Access current_user in model

Rails 5

Declare a module

module Current
  thread_mattr_accessor :user
end

Assign the current user

class ApplicationController < ActionController::Base
  around_action :set_current_user
  def set_current_user
    Current.user = current_user
    yield
  ensure
    # to address the thread variable leak issues in Puma/Thin webserver
    Current.user = nil
  end             
end

Now you can refer to the current user as Current.user

Documentation about thread_mattr_accessor

Rails 3,4

It is not a common practice to access the current_user within a model. That being said, here is a solution:

class User < ActiveRecord::Base
  def self.current
    Thread.current[:current_user]
  end

  def self.current=(usr)
    Thread.current[:current_user] = usr
  end
end

Set the current_user attribute in a around_filter of ApplicationController.

class ApplicationController < ActionController::Base
  around_filter :set_current_user

  def set_current_user
    User.current = User.find_by_id(session[:user_id])
    yield
  ensure
    # to address the thread variable leak issues in Puma/Thin webserver
    User.current = nil
  end             
end

Set the current_user after successful authentication:

def login
  if User.current=User.authenticate(params[:username], params[:password])
    session[:user_id] = User.current.id
    flash[:message] = "Successfully logged in "
    redirect_to( :action=>'home')
  else
    flash[:notice] = "Incorrect user/password combination"
    redirect_to(:action=>"login")
  end
end

Finally, refer to the current_user in update_history of Item.

class Item < ActiveRecord::Base
  has_many :histories
  after_create :update_history
  def update_history
    histories.create(:date=>Time.now, :username=> User.current.username) 
  end
end

The Controller should tell the model instance

Working with the database is the model's job. Handling web requests, including knowing the user for the current request, is the controller's job.

Therefore, if a model instance needs to know the current user, a controller should tell it.

def create
  @item = Item.new
  @item.current_user = current_user # or whatever your controller method is
  ...
end

This assumes that Item has an attr_accessor for current_user.

The Rails 5.2 approach for having global access to the user and other attributes is CurrentAttributes.

If the user creates an item, shouldn't the item have a belongs_to :user clause? This would allow you in your after_update to do

History.create :username => self.user.username