Add support for manuscripts with no current holdings

lib/digital_scriptorium.rb

@@ -4,6 +4,8 @@
 require 'digital_scriptorium/property_id'
 
 require 'digital_scriptorium/ds_item'
+require 'digital_scriptorium/no_current_holding_exception'
+require 'digital_scriptorium/multiple_current_holdings_exception'
 require 'digital_scriptorium/ds_meta'
 require 'digital_scriptorium/holding'
 require 'digital_scriptorium/manuscript'

lib/digital_scriptorium/ds_meta.rb

@@ -12,17 +12,26 @@ def initialize(record, export_hash)
       manuscript = export_hash[record.described_manuscript_id]
       current_holdings = current_holdings(manuscript, export_hash)
 
-      if current_holdings.size != 1
-        raise "Manuscripts must have exactly 1 current holding, found #{current_holdings.size}"
-      end
-
-      @holding = current_holdings.first
+      @holding = validate_holdings_count(manuscript, current_holdings)
       @manuscript = manuscript
       @record = record
     end
 
     def current_holdings(manuscript, export_hash)
       manuscript.holding_ids.filter_map { |id| export_hash[id] if export_hash[id]&.current? }
     end
+
+    private
+
+    def validate_holdings_count(manuscript, current_holdings)
+      case current_holdings.size
+      when 0
+        raise NoCurrentHoldingException, manuscript.id
+      when 1
+        current_holdings.first
+      else
+        raise MultipleCurrentHoldingsException.new(manuscript.id, current_holdings.size)
+      end
+    end
   end
 end

lib/digital_scriptorium/multiple_current_holdings_exception.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+module DigitalScriptorium
+  # Exception raised when a manuscript has multiple current holdings, which is an invalid state
+  # that requires correction in the data.
+  class MultipleCurrentHoldingsException < StandardError
+    attr_reader :manuscript_id, :holdings_count
+
+    def initialize(manuscript_id, holdings_count)
+      @manuscript_id = manuscript_id
+      @holdings_count = holdings_count
+      super("Manuscript #{manuscript_id} has #{holdings_count} current holdings, expected at most 1")
+    end
+  end
+end

lib/digital_scriptorium/no_current_holding_exception.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+module DigitalScriptorium
+  # Exception raised when a manuscript has no current holding, which is a valid state
+  # but requires special handling in processing workflows.
+  class NoCurrentHoldingException < StandardError
+    attr_reader :manuscript_id
+
+    def initialize(manuscript_id)
+      @manuscript_id = manuscript_id
+      super("Manuscript #{manuscript_id} has no current holding")
+    end
+  end
+end

spec/digital_scriptorium/ds_meta_spec.rb

@@ -6,14 +6,50 @@ module DigitalScriptorium
   include WikibaseRepresentable::Representers
 
   RSpec.describe DsMeta do
+    let(:record) { export_hash.fetch('Q544') }
+    let(:manuscript) { export_hash.fetch('Q543') }
+
     context 'with a record concerning a manuscript with a single current holding' do
       it 'correctly sets the holding' do
-        record = export_hash.fetch('Q544')
         meta = described_class.new(record, export_hash)
         expect(meta.holding).to eq export_hash.fetch('Q542')
       end
     end
 
+    context 'with a record concerning a manuscript with no current holdings' do
+      let(:manuscript_no_holdings) do
+        manuscript.tap { |m| allow(m).to receive(:holding_ids).and_return([]) }
+      end
+
+      it 'raises NoCurrentHoldingException' do
+        allow(export_hash).to receive(:[]).with(record.described_manuscript_id).and_return(manuscript_no_holdings)
+
+        expect { described_class.new(record, export_hash) }
+          .to raise_error(DigitalScriptorium::NoCurrentHoldingException,
+                          'Manuscript Q543 has no current holding')
+      end
+    end
+
+    context 'with a record concerning a manuscript with multiple current holdings' do
+      let(:first_holding) { instance_double(DigitalScriptorium::Holding, current?: true) }
+      let(:second_holding) { instance_double(DigitalScriptorium::Holding, current?: true) }
+      let(:manuscript_multi_holdings) do
+        manuscript.tap { |m| allow(m).to receive(:holding_ids).and_return(%w[H1 H2]) }
+      end
+
+      before do
+        allow(export_hash).to receive(:[]).with(record.described_manuscript_id).and_return(manuscript_multi_holdings)
+        allow(export_hash).to receive(:[]).with('H1').and_return(first_holding)
+        allow(export_hash).to receive(:[]).with('H2').and_return(second_holding)
+      end
+
+      it 'raises MultipleCurrentHoldingsException' do
+        expect { described_class.new(record, export_hash) }
+          .to raise_error(DigitalScriptorium::MultipleCurrentHoldingsException,
+                          'Manuscript Q543 has 2 current holdings, expected at most 1')
+      end
+    end
+
     # TODO: Create setters in wikibase_representable and use them to build objects for testing
     # (currently) counterfactual holdings scenarios (i.e., other than exactly one current holding)
   end