Encapsulate the commit.sha logic

app/models/wiki_page.rb

@@ -138,6 +138,10 @@ class WikiPage
     versions.first
   end
+  def last_commit_sha
+    commit&.sha
+  end
+
   # Returns the Date that this latest version was
   # created on.
   def created_at
@@ -196,7 +200,7 @@ class WikiPage
     @attributes[:content] = new_content
     @attributes[:format] = format
-    if last_commit_sha && last_commit_sha != commit.sha
+    if last_commit_sha && last_commit_sha != self.last_commit_sha
       raise PageChangedError.new("You are attempting to update a page that has changed since you started editing it.")
     end

app/views/projects/wikis/_form.html.haml

@@ -5,7 +5,7 @@
   = f.hidden_field :title, value: @page.title
   - if @page.persisted?
-    = f.hidden_field :last_commit_sha, value: @page.commit.sha
+    = f.hidden_field :last_commit_sha, value: @page.last_commit_sha
   .form-group
     .col-sm-12= f.label :format, class: 'control-label-full-width'
     .col-sm-12

spec/models/wiki_page_spec.rb

@@ -211,15 +211,13 @@ describe WikiPage, models: true do
     context 'with same last commit sha' do
       it 'returns true' do
-        last_commit_sha = @page.commit.sha
-        expect(@page.update('more content', last_commit_sha: last_commit_sha)).to be_truthy
+        expect(@page.update('more content', last_commit_sha: @page.last_commit_sha)).to be_truthy
       end
     end
     context 'with different last commit sha' do
       it 'raises exception' do
-        last_commit_sha = 'xxx'
-        expect { @page.update('more content', last_commit_sha: last_commit_sha) }.to raise_error(WikiPage::PageChangedError)
+        expect { @page.update('more content', last_commit_sha: 'xxx') }.to raise_error(WikiPage::PageChangedError)
       end
     end
   end
@@ -345,6 +343,30 @@ describe WikiPage, models: true do
     end
   end
+  describe '#last_commit_sha' do
+    before do
+      create_page("Update", "content")
+      @page = wiki.find_page("Update")
+    end
+
+    after do
+      destroy_page("Update")
+    end
+
+    it 'returns commit sha' do
+      expect(@page.last_commit_sha).to eq @page.commit.sha
+    end
+
+    it 'is changed after page updated' do
+      last_commit_sha_before_update = @page.last_commit_sha
+
+      @page.update("new content")
+      @page = wiki.find_page("Update")
+
+      expect(@page.last_commit_sha).not_to eq last_commit_sha_before_update
+    end
+  end
+
   private
   def remove_temp_repo(path)