# 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 that returns
an instance of the model, and a `.where(id:)` class method that returns an enumerable of
instance(s). Support is automatically included in Active Record.

```ruby
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">
```

`locate` returns `nil` for a blank or unparseable Global ID, and lets the
backend's own exceptions bubble up when a record can't be found. Use `fetch`
when you want to tell apart a record that's gone for good from a transient
backend failure:

```ruby
GlobalID::Locator.fetch person_gid
# => #<Person:0x007fae94bf6298 @id="1">           # found
# => raises GlobalID::Locator::RecordNotFound     # the record no longer exists
# => raises GlobalID::Locator::RecordUnavailable  # the backend failed; retry may succeed
```

Both errors extend `GlobalID::Locator::Error`, so you can rescue either at
once. This is useful, for example, to discard a background job whose argument
points at a deleted record, without also discarding jobs that hit a temporary
database error.

### Signed Global IDs

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

```ruby
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 sometime in the future. This is useful if there's a resource
people shouldn't have indefinite access to, like a share link.

```ruby
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
default in an initializer with:

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

You can assign a default SGID lifetime like so:

```ruby
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

```ruby
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,

```ruby
# 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

```ruby
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.

```ruby
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:

```ruby
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 an `includes` method of Active Record.
  See [Active Record eager loading associations](https://guides.rubyonrails.org/active_record_querying.html#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 those 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:

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

Using a class:

```ruby
class BarLocator
  def locate(gid, options = {})
    @search_client.search name: gid.model_name, id: gid.model_id
  end
end

GlobalID::Locator.use :bar, BarLocator.new
```

It's recommended to inherit from `GlobalID::Locator::BaseLocator` (or `GlobalID::Locator::UnscopedLocator` for Active Record models) to get default implementations of `model_class` and `locate_many`:

```ruby
class BarLocator < GlobalID::Locator::BaseLocator
  def locate(gid, options = {})
    @search_client.search name: gid.model_name, id: gid.model_id
  end
end

GlobalID::Locator.use :bar, BarLocator.new
```

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.

#### Custom Model Class Derivation

By default, GlobalID derives the model class by calling `constantize` on the model name from the GID. Custom locators can override this behavior by implementing a `model_class` method. This is useful when the model name in the GID doesn't match the actual class name, or when you want to redirect to a different model.

Inherit from `BaseLocator` and override `model_class`:

```ruby
class RemoteLocator < GlobalID::Locator::BaseLocator
  def model_class(gid)
    # Map remote model names to local models
    case gid.model_name
    when 'User'
      RemoteUser
    when 'Profile'
      RemoteProfile
    else
      super # Fall back to default constantize behavior
    end
  end

  def locate(gid, options = {})
    # Use the mapped model class to find the record
    model_class(gid).find_by(remote_id: gid.model_id)
  end
end

GlobalID::Locator.use :remote, RemoteLocator.new
```

This allows you to work with Global IDs that reference models that don't exist in your application, redirecting them to the appropriate local models.

**Note**: For backward compatibility, if a custom locator doesn't implement `model_class`, GlobalID will fall back to the default behavior (`constantize`) but will emit a deprecation warning. To avoid this, inherit from `GlobalID::Locator::BaseLocator` or `GlobalID::Locator::UnscopedLocator`.

### Custom Default Locator

A custom default locator can be set for an app by calling `GlobalID::Locator.default_locator=` and providing a default locator to use for that app.

```ruby
class MyCustomLocator < UnscopedLocator
  def locate(gid, options = {})
    ActiveRecord::Base.connected_to(role: :reading) do
      super(gid, options)
    end
  end

  def locate_many(gids, options = {})
    ActiveRecord::Base.connected_to(role: :reading) do
      super(gids, options)
    end
  end
end

GlobalID::Locator.default_locator = MyCustomLocator.new
```

## Contributing to GlobalID

GlobalID is work of many contributors. You're encouraged to submit pull requests, propose
features and discuss issues.

See [CONTRIBUTING](CONTRIBUTING.md).

## License
GlobalID is released under the [MIT License](http://www.opensource.org/licenses/MIT).