Enhance human_attribute_name to support explicit locale options for nested attributes

Author: Syati

lib/structured_params/i18n.rb

@@ -46,22 +46,28 @@ def human_attribute_name(attribute, options = {})
         return super if parts.length == 1
         return super unless structured_attributes.key?(parts.first)
 
-        resolve_nested_human_attribute_name(parts)
+        resolve_nested_human_attribute_name(parts, options)
       end
 
       private
 
       # Walk +parts+ (e.g. ["hobbies", "0", "name"]) and build a human-readable
       # label by delegating each segment to the appropriate nested class.
       #
-      #: (Array[String]) -> String
-      def resolve_nested_human_attribute_name(parts)
+      # Only +:locale+ is forwarded to inner +human_attribute_name+ calls.
+      # Options such as +:default+ are specific to the outer call (e.g. from
+      # +full_messages+) and must not bleed into individual segment lookups,
+      # where they would replace the segment's own translation fallback.
+      #
+      #: (Array[String], Hash[untyped, untyped]) -> String
+      def resolve_nested_human_attribute_name(parts, options)
         label = nil
         klass = self
+        inner_opts = options.slice(:locale)
 
         attr_segments(parts).each do |index, attr|
-          human = klass&.human_attribute_name(attr) || attr.humanize
-          label = build_nested_label(label, index, human)
+          human = klass&.human_attribute_name(attr, inner_opts) || attr.humanize
+          label = build_nested_label(label, index, human, options)
           klass &&= klass.structured_attributes[attr]
         end
 
@@ -93,24 +99,31 @@ def attr_segments(parts)
       #   activemodel.errors.nested_attribute.array  (parent, index, child)
       #   activemodel.errors.nested_attribute.object (parent, child)
       #
-      #: (String?, String?, String) -> String
-      def build_nested_label(result, index, attr_human)
-        if result.nil?
-          attr_human
-        elsif index
+      # The +locale:+ key from +options+ is forwarded to ::I18n.t so that an
+      # explicit locale passed to human_attribute_name is honoured.
+      #
+      #: (String?, String?, String, Hash[untyped, untyped]) -> String
+      def build_nested_label(result, index, attr_human, options)
+        return attr_human if result.nil?
+
+        i18n_opts = options.slice(:locale)
+
+        if index
           ::I18n.t(
             'activemodel.errors.nested_attribute.array',
             parent: result,
             index: index,
             child: attr_human,
-            default: "#{result} #{index} #{attr_human}"
+            default: "#{result} #{index} #{attr_human}",
+            **i18n_opts
           )
         else
           ::I18n.t(
             'activemodel.errors.nested_attribute.object',
             parent: result,
             child: attr_human,
-            default: "#{result} #{attr_human}"
+            default: "#{result} #{attr_human}",
+            **i18n_opts
           )
         end
       end

spec/i18n_spec.rb

@@ -190,4 +190,45 @@
       end
     end
   end
+
+  describe 'explicit locale: option threading' do
+    include_context 'with ja locale'
+
+    let(:ja_overrides) do
+      {
+        activemodel: {
+          errors: {
+            nested_attribute: {
+              array: '%<parent>s %<index>s 番目の%<child>s',
+              object: '%<parent>sの%<child>s'
+            }
+          }
+        }
+      }
+    end
+
+    context 'when locale: :ja is passed while the current locale is :en' do
+      it 'resolves array attribute labels in ja' do
+        I18n.with_locale(:en) do
+          expect(UserParameter.human_attribute_name(:'hobbies.0.name', locale: :ja)).to eq('趣味 0 番目の名前')
+        end
+      end
+
+      it 'resolves object attribute labels in ja' do
+        I18n.with_locale(:en) do
+          expect(UserParameter.human_attribute_name(:'address.postal_code', locale: :ja)).to eq('住所の郵便番号')
+        end
+      end
+    end
+
+    context 'when locale: :en is passed while the current locale is :ja' do
+      it 'resolves array attribute labels in en' do
+        expect(UserParameter.human_attribute_name(:'hobbies.0.name', locale: :en)).to eq('Hobbies 0 Name')
+      end
+
+      it 'resolves object attribute labels in en' do
+        expect(UserParameter.human_attribute_name(:'address.postal_code', locale: :en)).to eq('Address Postal code')
+      end
+    end
+  end
 end