Tomasz

forum_threads_controller
class ForumThreadsController <ApplicationController

before_action :authenticate_user!, except: [:index,:show]
before_action :set_forum_thread,except: [:index,:new,:create]

def index
@forum_threads = ForumThread.all

end

def new
@forum_thread = ForumThread.new
@forum_thread.forum_posts.new
end

def create
@forum_thread = current_user.forum_threads.new forum_thread_params
if @forum_thread.save
redirect_to @forum_thread
else
render action: :new
end

end

def show

end

private
def set_forum_thread
@forum_thread = ForumThread.find(params[:id])
end

def forum_thread_params
params.require(:forum_thread).permit(:subject,forum_posts_attributes:[:body])
end
end

forum_posts_controller
class ForumPostController < ApplicationController

def index
@forum_posts = @ForumPost.all
end

def new
@forum_post = @ForumPost.new
end

def edit

end

def show

end

def create

end
end

form partial

<%= form_with model: @forum_thread do |form| %>

<%= form.text_field :subject, palceholder: "subject", class: "form-control" %>

<%= form.fields_for :forum_posts do |p| %>

<% end %>

forum_thread> new.html.erb
<%= render 'forum_threads/form' %>

Shows
unknown attribute 'forum_thread_id' for ForumPost.