Beginner stuck on Ruby on Rails project "...must exist" - ruby

Question

So my assignment asks me to generate a page that handles students, courses, and sections. I'm supposed to be using many_to_many or some similar method to be able to list all this shared data. My specific problem is that I created a new table (rails g scaffold enrollment students:references sections:references) that doesn't seem to work. when I attempt to create a new enrollment using a student and section, I get an error stating that "Section must exist" error . I have no idea where this error is coming from. The sections field in this view is only populated with existing sections, so the fact that it says "must exist" is very...confusing. Can anyone point me in the right direction to solve this? I've rebuilt this project 3 times trying different methods and am just... stuck. I'll post the relevant code, but if I'm wrong about which sections you may need to see, I'll happily post the rest.
class Enrollment < ApplicationRecord
belongs_to :section
belongs_to :student
end
<%= form_with(model: enrollment, local: true) do |form| %>
<% if enrollment.errors.any? %>
<div id="error_explanation">
<h2><%= pluralize(enrollment.errors.count, "error") %>
prohibited this enrollment from being saved:</h2>
<ul>
<% enrollment.errors.full_messages.each do |message| %>
<li><%= message %></li>
<% end %>
</ul>
</div>
<% end %>
<div class="field">
<%= form.label :student_id %>
<%= form.collection_select :student_id, Student.order(:student_name), :id,
:student_id, include_blank:true %>
</div>
<div class="field">
<%= form.label :course_id %>
<%= form.collection_select :course_id, Course.order(:name), :id, :name,
include_blank: true %>
</div>
<div class="field">
<%= form.label :sections_number %>
<%= form.collection_select :section_number, Section.all, :id,
:section_number, include_blank:false %>
</div>
<div class="actions">
<%= form.submit %>
</div>
<% end %>
class Section < ApplicationRecord
has_many :enrollments
has_and_belongs_to_many :students, through: :enrollments
belongs_to :course
def numsem
"#{course.name} #{course_id}"
end
end
L
-----------------------EDIT TO ADD CONTROLLER CODE----------------
class EnrollmentsController < ApplicationController
before_action :set_enrollment, only: [:show, :edit, :update, :destroy]
# GET /enrollments
# GET /enrollments.json
def index
#enrollments = Enrollments.all
end
# GET /enrollments/1
# GET /enrollments/1.json
def show
end
# GET /enrollments/new
def new
#enrollment = Enrollments.new
#sections = Section.all
#students = Student.all
end
# GET /enrollments/1/edit
def edit
end
# POST /enrollments
# POST /enrollments.json
def create
#enrollment = Enrollments.new(enrollment_params)
respond_to do |format|
if #enrollment.save
format.html { redirect_to #enrollment, notice: 'Enrollments was
successfully created.' }
format.json { render :show, status: :created, location: #enrollment }
else
format.html { render :new }
format.json { render json: #enrollment.errors, status:
:unprocessable_entity }
end
end
end
# PATCH/PUT /enrollments/1
# PATCH/PUT /enrollments/1.json
def update
respond_to do |format|
if #enrollment.update(enrollment_params)
format.html { redirect_to #enrollment, notice: 'Enrollments was
successfully updated.' }
format.json { render :show, status: :ok, location: #enrollment }
else
format.html { render :edit }
format.json { render json: #enrollment.errors, status:
:unprocessable_entity }
end
end
end
# DELETE /enrollments/1
# DELETE /enrollments/1.json
def destroy
#enrollment.destroy
respond_to do |format|
format.html { redirect_to enrollments_index_url, notice: 'Enrollments was
successfully destroyed.' }
format.json { head :no_content }
end
end
private
# Use callbacks to share common setup or constraints between actions.
def set_enrollment
#enrollment = Enrollments.find(params[:id])
end
# Never trust parameters from the scary internet, only allow the white list
through.
def enrollment_params
params.require(:enrollment).permit(:student_id, :section_id)
end
end

Answer 1

Section must exist
The error is because you are trying to save the #enrollment with a nil section_id which raises that exception. The problem lies with your form where you have this
<%= form.collection_select :section_number, Section.all, :id,
:section_number, include_blank:false %>
which should be
<%= form.collection_select :section_id, Section.all, :id,
:section_number, include_blank:false %>