marginalia

Attach comments to your ActiveRecord queries. By default, it adds the application, controller, and action names as a
comment at the end of each query.

This helps when searching log files for queries, and seeing where slow queries came from.

For example, once enabled, your logs will look like:

Account Load (0.3ms)  SELECT `accounts`.* FROM `accounts` 
WHERE `accounts`.`queenbee_id` = 1234567890 
LIMIT 1 
/*application:BCX,controller:project_imports,action:show*/

You can also use these query comments along with a tool like pt-query-digest
to automate identification of controllers and actions that are hotspots for slow queries.

This gem was created at 37signals. You can read more about how we use it on
our blog.

This has been tested and used in production with the mysql2 and pg gems, and is
tested on Rails 5.2 through 6.1, and Ruby 2.6 through 3.0. It is also tested
for sqlite3. As of Rails 7, Marginalia is a part of Rails itself and does not need to be separately included.

Rails version support will follow supported versions in the Ruby on Rails maintenance policy
and Ruby support will follow maintained versions in the Ruby maintenance policy.

Patches are welcome for other database adapters.

Installation

# Gemfile
gem 'marginalia'

Customization

Optionally, you can set the application name shown in the log like so in an initializer (e.g. config/initializers/marginalia.rb):

Marginalia.application_name = "BCX"

The name will default to your Rails application name.

Components

You can also configure the components of the comment that will be appended,
by setting Marginalia::Comment.components. By default, this is set to:

Marginalia::Comment.components = [:application, :controller, :action]

Which results in a comment of
application:#{application_name},controller:#{controller.name},action:#{action_name}.

You can re-order or remove these components. You can also add additional
comment components of your desire by defining new module methods for
Marginalia::Comment which return a string. For example:

module Marginalia
  module Comment
    def self.mycommentcomponent
      "TEST"
    end
  end
end

Marginalia::Comment.components = [:application, :mycommentcomponent]

Which will result in a comment like
application:#{application_name},mycommentcomponent:TEST
The calling controller is available to these methods via @controller.

Marginalia ships with :application, :controller, and :action enabled by
default. In addition, implementation is provided for:

• :line (for file and line number calling query). :line supports
  a configuration by setting a regexp in Marginalia::Comment.lines_to_ignore
  to exclude parts of the stacktrace from inclusion in the line comment.
• :controller_with_namespace to include the full classname (including namespace)
  of the controller.
• :job to include the classname of the ActiveJob being performed.
• :hostname to include Socket.gethostname.
• :pid to include current process id.
• :db_host to include the configured database hostname.
• :socket to include the configured database socket.
• :database to include the configured database name.

Pull requests for other included comment components are welcome.

Prepend comments

By default marginalia appends the comments at the end of the query. Certain databases, such as MySQL will truncate
the query text. This is the case for slow query logs and the results of querying some InnoDB internal tables where the
length of the query is more than 1024 bytes.

In order to not lose the marginalia comments from your logs, you can prepend the comments using this option:

Marginalia::Comment.prepend_comment = true

Inline query annotations

In addition to the request or job-level component-based annotations,
Marginalia may be used to add inline annotations to specific queries using a
block-based API.

For example, the following code:

Marginalia.with_annotation("foo") do
  Account.where(queenbee_id: 1234567890).first
end

will issue this query:

Account Load (0.3ms)  SELECT `accounts`.* FROM `accounts`
WHERE `accounts`.`queenbee_id` = 1234567890
LIMIT 1
/*application:BCX,controller:project_imports,action:show*/ /*foo*/

Nesting with_annotation blocks will concatenate the comment strings.

Caveats

Prepared statements

Be careful when using Marginalia with prepared statements. If you use a component
like request_id then every query will be unique and so ActiveRecord will create
a new prepared statement for each potentially exhausting system resources.
Disable prepared statements
if you wish to use components with high cardinality values.

Contributing

Start by bundling and creating the test database:

bundle
rake db:mysql:create
rake db:postgresql:create

Then, running rake will run the tests on all the database adapters (mysql, mysql2, postgresql and sqlite):

rake