Add declarative_policy_class to Declarative Policy

doc/development/policies.md

@@ -157,3 +157,18 @@ end
 ```
 
 will include all rules from `ProjectPolicy`. The delegated conditions will be evaluated with the correct delegated subject, and will be sorted along with the regular rules in the policy. Note that only the relevant rules for a particular ability will actually be considered.
+
+## Specifying Policy Class
+
+You can also override the Policy used for a given subject:
+
+```ruby
+class Foo
+
+  def self.declarative_policy_class
+    'SomeOtherPolicy'
+  end
+end
+```
+
+This will use & check permissions on the `SomeOtherPolicy` class rather than the usual calculated `FooPolicy` class.

lib/declarative_policy.rb

@@ -74,7 +74,14 @@ module DeclarativePolicy
         next unless klass.name
 
         begin
-          policy_class = "#{klass.name}Policy".constantize
+          klass_name =
+            if subject_class.respond_to?(:declarative_policy_class)
+              subject_class.declarative_policy_class
+            else
+              "#{klass.name}Policy"
+            end
+
+          policy_class = klass_name.constantize
 
           # NOTE: the < operator here tests whether policy_class
           # inherits from Base. We can't use #is_a? because that