Rails Rollback Transaction - ruby-on-rails

Question

I have problems with my Rails Block. After I implemented a comment-section I am not able to create posts anymore. The console gives me a rollback transaction. So I did
p = Post.new
p.valid? # false
p.errors.messages
It seems I have some validation problems with user :user=>["must exist"]. But before I implemented comments it did work. Can someone help me out?
User.rb
class User < ApplicationRecord
has_many :posts
# Include default devise modules. Others available are:
# :confirmable, :lockable, :timeoutable and :omniauthable
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :trackable, :validatable
end
Post.rb
class Post < ApplicationRecord
belongs_to :user
has_many :comments, dependent: :destroy
validates :title, presence: true, length: {minimum: 5}
validates :body, presence: true
has_attached_file :image #, :styles => { :medium => "300x300>", :thumb => "100x100>" }
validates_attachment_content_type :image, :content_type => /\Aimage\/.*\Z/
end
Post-migrate
class CreatePosts < ActiveRecord::Migration[5.1]
def change
create_table :posts do |t|
t.string :title
t.text :body
t.timestamps
end
end
end
Post_controller
class PostsController < ApplicationController
def index
#posts = Post.all.order("created_at DESC")
end
def new
#post = Post.new
end
def create
#post = Post.new(post_params)
if #post.save
redirect_to #post
else
render 'new'
end
end
def show
#post = Post.find(params[:id])
end
def edit
#post = Post.find(params[:id])
end
def update
#post = Post.find(params[:id])
if #post.update(post_params)
redirect_to #post
else
render 'edit'
end
end
def destroy
#post = Post.find(params[:id])
#post.destroy
redirect_to posts_path
end
private
def post_params
params.require(:post).permit(:title, :body, :theme)
end
end

Answer 1

When you are creating a post you need to assign a user to that post in the create method under your posts controller. You could try something like this.
def create
if current_user
#post.user_id = current_user.id
end
## More create method stuff
end
By default, in a belongs_to association a user is required to create the post otherwise you will not be able to create the post. Since, from the looks of it, you do not have anything that assigns the user to that post in the create method.