Ruby on Rails, undefined method merge

Use this (if you already have current_user method available):

<%= f.hidden_field :user_id, :value => current_user.id %>

If you don't have current_user method implemented, in your corresponding controller, you can have something like this:

@current_user = User.find(params[:user_id])

Then, in your view, you can do:

<%= f.hidden_field :user_id, :value => @current_user.id %>

Update

After the above conversation, if you want to use session to store the user_id, then you can do something like this.

You can create a SessionsHelper module (which you can include in your ApplicationController) where you can define a log_in method:

  # Logs in the given user.
  def log_in(user)
    session[:user_id] = user.id
  end

(You can also put this: session[:user_id] = user.id in the create action where you create an user.)

You can also define a current_user method in this module:

  # Returns the current logged-in user (if any).
  def current_user
    @current_user ||= User.find_by(id: session[:user_id])
  end

Here are some other useful helper methods that you can add in this module:

  # Returns true if the given user is the current user.
  def current_user?(user)
    user == current_user
  end

  # Returns true if the user is logged in, false otherwise.
  def logged_in?
    !current_user.nil?
  end

  # Logs out the current user.
  def log_out
    session.delete(:user_id)
    @current_user = nil
  end

Finally, I would suggest you to take a look at Devise gem which is a very popular authentication solution for Rails application.