How to log user_name in Rails?
config.log_tags = [:user_name]

config.log_tags = [ lambda {|req| req.cookie_jar["_session_id"].to_s ]

config.log_tags = [ lambda {|req| req.cookie_jar["user_name"] || 'Noone' } ]

config.log_tags = [ lambda { |req| user = req.env['warden'].user; user && user.name || 'Noone'; } ]

config.log_tags = [ lambda { |req| user = User.find_by_id(req.env['rack.session']['user_id']); user && user.name || 'Noone'; }

Rails.configuration.log_tags = [
  :uuid,   # request UUID
  lambda { |req|
    # Credentials are (currently) in the format of:
    #
    #   <session_hash>::<user_id>
    #
    # So we have to split by '::' to obtain the user_id for logging.
    #
    # This will just output "User: nil" if there is no current session.
    "User: #{req.cookies['user_credentials'].to_s.split('::')[1]}"
  }
]

Rails.configuration.log_tags = [
  lambda { |req|
    req.inspect
  }
]

Rails.configuration.log_tags = [
  lambda { |req|
    req.cookies.inspect
  }
]

config.middleware.delete(ActionDispatch::Cookies)
config.middleware.delete(ActionDispatch::Session::CookieStore)
config.middleware.insert_before(Rails::Rack::Logger, ActionDispatch::Session::CookieStore)
config.middleware.insert_before(ActionDispatch::Session::CookieStore, ActionDispatch::Cookies)

Rails.configuration.log_tags = [
  proc do |req|
    if req.session["warden.user.user.key"].nil?
      "Anonym"
    else
      "user_id:#{req.session["warden.user.user.key"][1][0]}"
    end
  end
]

[Anonym] Served asset ...

[user_id:1] Served asset ...