Extracted CSV generation logic into CsvBuilder

app/views/projects/issues/index.csv.ruby

@@ -13,18 +13,4 @@ columns = {
   'Labels' => -> (issue) { issue.label_names.join(',').presence },
 }
 
-CSV.generate do |csv|
-  csv << columns.keys
-
-  @issues.each do |issue|
-    row = columns.values.map do |attribute|
-      if attribute.respond_to?(:call)
-        attribute.call(issue)
-      else
-        issue.send(attribute)
-      end
-    end
-
-    csv << row
-  end
-end
+CsvBuilder.new(columns).render(@issues)

lib/csv_builder.rb

+class CsvBuilder
+  def initialize(header_to_value_hash)
+    @header_to_value_hash = header_to_value_hash
+  end
+
+  def render(collection)
+    CSV.generate do |csv|
+      csv << headers
+
+      collection.each do |object|
+        csv << row(object)
+      end
+    end
+  end
+
+  private
+
+  def headers
+    @header_to_value_hash.keys
+  end
+
+  def attributes
+    @header_to_value_hash.values
+  end
+
+  def row(object)
+    attributes.map do |attribute|
+      if attribute.respond_to?(:call)
+        attribute.call(object)
+      else
+        object.send(attribute)
+      end
+    end
+  end
+end

spec/lib/csv_builder_spec.rb

+require 'spec_helper'
+
+describe CsvBuilder, lib: true do
+  let(:object) { double(question: :answer) }
+  let(:subject) { CsvBuilder.new('Q & A' => :question, 'Reversed' => -> (o) { o.question.to_s.reverse }) }
+  let(:csv_data) { subject.render([object]) }
+
+  it 'generates a csv' do
+    expect(csv_data.scan(/(,|\n)/).join).to include ",\n,"
+  end
+
+  it 'uses hash keys as headers' do
+    expect(csv_data).to start_with 'Q & A'
+  end
+
+  it 'gets data by calling method provided as hash value' do
+    expect(csv_data).to include 'answer'
+  end
+
+  it 'allows lamdas to look up more complicated data' do
+    expect(csv_data).to include 'rewsna'
+  end
+end