Associating a reply with a post - ruby-on-rails

Question

I'm trying to build a very basic forum-like app, where Users can create Topics and reply to existing Topics.
The Topic creation works fine and I'm able to display Reply form, however, the Reply create action is not working properly. I don't have any errors, it just redirects_to topics_path.
This is following a tutorial, so the code is not mine. Is anyone able to spot the obvious cause for this? Any help much appreciated!
replies_controller.rb
def create
#topic = Topic.find(params[:topic_id])
#reply = #topic.replies.create(params[:reply].permit(:reply))
#reply.user_id = current_user.id if current_user
#reply.save
if #reply.save
redirect_to topic_path(#topic)
else
flash[:notice] = "Error."
redirect_to topics_path
end
end
reply.rb
class Reply < ApplicationRecord
belongs_to :post
belongs_to :user
end
replies/_form.html.erb
<%= form_for [#topic, #topic.replies.create] do |f| %>
<%= f.label :reply %>
<%= f.text_area :reply, class: "textarea", rows: "10" %>
<%= f.submit class: "button is-primary" %>
<% end %>
topic.rb
class Topic < ApplicationRecord
belongs_to :user
has_many :replies
end
schema.rb
create_table "topics", force: :cascade do |t|
t.string "title"
t.text "content"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.integer "user_id"
end
create_table "replies", force: :cascade do |t|
t.text "reply"
t.bigint "topic_id"
t.bigint "user_id"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.index ["topic_id"], name: "index_replies_on_topic_id"
t.index ["user_id"], name: "index_replies_on_user_id"
end

Answer 1

In file replies/_form.html.erb
you should use build method build instead of create. Replace line:
<%= form_for [#topic, #topic.replies.create] do |f| %>
to
<%= form_for [#topic, #topic.replies.build] do |f| %>
There are some another problems with the code:
#reply = #topic.replies.create(params[:reply].permit(:reply))
In this line you call new + save, without user, which is required.
Change this to:
#reply = #topic.replies.new(params[:reply].permit(:reply))
Then, you call save twice:
#reply.save
if #reply.save
...
First line is unnecessary.
And finally, what is the cause of rollback, in your Reply model you have:
belongs_to :post
But in the schema.rb and in the params you have topic:
Schema.rb:
t.bigint "topic_id"
Params:
"reply"=>{"reply"=>"Test reply"}, "commit"=>"Create Reply", "topic_id"=>"4"}