:end_at, :room_id, :teacher_id)
end
end
end
(Actions relevant for us in this situation are new and edit)
Here are the relevant models:
room.rb
class Room < ApplicationRecord
belongs_to :building
has_many :lessons, dependent: :destroy
before_save {self.title = title.upcase_first}
before_save {self.code = code.upcase}
validates :title, presence: true,
length: { minimum: 3, maximum: 50},
uniqueness: {case_sensitive: false}
validates :code, presence: true,
length: {minimum: 2},
uniqueness: {case_sensitive: false}
validates :building_id, presence: true
end
building.rb
class Building < ApplicationRecord
has_many :rooms, dependent: :destroy
before_save {self.title = title.upcase_first}
before_save {self.code = code.upcase}
validates :title, presence: true,
length: { minimum: 3, maximum: 50},
uniqueness: true
validates :code, presence: true,
length: {minimum: 3},
uniqueness: {case_sensitive: false}
end
Sorry for this long post and my lack of knowledge. I will appreciate any feedback.

Answer 1

You can specify the collection option using includes in the f.association. Like this:
f.association :room, :label_method => lambda { |room| "#{room.title} (#{room.building.title})", :collection => Room.includes(:building).all }