Refactor class lookup

Avoid rechecking presence of declarative_policy_class.

lib/declarative_policy.rb

@@ -72,18 +72,17 @@ module DeclarativePolicy
     end
 
     def compute_class_for_class(subject_class)
+      if subject_class.respond_to?(:declarative_policy_class)
+        return subject_class.declarative_policy_class.constantize
+      end
+
       subject_class.ancestors.each do |klass|
-        next unless klass.name
+        name = klass.name
+
+        next unless name
 
         begin
-          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
+          policy_class = "#{name}Policy".constantize
 
           # NOTE: the < operator here tests whether policy_class
           # inherits from Base. We can't use #is_a? because that

spec/lib/declarative_policy_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe DeclarativePolicy do
+  describe '.class_for' do
+    it 'uses declarative_policy_class if present' do
+      instance = Gitlab::ErrorTracking::ErrorEvent.new
+
+      expect(described_class.class_for(instance)).to eq(ErrorTracking::BasePolicy)
+    end
+
+    it 'infers policy class from name' do
+      instance = PersonalSnippet.new
+
+      expect(described_class.class_for(instance)).to eq(PersonalSnippetPolicy)
+    end
+
+    it 'raises error if not found' do
+      instance = Object.new
+
+      expect { described_class.class_for(instance) }.to raise_error('no policy for Object')
+    end
+
+    context 'when found policy class does not inherit base' do
+      class Foo; end
+      class FooPolicy; end
+
+      it 'raises error if inferred class does not inherit Base' do
+        instance = Foo.new
+
+        expect { described_class.class_for(instance) }.to raise_error('no policy for Foo')
+      end
+    end
+  end
+end