Add support for Identifier relation types.

These are introduced in CFF schema version 1.3.0.

lib/cff/identifier.rb

@@ -44,6 +44,11 @@ class Identifier < ModelPart
       end.freeze
     end.freeze
 
+    # The defined set of identifier relation types (link to come).
+    IDENTIFIER_RELATION_TYPES = Schemas.read_defs('identifier-relation', 'enum') do |obj|
+      obj.dup.freeze
+    end.compact.freeze
+
     # :call-seq:
     #   new -> Identifier
     #   new { |id| block } -> Identifier
@@ -69,6 +74,17 @@ def initialize(param = nil, value = nil)
       yield self if block_given?
     end
 
+    # :call-seq:
+    #   relation = relation
+    #
+    # Sets the relation type of this Identifier. The relation is restricted to
+    # a defined set of identifier relation types (link to come).
+    #
+    # *Note:* This field is only available since version 1.3.0.
+    def relation=(relation)
+      @fields['relation'] = relation if IDENTIFIER_RELATION_TYPES.include?(relation)
+    end
+
     # :call-seq:
     #   type = type
     #

test/identifier_test.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-# Copyright (c) 2018-2022 The Ruby Citation File Format Developers.
+# Copyright (c) 2018-2024 The Ruby Citation File Format Developers.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -72,6 +72,22 @@ def test_bad_methods_not_allowed
     end
   end
 
+  def test_relation_type_restricted_to_allowed_types
+    id = ::CFF::Identifier.new
+
+    id.relation = 'IsCitedBy'
+    assert_equal('IsCitedBy', id.relation)
+
+    id.relation = 'xxx'
+    assert_equal('IsCitedBy', id.relation)
+
+    id.relation = 'compiles' # Incorrect case.
+    assert_equal('IsCitedBy', id.relation)
+
+    id.relation = 'Obsoletes'
+    assert_equal('Obsoletes', id.relation)
+  end
+
   def test_type_restricted_to_allowed_types
     id = ::CFF::Identifier.new