Added EachBatch for iterating tables in batchesactive-record-each-batch

This module provides a class method called `each_batch` that can be used
to iterate tables in batches in a more efficient way compared to Rails'
`in_batches` method. This commit also includes a RuboCop cop to
blacklist the use of `in_batches` in favour of this new method.

2 files changed, 72 insertions, 0 deletions

diff --git a/spec/models/concerns/each_batch_spec.rb b/spec/models/concerns/each_batch_spec.rb
new file mode 100644
index 00000000000..951690a217b
--- /dev/null
+++ b/spec/models/concerns/each_batch_spec.rb
@@ -0,0 +1,53 @@
+require 'spec_helper'
+
+describe EachBatch do
+  describe '.each_batch' do
+    let(:model) do
+      Class.new(ActiveRecord::Base) do
+        include EachBatch
+
+        self.table_name = 'users'
+      end
+    end
+
+    before do
+      5.times { create(:user, updated_at: 1.day.ago) }
+    end
+
+    it 'yields an ActiveRecord::Relation when a block is given' do
+      model.each_batch do |relation|
+        expect(relation).to be_a_kind_of(ActiveRecord::Relation)
+      end
+    end
+
+    it 'yields a batch index as the second argument' do
+      model.each_batch do |_, index|
+        expect(index).to eq(1)
+      end
+    end
+
+    it 'accepts a custom batch size' do
+      amount = 0
+
+      model.each_batch(of: 1) { amount += 1 }
+
+      expect(amount).to eq(5)
+    end
+
+    it 'does not include ORDER BYs in the yielded relations' do
+      model.each_batch do |relation|
+        expect(relation.to_sql).not_to include('ORDER BY')
+      end
+    end
+
+    it 'allows updating of the yielded relations' do
+      time = Time.now
+
+      model.each_batch do |relation|
+        relation.update_all(updated_at: time)
+      end
+
+      expect(model.where(updated_at: time).count).to eq(5)
+    end
+  end
+end
diff --git a/spec/rubocop/cop/in_batches_spec.rb b/spec/rubocop/cop/in_batches_spec.rb
new file mode 100644
index 00000000000..072481984c6
--- /dev/null
+++ b/spec/rubocop/cop/in_batches_spec.rb
@@ -0,0 +1,19 @@
+require 'spec_helper'
+require 'rubocop'
+require 'rubocop/rspec/support'
+require_relative '../../../rubocop/cop/in_batches'
+
+describe RuboCop::Cop::InBatches do
+  include CopHelper
+
+  subject(:cop) { described_class.new }
+
+  it 'registers an offense when in_batches is used' do
+    inspect_source(cop, 'foo.in_batches do; end')
+
+    aggregate_failures do
+      expect(cop.offenses.size).to eq(1)
+      expect(cop.offenses.map(&:line)).to eq([1])
+    end
+  end
+end