diff --git a/CHANGELOG.md b/CHANGELOG.md
index ace50817c5..cc8ddb1060 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@
 - Fixed a bug in the deep copy of plans where the old identifier was being copied into the new plan. We now copy the generated id of the new plan to the identifier field.
 - Fixed bar chart click function in the Usage dashboard (GitHub issue #3443)
 - Fixed broken link for the V1 API documentation.
+- Fix `hidden_field_tag` Nested Attributes Format For Rails 7 Upgrade and Add Test Coverage [#3479](https://github.com/DMPRoadmap/roadmap/pull/3479)
 
 
 **Note this upgrade is mainly a migration from Bootstrap 3 to Bootstrap 5.** 
diff --git a/app/views/research_outputs/metadata_standards/_search_result.html.erb b/app/views/research_outputs/metadata_standards/_search_result.html.erb
index 9d8ed5e102..5ec6059f2e 100644
--- a/app/views/research_outputs/metadata_standards/_search_result.html.erb
+++ b/app/views/research_outputs/metadata_standards/_search_result.html.erb
@@ -1,7 +1,7 @@
 <%# locals: result %>
 
 <% if result.present? %>
-  <%= hidden_field_tag "research_output[metadata_standards_attributes[#{result.id}][id]]", result.id %>
+  <%= hidden_field_tag "research_output[metadata_standards_attributes][#{result.id}][id]", result.id %>
 
   <p><%= sanitize(result.description) %></p>
 
diff --git a/app/views/research_outputs/repositories/_search_result.html.erb b/app/views/research_outputs/repositories/_search_result.html.erb
index f15efab19e..1b01303a22 100644
--- a/app/views/research_outputs/repositories/_search_result.html.erb
+++ b/app/views/research_outputs/repositories/_search_result.html.erb
@@ -3,7 +3,7 @@
 %>
 
 <% if result.present? %>
-  <%= hidden_field_tag "research_output[repositories_attributes[#{result.id}][id]]", result.id %>
+  <%= hidden_field_tag "research_output[repositories_attributes][#{result.id}][id]", result.id %>
 
   <p><%= result.description %></p>
 
diff --git a/spec/features/modal_search_spec.rb b/spec/features/modal_search_spec.rb
index f4df729692..8867e58a51 100644
--- a/spec/features/modal_search_spec.rb
+++ b/spec/features/modal_search_spec.rb
@@ -49,4 +49,35 @@
     click_link 'Remove'
     expect(page).not_to have_text(@model.description)
   end
+
+  it 'saves research output and selected repository association', :js do
+    # Fill in required fields
+    fill_in 'Title', with: 'Test Output'
+    select 'Audiovisual', from: 'research_output_output_type'
+
+    # Open the modal and select repository
+    click_button 'Add a repository'
+    within('#modal-search-repositories') do
+      fill_in 'research_output_search_term', with: @model.name
+      click_button 'Apply filter(s)'
+      click_link 'Select'
+      # (execute_script('arguments[0].click();', modal_close_button) works locally here, but not as GitHub Action)
+      find('[data-bs-dismiss="modal"]').click
+    end
+
+    click_button 'Save'
+
+    # Verify UI changes
+    expect(page).to have_css('.fas.fa-circle-check + span + span',
+                             text: 'Successfully added the researchoutput.')
+    expect(page).to have_content('Test Output')
+    expect(page).to have_content(@model.name)
+
+    # Verify DB changes
+    research_output = ResearchOutput.last
+    expect(research_output.title).to eq('Test Output')
+    expect(research_output.output_type).to eq('audiovisual')
+    expect(research_output.repositories).to include(@model)
+    expect(research_output.plan).to eq(@plan)
+  end
 end