Adds a great refactoring idea and clarified specs

app/serializers/diffs_entity.rb

@@ -80,20 +80,19 @@ class DiffsEntity < Grape::Entity
   private
 
   def commit_ids
-    strong_memoize(:commit_ids) do
-      [nil] + merge_request.commits.collect(&:id) + [nil]
-    end
+    @commit_ids ||= merge_request.commits.collect(&:id)
   end
 
-  def commit_with_neighbors(id)
-    commits = commit_ids.each_cons(3).find { |prev_commit, commit, next_commit| commit == id }
-    { prev_commit_id: commits.first, next_commit_id: commits.last } if commits
+  def commit_neighbors(commit_id)
+    index = commit_ids.index(commit_id)
+
+    return [] unless index
+
+    [(index > 0 ? commit_ids[index - 1] : nil), commit_ids[index + 1]]
   end
 
   def commit_options(options)
-    neighbors = commit_with_neighbors(options[:commit]&.id)
-    prev_commit_id = neighbors&.dig(:prev_commit_id)
-    next_commit_id = neighbors&.dig(:next_commit_id)
+    prev_commit_id, next_commit_id = *commit_neighbors(options[:commit]&.id)
 
     options.merge(
       type: :full,

spec/serializers/diffs_entity_spec.rb

@@ -29,21 +29,50 @@ describe DiffsEntity do
 
     context "when a commit_id is passed" do
       let(:commits) { [nil] + merge_request.commits + [nil] }
-      let(:commit) { commits.compact.sample }
       let(:entity) do
-        described_class.new(merge_request_diffs.first.diffs, request: request, merge_request: merge_request, merge_request_diffs: merge_request_diffs, commit: commit)
+        described_class.new(
+          merge_request_diffs.first.diffs,
+          request: request,
+          merge_request: merge_request,
+          merge_request_diffs: merge_request_diffs,
+          commit: @commit
+        )
       end
 
+      subject { entity.as_json }
+
+      context "when the passed commit is not the first or last in the group" do
         it 'includes commit references for previous and next' do
-        expect(subject[:commit]).to include(:prev_commit_id, :next_commit_id)
+          @commit = commits.third
+          check_neighbor_commits(commits, @commit)
+        end
+      end
+
+      context "when the passed commit is the first in the group" do
+        it 'includes commit references for nil and next' do
+          @commit = commits.compact.last
+          check_neighbor_commits(commits, @commit)
+        end
+      end
 
+      context "when the passed commit is the last in the group" do
+        it 'includes commit references for previous and nil' do
+          @commit = commits.compact.first
+          check_neighbor_commits(commits, @commit)
+        end
+      end
+    end
+  end
+
+  private
+
+  def check_neighbor_commits(commits, commit)
     index = commits.index(commit)
     prev_commit = commits[index - 1]&.id
     next_commit = commits[index + 1]&.id
 
+    expect(subject[:commit]).to include(:prev_commit_id, :next_commit_id)
     expect(subject[:commit][:prev_commit_id]).to eq(prev_commit)
     expect(subject[:commit][:next_commit_id]).to eq(next_commit)
   end
-    end
-  end
 end