diff --git a/app/assets/stylesheets/pages/diff.scss b/app/assets/stylesheets/pages/diff.scss
index 1b4694377b30ef6e3dab1586a2d87a0c521e730f..feefaad8a15c65c0b7e6b8eed5046d25f490ebb5 100644
--- a/app/assets/stylesheets/pages/diff.scss
+++ b/app/assets/stylesheets/pages/diff.scss
@@ -425,12 +425,6 @@
   float: right;
 }
 
-.diffs {
-  .content-block {
-    border-bottom: none;
-  }
-}
-
 .files-changed {
   border-bottom: none;
 }
diff --git a/app/assets/stylesheets/pages/merge_requests.scss b/app/assets/stylesheets/pages/merge_requests.scss
index 6a419384a3472bbee48853ee41afc395f452dc69..bca62b7fc3137f2bd2284bcf71af65273030871f 100644
--- a/app/assets/stylesheets/pages/merge_requests.scss
+++ b/app/assets/stylesheets/pages/merge_requests.scss
@@ -511,7 +511,6 @@
 
 .mr-version-controls {
   background: $gray-light;
-  border-bottom: 1px solid $border-color;
   color: $gl-text-color;
 
   .mr-version-menus-container {
diff --git a/app/controllers/projects/merge_requests_controller.rb b/app/controllers/projects/merge_requests_controller.rb
index 09dc8b38229bc3311faebb8d8898d80fcb7dbd04..a63b7ff0bed51dd31c3a21189da33e9011cfe2d8 100755
--- a/app/controllers/projects/merge_requests_controller.rb
+++ b/app/controllers/projects/merge_requests_controller.rb
@@ -120,7 +120,8 @@ class Projects::MergeRequestsController < Projects::ApplicationController
       define_diff_comment_vars
     else
       build_merge_request
-      @diffs = @merge_request.diffs(diff_options)
+      @compare = @merge_request
+      @diffs = @compare.diffs(diff_options)
       @diff_notes_disabled = true
     end
 
@@ -584,12 +585,14 @@ class Projects::MergeRequestsController < Projects::ApplicationController
       end
     end
 
-    @diffs =
+    @compare =
       if @start_sha
-        @merge_request_diff.compare_with(@start_sha).diffs(diff_options)
+        @merge_request_diff.compare_with(@start_sha)
       else
-        @merge_request_diff.diffs(diff_options)
+        @merge_request_diff
       end
+
+    @diffs = @compare.diffs(diff_options)
   end
 
   def define_diff_comment_vars
@@ -598,11 +601,11 @@ class Projects::MergeRequestsController < Projects::ApplicationController
       noteable_id: @merge_request.id
     }
 
-    @diff_notes_disabled = !@merge_request_diff.latest? || @start_sha
+    @diff_notes_disabled = false
 
     @use_legacy_diff_notes = !@merge_request.has_complete_diff_refs?
 
-    @grouped_diff_discussions = @merge_request.grouped_diff_discussions(@merge_request_diff.diff_refs)
+    @grouped_diff_discussions = @merge_request.grouped_diff_discussions(@compare.diff_refs)
     @notes = prepare_notes_for_rendering(@grouped_diff_discussions.values.flatten.flat_map(&:notes))
   end
 
diff --git a/app/helpers/notes_helper.rb b/app/helpers/notes_helper.rb
index eab0738a368f1e71032656a9cdc9881d69727069..08180883eb9e0571d47909fb3ad99a3b02a63050 100644
--- a/app/helpers/notes_helper.rb
+++ b/app/helpers/notes_helper.rb
@@ -60,20 +60,16 @@ module NotesHelper
     note.project.team.human_max_access(note.author_id)
   end
 
-  def discussion_diff_path(discussion)
-    if discussion.for_merge_request? && discussion.diff_discussion?
-      if discussion.active?
-        # Without a diff ID, the link always points to the latest diff version
-        diff_id = nil
-      elsif merge_request_diff = discussion.latest_merge_request_diff
-        diff_id = merge_request_diff.id
-      else
-        # If the discussion is not active, and we cannot find the latest
-        # merge request diff for this discussion, we return no path at all.
-        return
-      end
-
-      diffs_namespace_project_merge_request_path(discussion.project.namespace, discussion.project, discussion.noteable, diff_id: diff_id, anchor: discussion.line_code)
+  def discussion_path(discussion)
+    if discussion.for_merge_request?
+      return unless discussion.diff_discussion?
+
+      version_params = discussion.merge_request_version_params
+      return unless version_params
+
+      path_params = version_params.merge(anchor: discussion.line_code)
+
+      diffs_namespace_project_merge_request_path(discussion.project.namespace, discussion.project, discussion.noteable, path_params)
     elsif discussion.for_commit?
       anchor = discussion.line_code if discussion.diff_discussion?
 
diff --git a/app/models/concerns/discussion_on_diff.rb b/app/models/concerns/discussion_on_diff.rb
index 8ee42875670c43717d10ae8a6f316c835c0405af..a7bdf5587b2f348443ed4baddf45e513b5eebee5 100644
--- a/app/models/concerns/discussion_on_diff.rb
+++ b/app/models/concerns/discussion_on_diff.rb
@@ -11,6 +11,7 @@ module DiscussionOnDiff
               :diff_line,
               :for_line?,
               :active?,
+              :created_at_diff?,
 
               to: :first_note
 
diff --git a/app/models/concerns/note_on_diff.rb b/app/models/concerns/note_on_diff.rb
index 6c27dd5aa5c2f0bade9f994d9a96b93319e4985d..6359f7596b1d5af26049cdb612eb407d2855304a 100644
--- a/app/models/concerns/note_on_diff.rb
+++ b/app/models/concerns/note_on_diff.rb
@@ -30,6 +30,10 @@ module NoteOnDiff
     raise NotImplementedError
   end
 
+  def created_at_diff?(diff_refs)
+    false
+  end
+
   private
 
   def noteable_diff_refs
diff --git a/app/models/diff_discussion.rb b/app/models/diff_discussion.rb
index 6a6466b493b7de971fee7539484f8d2b4b09bad1..d627fbe327fef7a1ce85ee988e6a8299cf170a45 100644
--- a/app/models/diff_discussion.rb
+++ b/app/models/diff_discussion.rb
@@ -10,7 +10,6 @@ class DiffDiscussion < Discussion
 
   delegate  :position,
             :original_position,
-            :latest_merge_request_diff,
 
             to: :first_note
 
@@ -18,6 +17,25 @@ class DiffDiscussion < Discussion
     false
   end
 
+  def merge_request_version_params
+    return unless for_merge_request?
+
+    if active?
+      {}
+    else
+      diff_refs = position.diff_refs
+
+      if diff = noteable.merge_request_diff_for(diff_refs)
+        { diff_id: diff.id }
+      elsif diff = noteable.merge_request_diff_for(diff_refs.head_sha)
+        {
+          diff_id: diff.id,
+          start_sha: diff_refs.start_sha
+        }
+      end
+    end
+  end
+
   def reply_attributes
     super.merge(
       original_position: original_position.to_json,
diff --git a/app/models/diff_note.rb b/app/models/diff_note.rb
index abe4518d62ab83ba97042aa138cd718c42e68784..76c59199afdac8809db7fc3393ffb17160ca70c9 100644
--- a/app/models/diff_note.rb
+++ b/app/models/diff_note.rb
@@ -65,10 +65,11 @@ class DiffNote < Note
     self.position.diff_refs == diff_refs
   end
 
-  def latest_merge_request_diff
-    return unless for_merge_request?
+  def created_at_diff?(diff_refs)
+    return false unless supported?
+    return true if for_commit?
 
-    self.noteable.merge_request_diff_for(self.position.diff_refs)
+    self.original_position.diff_refs == diff_refs
   end
 
   private
diff --git a/app/models/legacy_diff_discussion.rb b/app/models/legacy_diff_discussion.rb
index e617ce36f567e3c675bd03ec645e29e1f619cd83..3c1d34db5fa17c017dae266811d9b67ec6a092d0 100644
--- a/app/models/legacy_diff_discussion.rb
+++ b/app/models/legacy_diff_discussion.rb
@@ -9,14 +9,14 @@ class LegacyDiffDiscussion < Discussion
 
   memoized_values << :active
 
-  def legacy_diff_discussion?
-    true
-  end
-
   def self.note_class
     LegacyDiffNote
   end
 
+  def legacy_diff_discussion?
+    true
+  end
+
   def active?(*args)
     return @active if @active.present?
 
@@ -27,6 +27,16 @@ class LegacyDiffDiscussion < Discussion
     !active?
   end
 
+  def merge_request_version_params
+    return unless for_merge_request?
+
+    if active?
+      {}
+    else
+      nil
+    end
+  end
+
   def reply_attributes
     super.merge(line_code: line_code)
   end
diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 365fa4f1e7059c1f08c4a5bebfdd222c883c637e..12c5481cd6d24c7bcd841820652c2bfc27d9579e 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -374,12 +374,18 @@ class MergeRequest < ActiveRecord::Base
     merge_request_diff(true)
   end
 
-  def merge_request_diff_for(diff_refs)
-    @merge_request_diffs_by_diff_refs ||= Hash.new do |h, diff_refs|
-      h[diff_refs] = merge_request_diffs.viewable.select_without_diff.find_by_diff_refs(diff_refs)
-    end
-
-    @merge_request_diffs_by_diff_refs[diff_refs]
+  def merge_request_diff_for(diff_refs_or_sha)
+    @merge_request_diffs_by_diff_refs_or_sha ||= Hash.new do |h, diff_refs_or_sha|
+      diffs = merge_request_diffs.viewable.select_without_diff
+      h[diff_refs_or_sha] =
+        if diff_refs_or_sha.is_a?(Gitlab::Diff::DiffRefs)
+          diffs.find_by_diff_refs(diff_refs_or_sha)
+        else
+          diffs.find_by(head_commit_sha: diff_refs_or_sha)
+        end
+    end
+
+    @merge_request_diffs_by_diff_refs_or_sha[diff_refs_or_sha]
   end
 
   def reload_diff_if_branch_changed
diff --git a/app/models/note.rb b/app/models/note.rb
index e720bfba030c8a75ee8af3ef73363b69ee0e94f3..b06985b4a6f814168db84c057dff8c58356cd70b 100644
--- a/app/models/note.rb
+++ b/app/models/note.rb
@@ -115,11 +115,19 @@ class Note < ActiveRecord::Base
     end
 
     def grouped_diff_discussions(diff_refs = nil)
-      diff_notes.
-        fresh.
-        discussions.
-        select { |n| n.active?(diff_refs) }.
-        group_by(&:line_code)
+      groups = {}
+
+      diff_notes.fresh.discussions.each do |discussion|
+        if discussion.active?(diff_refs)
+          discussions = groups[discussion.line_code] ||= []
+        elsif diff_refs && discussion.created_at_diff?(diff_refs)
+          discussions = groups[discussion.original_line_code] ||= []
+        end
+
+        discussions << discussion if discussions
+      end
+
+      groups
     end
 
     def count_for_collection(ids, type)
@@ -141,10 +149,6 @@ class Note < ActiveRecord::Base
     true
   end
 
-  def latest_merge_request_diff
-    nil
-  end
-
   def max_attachment_size
     current_application_settings.max_attachment_size.megabytes.to_i
   end
diff --git a/app/views/discussions/_diff_with_notes.html.haml b/app/views/discussions/_diff_with_notes.html.haml
index 549364761e6d1f4852f763c5b13710024f917d76..78c5b0c1dda5502d767310704671f061a12f2a2d 100644
--- a/app/views/discussions/_diff_with_notes.html.haml
+++ b/app/views/discussions/_diff_with_notes.html.haml
@@ -3,7 +3,7 @@
 
 .diff-file.file-holder
   .js-file-title.file-title
-    = render "projects/diffs/file_header", diff_file: diff_file, blob: blob, diff_commit: diff_file.content_commit, project: discussion.project, url: discussion_diff_path(discussion)
+    = render "projects/diffs/file_header", diff_file: diff_file, blob: blob, diff_commit: diff_file.content_commit, project: discussion.project, url: discussion_path(discussion)
 
   .diff-content.code.js-syntax-highlight
     %table
diff --git a/app/views/discussions/_discussion.html.haml b/app/views/discussions/_discussion.html.haml
index 8440fb3d78528c1af727f80fa7696c7520d0b471..38e85168f40500484c771c3f822b3f8d77a449a7 100644
--- a/app/views/discussions/_discussion.html.haml
+++ b/app/views/discussions/_discussion.html.haml
@@ -20,7 +20,7 @@
             = discussion.author.to_reference
             started a discussion
 
-            - url = discussion_diff_path(discussion)
+            - url = discussion_path(discussion)
             - if discussion.for_commit? && @noteable != discussion.noteable
               on
               - commit = discussion.noteable
diff --git a/app/views/projects/diffs/_line.html.haml b/app/views/projects/diffs/_line.html.haml
index 3e426ee9e7d380cbf2f6966aa1be2f582941fa1d..7439b8a66f7e49600bc1c9316f6fe53fcbef1832 100644
--- a/app/views/projects/diffs/_line.html.haml
+++ b/app/views/projects/diffs/_line.html.haml
@@ -35,6 +35,6 @@
       - else
         = diff_line_content(line.text)
 
-- if line_discussions
+- if line_discussions&.any?
   - discussion_expanded = local_assigns.fetch(:discussion_expanded, line_discussions.any?(&:expanded?))
   = render "discussions/diff_discussion", discussions: line_discussions, expanded: discussion_expanded
diff --git a/app/views/projects/merge_requests/show/_versions.html.haml b/app/views/projects/merge_requests/show/_versions.html.haml
index 547be78992e988ef013365ebaff3ccfd097803b5..11b0c55be0bb800b8e239ebb4b5cf292f1703431 100644
--- a/app/views/projects/merge_requests/show/_versions.html.haml
+++ b/app/views/projects/merge_requests/show/_versions.html.haml
@@ -35,7 +35,7 @@
         %span.dropdown.inline.mr-version-compare-dropdown
           %a.btn.btn-default.dropdown-toggle{ data: {toggle: :dropdown} }
             %span
-              - if @start_sha
+              - if @start_version
                 version #{version_index(@start_version)}
               - else
                 #{@merge_request.target_branch}
@@ -59,7 +59,7 @@
                       %small
                         = time_ago_with_tooltip(merge_request_diff.created_at)
                 %li
-                  = link_to merge_request_version_path(@project, @merge_request, @merge_request_diff), class: ('is-active' unless @start_sha) do
+                  = link_to merge_request_version_path(@project, @merge_request, @merge_request_diff), class: ('is-active' unless @start_version) do
                     %strong
                       #{@merge_request.target_branch} (base)
                     .monospace= short_sha(@merge_request_diff.base_commit_sha)
@@ -75,13 +75,15 @@
         = succeed '.' do
           %code= @merge_request.target_branch
 
-    - if @diff_notes_disabled
+    - if @start_version || !@merge_request_diff.latest?
       .comments-disabled-notif.content-block
         = icon('info-circle')
-        - if @start_sha
-          Comments are disabled because you're comparing two versions of this merge request.
+        Not all comments are displayed because you're
+        - if @start_version
+          comparing two versions
         - else
-          Discussions on this version of the merge request are displayed but comment creation is disabled.
+          viewing an old version
+        of this merge request.
 
         .pull-right
           = link_to 'Show latest version', diffs_namespace_project_merge_request_path(@project.namespace, @project, @merge_request), class: 'btn btn-sm'
diff --git a/changelogs/unreleased/dm-comment-on-diff-versions.yml b/changelogs/unreleased/dm-comment-on-diff-versions.yml
new file mode 100644
index 0000000000000000000000000000000000000000..af299713ad3d76b5c38dcbed9485601e2f1e0a1d
--- /dev/null
+++ b/changelogs/unreleased/dm-comment-on-diff-versions.yml
@@ -0,0 +1,4 @@
+---
+title: Allow commenting on older versions of the diff and comparisons between diff versions
+merge_request:
+author:
diff --git a/spec/features/merge_requests/versions_spec.rb b/spec/features/merge_requests/versions_spec.rb
index 7a2da623c58b19c29b3195ef4d82144960fc47f4..2b5b803946c960df59a2c3112bfe75c1c8ee9c83 100644
--- a/spec/features/merge_requests/versions_spec.rb
+++ b/spec/features/merge_requests/versions_spec.rb
@@ -24,7 +24,12 @@ feature 'Merge Request versions', js: true, feature: true do
     before do
       page.within '.mr-version-dropdown' do
         find('.btn-default').click
-        find(:link, 'version 1').trigger('click')
+        click_link 'version 1'
+      end
+
+      # Wait for the page to load
+      page.within '.mr-version-dropdown' do
+        expect(page).to have_content 'version 1'
       end
     end
 
@@ -36,8 +41,8 @@ feature 'Merge Request versions', js: true, feature: true do
       expect(page).to have_content '5 changed files'
     end
 
-    it 'show the message about disabled comment creation' do
-      expect(page).to have_content 'comment creation is disabled'
+    it 'show the message about comments' do
+      expect(page).to have_content 'Not all comments are displayed'
     end
 
     it 'shows comments that were last relevant at that version' do
@@ -52,15 +57,41 @@ feature 'Merge Request versions', js: true, feature: true do
       outdated_diff_note.position = outdated_diff_note.original_position
       outdated_diff_note.save!
 
+      visit current_url
+
       expect(page).to have_css(".diffs .notes[data-discussion-id='#{outdated_diff_note.discussion_id}']")
     end
+
+    it 'allows commenting' do
+      diff_file_selector = ".diff-file[id='7445606fbf8f3683cd42bdc54b05d7a0bc2dfc44']"
+      line_code = '7445606fbf8f3683cd42bdc54b05d7a0bc2dfc44_2_2'
+
+      page.within(diff_file_selector) do
+        find(".line_holder[id='#{line_code}'] td:nth-of-type(1)").trigger 'mouseover'
+        find(".line_holder[id='#{line_code}'] button").trigger 'click'
+
+        page.within("form[data-line-code='#{line_code}']") do
+          fill_in "note[note]", with: "Typo, please fix"
+          find(".js-comment-button").click
+        end
+
+        wait_for_ajax
+
+        expect(page).to have_content("Typo, please fix")
+      end
+    end
   end
 
   describe 'compare with older version' do
     before do
       page.within '.mr-version-compare-dropdown' do
         find('.btn-default').click
-        find(:link, 'version 1').trigger('click')
+        click_link 'version 1'
+      end
+
+      # Wait for the page to load
+      page.within '.mr-version-compare-dropdown' do
+        expect(page).to have_content 'version 1'
       end
     end
 
@@ -80,8 +111,43 @@ feature 'Merge Request versions', js: true, feature: true do
       end
     end
 
-    it 'show the message about disabled comments' do
-      expect(page).to have_content 'Comments are disabled'
+    it 'show the message about comments' do
+      expect(page).to have_content 'Not all comments are displayed'
+    end
+
+    it 'shows comments that were last relevant at that version' do
+      position = Gitlab::Diff::Position.new(
+        old_path: ".gitmodules",
+        new_path: ".gitmodules",
+        old_line: 4,
+        new_line: 4,
+        diff_refs: merge_request_diff3.compare_with(merge_request_diff1.head_commit_sha).diff_refs
+      )
+      outdated_diff_note = create(:diff_note_on_merge_request, project: project, noteable: merge_request, position: position)
+
+      visit current_url
+      wait_for_ajax
+
+      expect(page).to have_css(".diffs .notes[data-discussion-id='#{outdated_diff_note.discussion_id}']")
+    end
+
+    it 'allows commenting' do
+      diff_file_selector = ".diff-file[id='7445606fbf8f3683cd42bdc54b05d7a0bc2dfc44']"
+      line_code = '7445606fbf8f3683cd42bdc54b05d7a0bc2dfc44_4_4'
+
+      page.within(diff_file_selector) do
+        find(".line_holder[id='#{line_code}'] td:nth-of-type(1)").trigger 'mouseover'
+        find(".line_holder[id='#{line_code}'] button").trigger 'click'
+
+        page.within("form[data-line-code='#{line_code}']") do
+          fill_in "note[note]", with: "Typo, please fix"
+          find(".js-comment-button").click
+        end
+
+        wait_for_ajax
+
+        expect(page).to have_content("Typo, please fix")
+      end
     end
 
     it 'show diff between new and old version' do
diff --git a/spec/helpers/notes_helper_spec.rb b/spec/helpers/notes_helper_spec.rb
index a427de32c4ccefff7476110e650e0b8f4d0035d8..6c990f9417508fcb5bc5cdbfb1352a9ec2b00bf7 100644
--- a/spec/helpers/notes_helper_spec.rb
+++ b/spec/helpers/notes_helper_spec.rb
@@ -1,6 +1,8 @@
 require "spec_helper"
 
 describe NotesHelper do
+  include RepoHelpers
+
   let(:owner) { create(:owner) }
   let(:group) { create(:group) }
   let(:project) { create(:empty_project, namespace: group) }
@@ -36,4 +38,141 @@ describe NotesHelper do
       expect(helper.note_max_access_for_user(other_note)).to eq('Reporter')
     end
   end
+
+  describe '#discussion_path' do
+    let(:project) { create(:project) }
+
+    context 'for a merge request discusion' do
+      let(:merge_request) { create(:merge_request, source_project: project, target_project: project, importing: true) }
+      let!(:merge_request_diff1) { merge_request.merge_request_diffs.create(head_commit_sha: '6f6d7e7ed97bb5f0054f2b1df789b39ca89b6ff9') }
+      let!(:merge_request_diff2) { merge_request.merge_request_diffs.create(head_commit_sha: nil) }
+      let!(:merge_request_diff3) { merge_request.merge_request_diffs.create(head_commit_sha: '5937ac0a7beb003549fc5fd26fc247adbce4a52e') }
+
+      context 'for a diff discussion' do
+        context 'when the discussion is active' do
+          let(:discussion) { create(:diff_note_on_merge_request, noteable: merge_request, project: project).to_discussion }
+
+          it 'returns the diff path with the line code' do
+            expect(helper.discussion_path(discussion)).to eq(diffs_namespace_project_merge_request_path(project.namespace, project, merge_request, anchor: discussion.line_code))
+          end
+        end
+
+        context 'when the discussion is on an older merge request version' do
+          let(:position) do
+            Gitlab::Diff::Position.new(
+              old_path: ".gitmodules",
+              new_path: ".gitmodules",
+              old_line: nil,
+              new_line: 4,
+              diff_refs: merge_request_diff1.diff_refs
+            )
+          end
+
+          let(:diff_note) { create(:diff_note_on_merge_request, noteable: merge_request, project: project, position: position) }
+          let(:discussion) { diff_note.to_discussion }
+
+          before do
+            diff_note.position = diff_note.original_position
+            diff_note.save!
+          end
+
+          it 'returns the diff version path with the line code' do
+            expect(helper.discussion_path(discussion)).to eq(diffs_namespace_project_merge_request_path(project.namespace, project, merge_request, diff_id: merge_request_diff1, anchor: discussion.line_code))
+          end
+        end
+
+        context 'when the discussion is on a comparison between merge request versions' do
+          let(:position) do
+            Gitlab::Diff::Position.new(
+              old_path: ".gitmodules",
+              new_path: ".gitmodules",
+              old_line: 4,
+              new_line: 4,
+              diff_refs: merge_request_diff3.compare_with(merge_request_diff1.head_commit_sha).diff_refs
+            )
+          end
+
+          let(:discussion) { create(:diff_note_on_merge_request, noteable: merge_request, project: project, position: position).to_discussion }
+
+          it 'returns the diff version comparison path with the line code' do
+            expect(helper.discussion_path(discussion)).to eq(diffs_namespace_project_merge_request_path(project.namespace, project, merge_request, diff_id: merge_request_diff3, start_sha: merge_request_diff1.head_commit_sha, anchor: discussion.line_code))
+          end
+        end
+
+        context 'when the discussion does not have a merge request version' do
+          let(:outdated_diff_note) { create(:diff_note_on_merge_request, noteable: merge_request, project: project, diff_refs: project.commit(sample_commit.id).diff_refs) }
+          let(:discussion) { outdated_diff_note.to_discussion }
+
+          before do
+            outdated_diff_note.position = outdated_diff_note.original_position
+            outdated_diff_note.save!
+          end
+
+          it 'returns nil' do
+            expect(helper.discussion_path(discussion)).to be_nil
+          end
+        end
+      end
+
+      context 'for a legacy diff discussion' do
+        let(:discussion) { create(:legacy_diff_note_on_merge_request, noteable: merge_request, project: project).to_discussion }
+
+        context 'when the discussion is active' do
+          before do
+            allow(discussion).to receive(:active?).and_return(true)
+          end
+
+          it 'returns the diff path with the line code' do
+            expect(helper.discussion_path(discussion)).to eq(diffs_namespace_project_merge_request_path(project.namespace, project, merge_request, anchor: discussion.line_code))
+          end
+        end
+
+        context 'when the discussion is outdated' do
+          before do
+            allow(discussion).to receive(:active?).and_return(false)
+          end
+
+          it 'returns nil' do
+            expect(helper.discussion_path(discussion)).to be_nil
+          end
+        end
+      end
+
+      context 'for a non-diff discussion' do
+        let(:discussion) { create(:discussion_note_on_merge_request, noteable: merge_request, project: project).to_discussion }
+
+        it 'returns nil' do
+          expect(helper.discussion_path(discussion)).to be_nil
+        end
+      end
+    end
+
+    context 'for a commit discussion' do
+      let(:commit) { discussion.noteable }
+
+      context 'for a diff discussion' do
+        let(:discussion) { create(:diff_note_on_commit, project: project).to_discussion }
+
+        it 'returns the commit path with the line code' do
+          expect(helper.discussion_path(discussion)).to eq(namespace_project_commit_path(project.namespace, project, commit, anchor: discussion.line_code))
+        end
+      end
+
+      context 'for a legacy diff discussion' do
+        let(:discussion) { create(:legacy_diff_note_on_commit, project: project).to_discussion }
+
+        it 'returns the commit path with the line code' do
+          expect(helper.discussion_path(discussion)).to eq(namespace_project_commit_path(project.namespace, project, commit, anchor: discussion.line_code))
+        end
+      end
+
+      context 'for a non-diff discussion' do
+        let(:discussion) { create(:discussion_note_on_commit, project: project).to_discussion }
+
+        it 'returns the commit path' do
+          expect(helper.discussion_path(discussion)).to eq(namespace_project_commit_path(project.namespace, project, commit))
+        end
+      end
+    end
+  end
 end
diff --git a/spec/models/diff_discussion_spec.rb b/spec/models/diff_discussion_spec.rb
index 48e7c0a822c66b562cdd9d9630da4e0bd7387330..81f338745b1943cd640ebda0fb9eeef4c18426dd 100644
--- a/spec/models/diff_discussion_spec.rb
+++ b/spec/models/diff_discussion_spec.rb
@@ -1,19 +1,86 @@
 require 'spec_helper'
 
 describe DiffDiscussion, model: true do
-  subject { described_class.new([first_note, second_note, third_note]) }
+  include RepoHelpers
 
-  let(:first_note) { create(:diff_note_on_merge_request) }
-  let(:merge_request) { first_note.noteable }
-  let(:project) { first_note.project }
-  let(:second_note) { create(:diff_note_on_merge_request, noteable: merge_request, project: project, in_reply_to: first_note) }
-  let(:third_note) { create(:diff_note_on_merge_request, noteable: merge_request, project: project, in_reply_to: first_note) }
+  subject { described_class.new([diff_note]) }
+
+  let(:project) { create(:project) }
+  let(:merge_request) { create(:merge_request, source_project: project, target_project: project) }
+  let(:diff_note) { create(:diff_note_on_merge_request, noteable: merge_request, project: project) }
 
   describe '#reply_attributes' do
     it 'includes position and original_position' do
       attributes = subject.reply_attributes
-      expect(attributes[:position]).to eq(first_note.position.to_json)
-      expect(attributes[:original_position]).to eq(first_note.original_position.to_json)
+      expect(attributes[:position]).to eq(diff_note.position.to_json)
+      expect(attributes[:original_position]).to eq(diff_note.original_position.to_json)
+    end
+  end
+
+  describe '#merge_request_version_params' do
+    let(:merge_request) { create(:merge_request, source_project: project, target_project: project, importing: true) }
+    let!(:merge_request_diff1) { merge_request.merge_request_diffs.create(head_commit_sha: '6f6d7e7ed97bb5f0054f2b1df789b39ca89b6ff9') }
+    let!(:merge_request_diff2) { merge_request.merge_request_diffs.create(head_commit_sha: nil) }
+    let!(:merge_request_diff3) { merge_request.merge_request_diffs.create(head_commit_sha: '5937ac0a7beb003549fc5fd26fc247adbce4a52e') }
+
+    context 'when the discussion is active' do
+      it 'returns an empty hash, which will end up showing the latest version' do
+        expect(subject.merge_request_version_params).to eq({})
+      end
+    end
+
+    context 'when the discussion is on an older merge request version' do
+      let(:position) do
+        Gitlab::Diff::Position.new(
+          old_path: ".gitmodules",
+          new_path: ".gitmodules",
+          old_line: nil,
+          new_line: 4,
+          diff_refs: merge_request_diff1.diff_refs
+        )
+      end
+
+      let(:diff_note) { create(:diff_note_on_merge_request, noteable: merge_request, project: project, position: position) }
+
+      before do
+        diff_note.position = diff_note.original_position
+        diff_note.save!
+      end
+
+      it 'returns the diff ID for the version to show' do
+        expect(diff_id: merge_request_diff1.id)
+      end
+    end
+
+    context 'when the discussion is on a comparison between merge request versions' do
+      let(:position) do
+        Gitlab::Diff::Position.new(
+          old_path: ".gitmodules",
+          new_path: ".gitmodules",
+          old_line: 4,
+          new_line: 4,
+          diff_refs: merge_request_diff3.compare_with(merge_request_diff1.head_commit_sha).diff_refs
+        )
+      end
+
+      let(:diff_note) { create(:diff_note_on_merge_request, noteable: merge_request, project: project, position: position) }
+
+      it 'returns the diff ID and start sha of the versions to compare' do
+        expect(subject.merge_request_version_params).to eq(diff_id: merge_request_diff3.id, start_sha: merge_request_diff1.head_commit_sha)
+      end
+    end
+
+    context 'when the discussion does not have a merge request version' do
+      let(:diff_note) { create(:diff_note_on_merge_request, noteable: merge_request, project: project, diff_refs: project.commit(sample_commit.id).diff_refs) }
+
+      before do
+        diff_note.position = diff_note.original_position
+        diff_note.save!
+      end
+
+      it 'returns nil' do
+        expect(subject.merge_request_version_params).to be_nil
+      end
     end
   end
 end
diff --git a/spec/models/diff_note_spec.rb b/spec/models/diff_note_spec.rb
index f32b6b99b3d6be64cb4884324e0b4840d4b54ef1..ab4c51a87b0eea4a8c21a5a7d0eb71c20e0d41c2 100644
--- a/spec/models/diff_note_spec.rb
+++ b/spec/models/diff_note_spec.rb
@@ -155,23 +155,6 @@ describe DiffNote, models: true do
     end
   end
 
-  describe '#latest_merge_request_diff' do
-    context 'when active' do
-      it 'returns the current merge request diff' do
-        expect(subject.latest_merge_request_diff).to eq(merge_request.merge_request_diff)
-      end
-    end
-
-    context 'when outdated' do
-      let!(:old_merge_request_diff) { merge_request.merge_request_diff }
-      let!(:new_merge_request_diff) { merge_request.merge_request_diffs.create(diff_refs: commit.diff_refs) }
-
-      it 'returns the latest merge request diff that this diff note applied to' do
-        expect(subject.latest_merge_request_diff).to eq(old_merge_request_diff)
-      end
-    end
-  end
-
   describe "creation" do
     describe "updating of position" do
       context "when noteable is a commit" do
@@ -256,4 +239,39 @@ describe DiffNote, models: true do
       end
     end
   end
+
+  describe '#created_at_diff?' do
+    let(:diff_refs) { project.commit(sample_commit.id).diff_refs }
+    let(:position) do
+      Gitlab::Diff::Position.new(
+        old_path: "files/ruby/popen.rb",
+        new_path: "files/ruby/popen.rb",
+        old_line: nil,
+        new_line: 14,
+        diff_refs: diff_refs
+      )
+    end
+
+    context "when noteable is a commit" do
+      subject { build(:diff_note_on_commit, project: project, position: position) }
+
+      it "returns true" do
+        expect(subject.created_at_diff?(diff_refs)).to be true
+      end
+    end
+
+    context "when noteable is a merge request" do
+      context "when the diff refs match the original one of the diff note" do
+        it "returns true" do
+          expect(subject.created_at_diff?(diff_refs)).to be true
+        end
+      end
+
+      context "when the diff refs don't match the original one of the diff note" do
+        it "returns false" do
+          expect(subject.created_at_diff?(merge_request.diff_refs)).to be false
+        end
+      end
+    end
+  end
 end
diff --git a/spec/models/legacy_diff_discussion_spec.rb b/spec/models/legacy_diff_discussion_spec.rb
index 153e757a0efae94805e917b8ae41a9c052a7a769..6eb4a2aaf399adac5368058524cc747d15429d13 100644
--- a/spec/models/legacy_diff_discussion_spec.rb
+++ b/spec/models/legacy_diff_discussion_spec.rb
@@ -8,4 +8,26 @@ describe LegacyDiffDiscussion, models: true do
       expect(subject.reply_attributes[:line_code]).to eq(subject.line_code)
     end
   end
+
+  describe '#merge_request_version_params' do
+    context 'when the discussion is active' do
+      before do
+        allow(subject).to receive(:active?).and_return(true)
+      end
+
+      it 'returns an empty hash, which will end up showing the latest version' do
+        expect(subject.merge_request_version_params).to eq({})
+      end
+    end
+
+    context 'when the discussion is outdated' do
+      before do
+        allow(subject).to receive(:active?).and_return(false)
+      end
+
+      it 'returns nil' do
+        expect(subject.merge_request_version_params).to be_nil
+      end
+    end
+  end
 end
diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index be08b96641a6df3a69d583bdc51e70b5225cb366..8b72125dd5d1cf39a4c201fc434451a56c9f5dd2 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -1554,4 +1554,23 @@ describe MergeRequest, models: true do
       expect(subject.has_no_commits?).to be_truthy
     end
   end
+
+  describe '#merge_request_diff_for' do
+    subject { create(:merge_request, importing: true) }
+    let!(:merge_request_diff1) { subject.merge_request_diffs.create(head_commit_sha: '6f6d7e7ed97bb5f0054f2b1df789b39ca89b6ff9') }
+    let!(:merge_request_diff2) { subject.merge_request_diffs.create(head_commit_sha: nil) }
+    let!(:merge_request_diff3) { subject.merge_request_diffs.create(head_commit_sha: '5937ac0a7beb003549fc5fd26fc247adbce4a52e') }
+
+    context 'with diff refs' do
+      it 'returns the diffs' do
+        expect(subject.merge_request_diff_for(merge_request_diff1.diff_refs)).to eq(merge_request_diff1)
+      end
+    end
+
+    context 'with a commit SHA' do
+      it 'returns the diffs' do
+        expect(subject.merge_request_diff_for(merge_request_diff3.head_commit_sha)).to eq(merge_request_diff3)
+      end
+    end
+  end
 end
diff --git a/spec/models/note_spec.rb b/spec/models/note_spec.rb
index 557ea97b008891a76ebf0832a8e9e3a6d37aba3f..7a01cef9b4b1f0bb1bc881799e87bb238b4539e6 100644
--- a/spec/models/note_spec.rb
+++ b/spec/models/note_spec.rb
@@ -272,9 +272,9 @@ describe Note, models: true do
       Gitlab::Diff::Position.new(
         old_path: "files/ruby/popen.rb",
         new_path: "files/ruby/popen.rb",
-        old_line: 16,
-        new_line: 22,
-        diff_refs: merge_request.diff_refs
+        old_line: nil,
+        new_line: 13,
+        diff_refs: project.commit(sample_commit.id).diff_refs
       )
     end
 
@@ -288,26 +288,78 @@ describe Note, models: true do
       )
     end
 
-    subject { merge_request.notes.grouped_diff_discussions }
+    context 'active diff discussions' do
+      subject { merge_request.notes.grouped_diff_discussions }
 
-    it "includes active discussions" do
-      discussions = subject.values.flatten
+      it "includes active discussions" do
+        discussions = subject.values.flatten
 
-      expect(discussions.count).to eq(2)
-      expect(discussions.map(&:id)).to eq([active_diff_note1.discussion_id, active_diff_note3.discussion_id])
-      expect(discussions.all?(&:active?)).to be true
+        expect(discussions.count).to eq(2)
+        expect(discussions.map(&:id)).to eq([active_diff_note1.discussion_id, active_diff_note3.discussion_id])
+        expect(discussions.all?(&:active?)).to be true
 
-      expect(discussions.first.notes).to eq([active_diff_note1, active_diff_note2])
-      expect(discussions.last.notes).to eq([active_diff_note3])
-    end
+        expect(discussions.first.notes).to eq([active_diff_note1, active_diff_note2])
+        expect(discussions.last.notes).to eq([active_diff_note3])
+      end
 
-    it "doesn't include outdated discussions" do
-      expect(subject.values.flatten.map(&:id)).not_to include(outdated_diff_note1.discussion_id)
+      it "doesn't include outdated discussions" do
+        expect(subject.values.flatten.map(&:id)).not_to include(outdated_diff_note1.discussion_id)
+      end
+
+      it "groups the discussions by line code" do
+        expect(subject[active_diff_note1.line_code].first.id).to eq(active_diff_note1.discussion_id)
+        expect(subject[active_diff_note3.line_code].first.id).to eq(active_diff_note3.discussion_id)
+      end
     end
 
-    it "groups the discussions by line code" do
-      expect(subject[active_diff_note1.line_code].first.id).to eq(active_diff_note1.discussion_id)
-      expect(subject[active_diff_note3.line_code].first.id).to eq(active_diff_note3.discussion_id)
+    context 'diff discussions for older diff refs' do
+      subject { merge_request.notes.grouped_diff_discussions(diff_refs) }
+
+      context 'for diff refs a discussion was created at' do
+        let(:diff_refs) { active_position2.diff_refs }
+
+        it "includes discussions that were created then" do
+          discussions = subject.values.flatten
+
+          expect(discussions.count).to eq(1)
+
+          discussion = discussions.first
+
+          expect(discussion.id).to eq(active_diff_note3.discussion_id)
+          expect(discussion.active?).to be true
+          expect(discussion.active?(diff_refs)).to be false
+          expect(discussion.created_at_diff?(diff_refs)).to be true
+
+          expect(discussion.notes).to eq([active_diff_note3])
+        end
+
+        it "groups the discussions by original line code" do
+          expect(subject[active_diff_note3.original_line_code].first.id).to eq(active_diff_note3.discussion_id)
+        end
+      end
+
+      context 'for diff refs a discussion was last active at' do
+        let(:diff_refs) { outdated_position.diff_refs }
+
+        it "includes discussions that were last active" do
+          discussions = subject.values.flatten
+
+          expect(discussions.count).to eq(1)
+
+          discussion = discussions.first
+
+          expect(discussion.id).to eq(outdated_diff_note1.discussion_id)
+          expect(discussion.active?).to be false
+          expect(discussion.active?(diff_refs)).to be true
+          expect(discussion.created_at_diff?(diff_refs)).to be true
+
+          expect(discussion.notes).to eq([outdated_diff_note1, outdated_diff_note2])
+        end
+
+        it "groups the discussions by line code" do
+          expect(subject[outdated_diff_note1.line_code].first.id).to eq(outdated_diff_note1.discussion_id)
+        end
+      end
     end
   end