Allow subclasses of BasePipeline to resolved

Currently, one can only pass Symbols to `Pipeline.[]` which is
convenient in most cases assuming that the pipeline is namespaced to
`Banzai::Pipeline`.

In some cases, we want a namespace a pipeline to a specific domain
namespace (e.g `StatusPage::PostProcessPipeline`). In those cases we had
to pass a underscored version of the namespaced class name (e.g.
`:'_status_page::_post_process_pipeline'` or
`:'status_page/post_process_pipeline'`).

This commit allows passing a subclass of
`Banzai::Pipeline::BasePipeline` to `Pipeline.[]`.

app/models/resource_label_event.rb

@@ -56,7 +56,7 @@ class ResourceLabelEvent < ResourceEvent
   end
 
   def banzai_render_context(field)
-    super.merge(pipeline: 'label', only_path: true)
+    super.merge(pipeline: :label, only_path: true)
   end
 
   def refresh_invalid_reference

lib/banzai/pipeline.rb

@@ -2,9 +2,33 @@
 
 module Banzai
   module Pipeline
+    # Resolve a pipeline by name
+    #
+    # name - nil, Class or Symbol. The name to be resolved.
+    #
+    # Examples:
+    #     Pipeline[nil] # => Banzai::Pipeline::FullPipeline
+    #     Pipeline[:label] # => Banzai::Pipeline::LabelPipeline
+    #     Pipeline[StatusPage::PostProcessPipeline] # => StatusPage::PostProcessPipeline
+    #
+    #     Pipeline['label'] # => raises ArgumentError - unsupport type
+    #     Pipeline[Project] # => raises ArgumentError - not a subclass of BasePipeline
+    #
+    # Returns a pipeline class which is a subclass of Banzai::Pipeline::BasePipeline.
     def self.[](name)
-      name ||= :full
-      const_get("#{name.to_s.camelize}Pipeline", false)
+      name ||= FullPipeline
+
+      pipeline = case name
+                 when Class
+                   name
+                 when Symbol
+                   const_get("#{name.to_s.camelize}Pipeline", false)
+                 end
+
+      return pipeline if pipeline && pipeline < BasePipeline
+
+      raise ArgumentError,
+        "unsupported pipeline name #{name.inspect} (#{name.class})"
     end
   end
 end

spec/lib/banzai/pipeline_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Banzai::Pipeline do
+  describe '.[]' do
+    subject { described_class[name] }
+
+    shared_examples 'error' do |exception, message|
+      it do
+        expect { subject }.to raise_error(exception, message)
+      end
+    end
+
+    context 'for nil' do
+      let(:name) { nil }
+
+      it { is_expected.to eq(Banzai::Pipeline::FullPipeline) }
+    end
+
+    context 'for symbols' do
+      context 'when known' do
+        let(:name) { :full }
+
+        it { is_expected.to eq(Banzai::Pipeline::FullPipeline) }
+      end
+
+      context 'when unknown' do
+        let(:name) { :unknown }
+
+        it_behaves_like 'error', NameError,
+          'uninitialized constant Banzai::Pipeline::UnknownPipeline'
+      end
+    end
+
+    context 'for classes' do
+      let(:name) { klass }
+
+      context 'subclassing Banzai::Pipeline::BasePipeline' do
+        let(:klass) { Class.new(Banzai::Pipeline::BasePipeline) }
+
+        it { is_expected.to eq(klass) }
+      end
+
+      context 'subclassing other types' do
+        let(:klass) { Class.new(Banzai::RenderContext) }
+
+        before do
+          stub_const('Foo', klass)
+        end
+
+        it_behaves_like 'error', ArgumentError,
+          'unsupported pipeline name Foo (Class)'
+      end
+    end
+
+    context 'for other types' do
+      let(:name) { 'label' }
+
+      it_behaves_like 'error', ArgumentError,
+        'unsupported pipeline name "label" (String)'
+    end
+  end
+end