Add a ChildTypes to manage selection display during work attachment

With `ActiveFedora` models, we use `MyWork.valid_child_concerns` to determine
which types the user can select to attach to an existing work.

Since this is a display-only issue, we break this out into a new service. For
legacy models, we delegate the configuration back to the model, avoiding any
need for configuration changes.

For Valkyrie models, we provide a default config (a work can nest works of its
own type), but punt on configuration issues. There are other configuration
matters to discuss.

app/presenters/hyrax/model_proxy.rb

@@ -4,7 +4,7 @@ module Hyrax
   module ModelProxy
     delegate :to_param, :to_key, :id, to: :solr_document
 
-    delegate :model_name, :valid_child_concerns, to: :_delegated_to
+    delegate :model_name, to: :_delegated_to
 
     def to_partial_path
       _delegated_to._to_partial_path
@@ -18,6 +18,10 @@ def to_model
       self
     end
 
+    def valid_child_concerns
+      Hyrax::ChildTypes.for(parent: solr_document.hydra_model)
+    end
+
     private
 
       def _delegated_to

app/services/hyrax/child_types.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+module Hyrax
+  ##
+  # A list of child work types a user may choose to attach for a given work type
+  #
+  # These lists are used when users select among work types to attach to an
+  # existing work, e.g. from Actions available on the show view.
+  #
+  # @example
+  #   child_types = Hyrax::ChildTypes.for(parent: MyWorkType)
+  #
+  class ChildTypes
+    include Enumerable
+    extend Forwardable
+
+    def_delegators :@types, :each
+
+    ##
+    # @!attribute [r] types
+    #   @return [Array<Class>]
+    attr_reader :types
+
+    ##
+    # @params [Class] parent
+    def self.for(parent:)
+      return new(parent.valid_child_concerns) if
+        parent.respond_to?(:valid_child_concerns)
+
+      new([parent])
+    end
+
+    ##
+    # @param [Array<Class>] types
+    def initialize(types)
+      @types = types.to_a
+    end
+  end
+end

spec/services/hyrax/child_types_spec.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+RSpec.describe Hyrax::ChildTypes do
+  subject(:child_types) { described_class.new(types) }
+  let(:types)           { [GenericWork] }
+
+  describe '.for' do
+    let(:parent) { Hyrax::Test::SimpleWork }
+
+    it 'can have itself as a child by default' do
+      expect(described_class.for(parent: parent)).to contain_exactly(parent)
+    end
+
+    context 'with an ActiveFedora work' do
+      let(:parent) { GenericWork }
+
+      it 'gives the configured valid_child_concerns' do
+        expect(described_class.for(parent: parent)).to contain_exactly(*parent.valid_child_concerns)
+      end
+    end
+  end
+
+  describe '#types' do
+    it 'returns the initialized types' do
+      expect(child_types.types).to contain_exactly(*types)
+    end
+  end
+
+  describe '#to_a' do
+    it 'returns the initialized types' do
+      expect(child_types.to_a).to contain_exactly(*types)
+    end
+  end
+end