Rails makes use of ruby's standard "logger":http://www.ruby-doc.org/stdlib/libdoc/logger/rdoc/index.html

From within a controller:

<pre><code>
  logger.debug "user.id = #{user.id}"
  logger.info "Starting process fubar..."
  logger.warn "No results found for XYZ"
  logger.error err.message
  logger.fatal "Database down"
</pre></code>

From within a view:
<pre><code>
  @logger.debug "user.id = #{user.id}"
</pre></code>

(This will hopefully be rationalised in a future version of Rails.)

Finally, there's a constant @RAILS_DEFAULT_LOGGER@ which can be used from anywhere, including the console.

In summary:
<pre>
                             Model     View     Controller    Console
    @logger                    -        X           -            -
    logger                     X        -           X            -
    RAILS_DEFAULT_LOGGER       X        X           X            X
</pre>

<div style="margin-left: 2em; padding: 1ex; color: #888; font-style: italic">
A better summary would be
<pre style="background: none">
                             Model     View     Controller    Console
    logger                     X        X           X            X
</pre>
but that's not the way it works at the moment (2005-04-21).
</div>

To expose the controller's logger to its views, including helper methods:

<pre>
  class ApplicationController < ActionController::Base
    helper_method :logger
  end
</pre>

See also HowtoConfigureLogging.

category: Howto