Global ID - Reference models by URI

A Global ID is an app wide URI that uniquely identifies a model instance:

gid://YourApp/Some::Model/id

This is helpful when you need a single identifier to reference different classes of objects.

One example is job scheduling. We need to reference a model object rather than serialize the object itself. We can pass a Global ID that can be used to locate the model when it's time to perform the job. The job scheduler doesn't need to know the details of model naming and IDs, just that it has a global identifier that references a model.

Another example is a drop-down list of options, consisting of both Users and Groups. Normally we'd need to come up with our own ad hoc scheme to reference them. With Global IDs, we have a universal identifier that works for objects of both classes.

Usage

Mix GlobalID::Identification into any model with a #find(id) class method. Support is automatically included in Active Record.

person_gid = Person.find(1).to_global_id
# => #<GlobalID ...

person_gid.uri
# => #<URI ...

person_gid.to_s
# => "gid://app/Person/1"

GlobalID::Locator.locate person_gid
# => #<Person:0x007fae94bf6298 @id="1">

Signed Global IDs

For added security GlobalIDs can also be signed to ensure that the data hasn't been tampered with.

person_sgid = Person.find(1).to_signed_global_id
# => #<SignedGlobalID:0x007fea1944b410>

person_sgid = Person.find(1).to_sgid
# => #<SignedGlobalID:0x007fea1944b410>

person_sgid.to_s
# => "BAhJIh5naWQ6Ly9pZGluYWlkaS9Vc2VyLzM5NTk5BjoGRVQ=--81d7358dd5ee2ca33189bb404592df5e8d11420e"

GlobalID::Locator.locate_signed person_sgid
# => #<Person:0x007fae94bf6298 @id="1">

Expiration

Signed Global IDs can expire some time in the future. This is useful if there's a resource people shouldn't have indefinite access to, like a share link.

expiring_sgid = Document.find(5).to_sgid(expires_in: 2.hours, for: 'sharing')
# => #<SignedGlobalID:0x008fde45df8937 ...>

# Within 2 hours...
GlobalID::Locator.locate_signed(expiring_sgid.to_s, for: 'sharing')
# => #<Document:0x007fae94bf6298 @id="5">

# More than 2 hours later...
GlobalID::Locator.locate_signed(expiring_sgid.to_s, for: 'sharing')
# => nil

In Rails, an auto-expiry of 1 month is set by default. You can alter that deal in an initializer with:

# config/initializers/global_id.rb
Rails.application.config.global_id.expires_in = 3.months

You can assign a default SGID lifetime like so:

SignedGlobalID.expires_in = 1.month

This way any generated SGID will use that relative expiry.

It's worth noting that expiring SGIDs are not idempotent because they encode the current timestamp; repeated calls to to_sgid will produce different results. For example, in Rails

Document.find(5).to_sgid.to_s == Document.find(5).to_sgid.to_s
# => false

You need to explicitly pass expires_in: nil to generate a permanent SGID that will not expire,

# Passing a false value to either expiry option turns off expiration entirely.
never_expiring_sgid = Document.find(5).to_sgid(expires_in: nil)
# => #<SignedGlobalID:0x008fde45df8937 ...>

# Any time later...
GlobalID::Locator.locate_signed never_expiring_sgid
# => #<Document:0x007fae94bf6298 @id="5">

It's also possible to pass a specific expiry time

explicit_expiring_sgid = SecretAgentMessage.find(5).to_sgid(expires_at: Time.now.advance(hours: 1))
# => #<SignedGlobalID:0x008fde45df8937 ...>

# 1 hour later...
GlobalID::Locator.locate_signed explicit_expiring_sgid.to_s
# => nil

Note that an explicit :expires_at takes precedence over a relative :expires_in.

Purpose

You can even bump the security up some more by explaining what purpose a Signed Global ID is for. In this way evildoers can't reuse a sign-up form's SGID on the login page. For example.

signup_person_sgid = Person.find(1).to_sgid(for: 'signup_form')
# => #<SignedGlobalID:0x007fea1984b520

GlobalID::Locator.locate_signed(signup_person_sgid.to_s, for: 'signup_form')
# => #<Person:0x007fae94bf6298 @id="1">

Locating many Global IDs

When needing to locate many Global IDs use GlobalID::Locator.locate_many or GlobalID::Locator.locate_many_signed for Signed Global IDs to allow loading Global IDs more efficiently.

For instance, the default locator passes every model_id per model_name thus using model_name.where(id: model_ids) versus GlobalID::Locator.locate's model_name.find(id).

In the case of looking up Global IDs from a database, it's only necessary to query once per model_name as shown here:

gids = users.concat(people).sort_by(&:id).map(&:to_global_id)
# => [#<GlobalID:0x00007ffd6a8411a0 @uri=#<URI::GID gid://app/User/1>>,
#<GlobalID:0x00007ffd675d32b8 @uri=#<URI::GID gid://app/Student/1>>,
#<GlobalID:0x00007ffd6a840b10 @uri=#<URI::GID gid://app/User/2>>,
#<GlobalID:0x00007ffd675d2c28 @uri=#<URI::GID gid://app/Student/2>>,
#<GlobalID:0x00007ffd6a840480 @uri=#<URI::GID gid://app/User/3>>,
#<GlobalID:0x00007ffd675d2598 @uri=#<URI::GID gid://app/Student/3>>]

GlobalID::Locator.locate_many gids
# SELECT "users".* FROM "users" WHERE "users"."id" IN ($1, $2, $3)  [["id", 1], ["id", 2], ["id", 3]]
# SELECT "students".* FROM "students" WHERE "students"."id" IN ($1, $2, $3)  [["id", 1], ["id", 2], ["id", 3]]
# => [#<User id: 1>, #<Student id: 1>, #<User id: 2>, #<Student id: 2>, #<User id: 3>, #<Student id: 3>]

Note the order is maintained in the returned results.

Options

Either GlobalID::Locator.locate or GlobalID::Locator.locate_many supports a hash of options as second parameter. The supported options are:

:includes - A Symbol, Array, Hash or combination of them The same structure you would pass into a includes method of Active Record. See Active Record eager loading associations If present, locate or locate_many will eager load all the relationships specified here. Note: It only works if all the gids models have that relationships.
:only - A class, module or Array of classes and/or modules that are allowed to be located. Passing one or more classes limits instances of returned classes to those classes or their subclasses. Passing one or more modules in limits instances of returned classes to those including that module. If no classes or modules match, +nil+ is returned.
:ignore_missing (Only for locate_many) - By default, locate_many will call #find on the model to locate the ids extracted from the GIDs. In Active Record (and other data stores following the same pattern), #find will raise an exception if a named ID can't be found. When you set this option to true, we will use #where(id: ids) instead, which does not raise on missing records.

Custom App Locator

A custom locator can be set for an app by calling GlobalID::Locator.use and providing an app locator to use for that app. A custom app locator is useful when different apps collaborate and reference each others' Global IDs. When finding a Global ID's model, the locator to use is based on the app name provided in the Global ID url.

A custom locator can either be a block or a class.

Using a block:

GlobalID::Locator.use :foo do |gid, options|
  FooRemote.const_get(gid.model_name).find(gid.model_id)
end

Using a class:

GlobalID::Locator.use :bar, BarLocator.new
class BarLocator
  def locate(gid, options = {})
    @search_client.search name: gid.model_name, id: gid.model_id
  end
end

After defining locators as above, URIs like "gid://foo/Person/1" and "gid://bar/Person/1" will now use the foo block locator and BarLocator respectively. Other apps will still keep using the default locator.