Could not save nested form in rails 5.1 - ruby-on-rails

Question

I used nested_form gem and I am trying to build a form which has fields from two tables(Project, Question).
My model:
class Project < ApplicationRecord
has_many :questions
accepts_nested_attributes_for :questions
end
class Question < ApplicationRecord
belongs_to :project
end
My controller:
class ProjectsController < ApplicationController
layout 'application'
def index
#projects = Project.all
end
def show
#project = Project.find(params[:id])
end
def new
#project = Project.new
#questions = #project.questions.build
end
def create
#project = Project.new(project_params)
#project.save
respond_to do |format|
if #project.save
format.html { redirect_to #project, notice: 'Project was successfully created.' }
format.json { render :show, status: :created, location: #project }
else
format.html { render :new }
format.json { render json: #project.errors, status: :unprocessable_entity }
end
end
end
private
def project_params
params.require(:project).permit(:name, question_attributes: [:id, :content, :_delete])
end
end
My view:
<%= nested_form_for(#project) do |f| %>
<% if #project.errors.any? %>
<div id="error_explanation">
<h2>
<%= pluralize(#project.errors.count, "error") %> prohibited this project from being saved:</h2>
<ul>
<% #project.errors.full_messages.each do |message| %>
<li>
<%= message %>
</li>
<% end %>
</ul>
</div>
<% end %>
<div class="field">
<%= f.label :name %><br>
<%= f.text_field :name %>
</div>
<%= f.fields_for :questions do |builder| %>
<%= render "question_fields", :ff => builder %>
<% end %>
<p>
<%= f.link_to_add "Add a questions",:questions %>
</p>
<div class="actions">
<%= f.submit %>
</div>
<% end %>
And my schema file :
create_table "projects", force: :cascade do |t|
t.string "name"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
end
create_table "questions", force: :cascade do |t|
t.integer "project_id"
t.string "content"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.index ["project_id"], name: "index_questions_on_project_id", using: :btree
end
And _question_fields file is :
<p>
<%= ff.label :content, "Question" %>
<%= ff.text_area :content%>
<%= ff.link_to_remove "Remove this task"%>
</p>
Table Project will be saved but the table Question could not be saved.Why?
After change this line of code
def project_params
params.require(:project).permit(:name, questions_attributes: [:id, :content, :_delete])
end
I get the following error:
1 error prohibited this project from being saved:
Questions project must exist
I applied the change too ,but this time nothing could not be saved.
def project_params
params.require(:project).permit(:name, questions_attributes: [:id, :content, :project_id, :_delete])
end

Answer 1

The issue is with the project_params. As you have has_many :questions, the question_attributes should be changed to questions_attributes
def project_params
params.require(:project).permit(:name, questions_attributes: [:id, :content, :_delete])
end
Update:
Questions project must exist
Either permit project_id in questions_attributes
def project_params
params.require(:project).permit(:name, questions_attributes: [:id, :content, :project_id, :_delete])
end
or set optional: true on the association
class Question < ApplicationRecord
belongs_to :project, optional: true
end