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ObjectTracer (previously called TappingDevice)

GitHub Action Gem Version Maintainability Test Coverage Open Source Helpers


As the name states, ObjectTracer allows you to secretly listen to different events of an object:

  • Method Calls - what does the object do
  • Traces - how is the object used by the application
  • State Mutations - what happens inside the object

After collecting the events, ObjectTracer will output them in a nice, readable format to either stdout or a file.

Ultimately, its goal is to let you know all the information you need for debugging with just 1 line of code.


Track Method Calls

By tracking an object's method calls, you'll be able to observe the object's behavior very easily

image of print_calls output

Each entry consists of 5 pieces of information:

  • method name
  • source of the method
  • call site
  • arguments
  • return value

explanation of individual entry


  • print_calls(object) - prints the result to stdout
  • write_calls(object, log_file: "file_name") - writes the result to a file
    • the default file is /tmp/object_tracer.log, but you can change it with log_file: "new_path" option

Use Cases

  • Understand a service object/form object's behavior
  • Debug a messy controller

Track Traces

By tracking an object's traces, you'll be able to observe the object's journey in your application

image of print_traces output


  • print_traces(object) - prints the result to stdout
  • write_traces(object, log_file: "file_name") - writes the result to a file
    • the default file is /tmp/object_tracer.log, but you can change it with log_file: "new_path" option

Use Cases

  • Debug argument related issues
  • Understand how a library uses your objects

Track State Mutations

By tracking an object's traces, you'll be able to observe the state changes happen inside the object between each method call

image of print_mutations output


  • print_mutations(object) - prints the result to stdout
  • write_mutations(object, log_file: "file_name") - writes the result to a file
    • the default file is /tmp/object_tracer.log, but you can change it with log_file: "new_path" option

Use Cases

  • Debug state related issues
  • Debug memoization issues

Track All Instances Of A Class

It's not always easy to directly access the objects we want to track, especially when they're managed by a library (e.g. ActiveRecord::Relation). In such cases, you can use these helpers to track the class's instances:

  • print_instance_calls(ObjectKlass)
  • print_instance_traces(ObjectKlass)
  • print_instance_mutations(ObjectKlass)
  • write_instance_calls(ObjectKlass)
  • write_instance_traces(ObjectKlass)
  • write_instance_mutations(ObjectKlass)

Use with_HELPER_NAME for chained method calls

In Ruby programs, we often chain multiple methods together like this:

And to debug it, we'll need to break the method chain into

service =
print_calls(service, options)

This kind of code changes are usually annoying, and that's one of the problems I want to solve with ObjectTracer.

So here's another option, just insert a with_HELPER_NAME call in between:

And it'll behave exactly like

service =
print_calls(service, options)


Add this line to your application's Gemfile:

gem 'object_tracer', group: :development

And then execute:

$ bundle

Or install it directly:

$ gem install object_tracer

Depending on the size of your application, ObjectTracer could harm the performance significantly. So make sure you don't put it inside the production group

Advance Usages & Options

Add Conditions With .with

Sometimes we don't need to know all the calls or traces of an object; we just want some of them. In those cases, we can chain the helpers with .with to filter the calls/traces.

# only prints calls with name matches /foo/
print_calls(object).with do |payload|


There are many options you can pass when using a helper method. You can list all available options and their default value with

ObjectTracer::Configurable::DEFAULTS #=> {

Here are some commonly used options:

colorize: false

  • default: true

By default print_calls and print_traces colorize their output. If you don't want the colors, you can use colorize: false to disable it.

print_calls(object, colorize: false)

inspect: true

  • default: false

As you might have noticed, all the objects are converted into strings with #to_s instead of #inspect. This is because when used on some Rails objects, #inspect can generate a significantly larger string than #to_s. For example:

post.to_s #=> #<Post:0x00007f89a55201d0>
post.inspect #=> #<Post id: 649, user_id: 3, topic_id: 600, post_number: 1, raw: "Hello world", cooked: "<p>Hello world</p>", created_at: "2020-05-24 08:07:29", updated_at: "2020-05-24 08:07:29", reply_to_post_number: nil, reply_count: 0, quote_count: 0, deleted_at: nil, off_topic_count: 0, like_count: 0, incoming_link_count: 0, bookmark_count: 0, score: nil, reads: 0, post_type: 1, sort_order: 1, last_editor_id: 3, hidden: false, hidden_reason_id: nil, notify_moderators_count: 0, spam_count: 0, illegal_count: 0, inappropriate_count: 0, last_version_at: "2020-05-24 08:07:29", user_deleted: false, reply_to_user_id: nil, percent_rank: 1.0, notify_user_count: 0, like_score: 0, deleted_by_id: nil, edit_reason: nil, word_count: 2, version: 1, cook_method: 1, wiki: false, baked_at: "2020-05-24 08:07:29", baked_version: 2, hidden_at: nil, self_edits: 0, reply_quoted: false, via_email: false, raw_email: nil, public_version: 1, action_code: nil, image_url: nil, locked_by_id: nil, image_upload_id: nil>

hijack_attr_methods: true

  • default: false
    • except for tap_mutation! and print_mutations

Because TracePoint doesn't track methods generated by attr_* helpers (see this issue for more info), we need to redefine those methods with the normal method definition.

For example, it generates

def name=(val)
  @name = val


attr_writer :name

This hack will only be applied to the target instance with instance_eval. So other instances of the class remain untouched.

The default is false because

  1. Checking what methods are generated by attr_* helpers isn't free. It's an O(n) operation, where n is the number of methods the target object has.
  2. It's still unclear if this hack safe enough for most applications.


Sometimes we use many private methods to perform trivial operations, like

class Operation
  def extras


  def data

  def dig_attribute(attr)
    data.dig("attributes", attr) 

And we may not be interested in those method calls. If that's the case, you can use the ignore_private option

operation =
print_calls(operation, ignore_private: true) #=> only prints the `extras` call


This option does the opposite of the ignore_private option does.

Global Configuration

If you don't want to pass options every time you use a helper, you can use global configuration to change the default values:

ObjectTracer.config[:colorize] = false
ObjectTracer.config[:hijack_attr_methods] = true

And if you're using Rails, you can put the configs under config/initializers/object_tracer.rb like this:

if defined?(ObjectTracer)
  ObjectTracer.config[:colorize] = false
  ObjectTracer.config[:hijack_attr_methods] = true

Lower-Level Helpers

print_calls and print_traces aren't the only helpers you can get from ObjectTracer. They are actually built on top of other helpers, which you can use as well. To know more about them, please check this page


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.


The gem is available as open-source under the terms of the MIT License.

Code of Conduct

Everyone interacting in the ObjectTracer project's codebases, issue trackers, chat rooms, and mailing lists is expected to follow the code of conduct.