add can_grant_permissions_to relations from object to subject

README.md

@@ -83,6 +83,25 @@ user.permitted_documents.with_permission ["viewer", "editor"]
 
 ```
 
+The inverse relationship is also possible by specifying `can_grant_permissions_to` on objects:
+
+```
+class Document < ApplicationRecord
+  include Caber::Object
+  can_grant_permissions_to User
+end
+
+document.permitted_users
+# => all users with any permission
+
+document.permitted_users.with_permission "viewer"
+# => all users with viewer permission
+
+document.permitted_users.with_permission ["viewer", "editor"]
+# => all users with viewer or editor permission
+
+```
+
 ## Development
 
 After checking out the repo, run `bin/setup` to install dependencies. Then, run `rake spec` to run the tests.

app/models/concerns/caber/object.rb

@@ -2,7 +2,12 @@ module Caber::Object
   extend ActiveSupport::Concern
 
   included do
+    has_many :caber_relations, as: :object, class_name: "Caber::Relation"
     scope :with_permission, ->(permission) { where("caber_relations.permission": permission) }
+
+    def self.can_grant_permissions_to(model)
+      has_many :"permitted_#{model.name.pluralize.parameterize}", through: :caber_relations, source: :subject, source_type: model.name
+    end
   end
 
   def grant_permission_to(permission, subject)

app/models/concerns/caber/subject.rb

@@ -3,6 +3,7 @@ module Caber::Subject
 
   included do
     has_many :caber_relations, as: :subject, class_name: "Caber::Relation"
+    scope :with_permission, ->(permission) { where("caber_relations.permission": permission) }
 
     def self.can_have_permissions_on(model)
       has_many :"permitted_#{model.name.pluralize.parameterize}", through: :caber_relations, source: :object, source_type: model.name

spec/dummy/app/models/document.rb

@@ -1,3 +1,5 @@
 class Document < ApplicationRecord
   include Caber::Object
+
+  can_grant_permissions_to User
 end

spec/models/caber/relation_spec.rb

@@ -37,6 +37,14 @@
     it "gives Alice a list of objects she can view" do
       expect(alice.permitted_documents.with_permission("viewer")).to include object
     end
+
+    it "includes Alice in a list of permitted users for the object" do
+      expect(object.permitted_users).to include alice
+    end
+
+    it "includes Alice in a list of users with viewer permission for the object" do
+      expect(object.permitted_users.with_permission("viewer")).to include alice
+    end
   end
 
   context "with multiple objects" do
@@ -75,6 +83,10 @@
     it "gives Alice a list of objects she can view or own" do
       expect(alice.permitted_documents.with_permission(["viewer", "owner"])).to include object
     end
+
+    it "includes Alice in a list of users with viewer or owner permission for the object" do
+      expect(object.permitted_users.with_permission(["viewer", "owner"])).to include alice
+    end
   end
 
   context "when anyone can view a object" do