CSV generated via Tempfile

app/workers/export_csv_worker.rb

@@ -10,6 +10,6 @@ class ExportCsvWorker
 
     issues = IssuesFinder.new(@current_user, params.symbolize_keys).execute
 
-    Issues::ExportCsvService.new(issues.limit(100)).email(@current_user, @project)
+    Issues::ExportCsvService.new(issues).email(@current_user, @project)
   end
 end

lib/csv_builder.rb

@@ -26,7 +26,7 @@ class CsvBuilder
 
   # Renders the csv to a string
   def render
-    CSV.generate do |csv|
+    generate_csv_with_tempfile do |csv|
       csv << headers
 
       @collection.find_each do |object|
@@ -38,11 +38,11 @@ class CsvBuilder
   private
 
   def headers
-    @header_to_value_hash.keys
+    @headers ||= @header_to_value_hash.keys
   end
 
   def attributes
-    @header_to_value_hash.values
+    @attributes ||= @header_to_value_hash.values
   end
 
   def row(object)
@@ -54,4 +54,17 @@ class CsvBuilder
       end
     end
   end
+
+  def generate_csv_with_tempfile
+    tempfile = Tempfile.new('issues_csv')
+    csv = CSV.new(tempfile)
+
+    yield(csv)
+
+    tempfile.rewind
+    tempfile.read
+  ensure
+    tempfile.close
+    tempfile.unlink
+  end
 end

spec/lib/csv_builder_spec.rb

@@ -14,6 +14,18 @@ describe CsvBuilder, lib: true do
     expect(csv_data.scan(/(,|\n)/).join).to include ",\n,"
   end
 
+  it 'uses a temporary file to reduce memory allocation' do
+    expect(CSV).to receive(:new).with(instance_of(Tempfile)).and_call_original
+
+    subject.render
+  end
+
+  it 'avoids loading all data in a single query' do
+    expect(fake_relation).to receive(:find_each)
+
+    subject.render
+  end
+
   it 'uses hash keys as headers' do
     expect(csv_data).to start_with 'Q & A'
   end

spec/services/issues/export_csv_service_spec.rb

@@ -5,7 +5,6 @@ describe Issues::ExportCsvService, services: true do
   let(:project) { create(:empty_project, :public) }
   let!(:issue)  { create(:issue, project: project, author: user) }
   let(:subject) { described_class.new(Issue.all) }
-  let(:email) { subject.email(user, project) }
 
   it 'renders csv to string' do
     expect(subject.render).to be_a String
@@ -13,15 +12,12 @@ describe Issues::ExportCsvService, services: true do
 
   describe '#email' do
     it 'emails csv' do
-      expect{ email }.to change(ActionMailer::Base.deliveries, :count)
+      expect{ subject.email(user, project) }.to change(ActionMailer::Base.deliveries, :count)
     end
   end
 
-  it 'renders csv to temporary file'
-
   def csv
-    attachment = email.attachments[0]
-    CSV.parse(attachment.decode_body, headers: true)
+    CSV.parse(subject.render, headers: true)
   end
 
   context 'includes' do