Make default_locator as configurable

README.md

@@ -210,6 +210,28 @@ 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.
 
+### 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

lib/global_id/global_id.rb

@@ -32,6 +32,10 @@ def app=(app)
       @app = URI::GID.validate_app(app)
     end
 
+    def default_locator(default_locator)
+      Locator.default_locator = default_locator
+    end
+
     private
       def parse_encoded_gid(gid, options)
         new(Base64.urlsafe_decode64(gid), options) rescue nil

lib/global_id/locator.rb

@@ -5,6 +5,9 @@ module Locator
     class InvalidModelIdError < StandardError; end
 
     class << self
+      # The default locator used when no app-specific locator is found.
+      attr_accessor :default_locator
+
       # Takes either a GlobalID or a string that can be turned into a GlobalID
       #
       # Options:
@@ -134,7 +137,7 @@ def use(app, locator = nil, &locator_block)
 
       private
         def locator_for(gid)
-          @locators.fetch(normalize_app(gid.app)) { DEFAULT_LOCATOR }
+          @locators.fetch(normalize_app(gid.app)) { default_locator }
         end
 
         def find_allowed?(model_class, only = nil)
@@ -223,7 +226,8 @@ def unscoped(model_class)
             end
           end
       end
-      DEFAULT_LOCATOR = UnscopedLocator.new
+
+      self.default_locator = UnscopedLocator.new
 
       class BlockLocator
         def initialize(block)

test/cases/global_locator_test.rb

@@ -386,13 +386,32 @@ def locate_many(gids, options = {}); gids.map(&:model_id); end
     assert_equal 2, found.length
   end
 
+  test "can set default_locator" do
+    class MyLocator
+      def locate(gid)
+        :my_locator
+      end
+    end
+
+    with_default_locator(MyLocator.new) do
+      assert_equal :my_locator, GlobalID::Locator.locate('gid://app/Person/1')
+    end
+  end
+
   private
     def with_app(app)
       old_app, GlobalID.app = GlobalID.app, app
       yield
     ensure
       GlobalID.app = old_app
     end
+
+    def with_default_locator(default_locator)
+      old_locator, GlobalID::Locator.default_locator = GlobalID::Locator.default_locator, default_locator
+      yield
+    ensure
+      GlobalID::Locator.default_locator = old_locator
+    end
 end
 
 class ScopedRecordLocatingTest < ActiveSupport::TestCase