Fixes #71 via configuration

* parallel-updates-bug:
  Describe the race condition context [skip ci]
  Fix #71 via configuration
  Make Race spec self-contained
  Add failing spec

Author: vjt

README.md

@@ -177,6 +177,19 @@ This is visible in `psql` if you issue a `\d+`. Example after a test run:
      public | test_table    | view     | chronomodel | 0 bytes    | {"temporal":true,"journal":["foo"],"chronomodel":"0.7.0.alpha"}
 
 
+**IMPORTANT**: Rails counter cache issues an UPDATE on the parent record
+table, thus triggering new history entries creation. You are **strongly**
+advised to NOT journal the counter cache columns, or race conditions will
+occur (see https://github.com/ifad/chronomodel/issues/71).
+
+In such cases, ensure to add `no_journal: %w( your_counter_cache_column_name )`
+to your `create_table`. Example:
+
+    create_table 'sections', temporal: true, no_journal: %w( articles_count ) do |t|
+      t.string :name
+      t.integer :articles_count, default: 0
+    end
+
 ## Data querying
 
 Include the `ChronoModel::TimeMachine` module in your model.

spec/chrono_model/adapter/counter_cache_race_spec.rb

@@ -0,0 +1,50 @@
+require 'spec_helper'
+require 'support/helpers'
+
+describe 'models with counter cache' do
+  include ChronoTest::Helpers::TimeMachine
+
+    adapter.create_table 'sections', temporal: true, no_journal: %w( articles_count ) do |t|
+      t.string :name
+      t.integer :articles_count, default: 0
+    end
+
+    adapter.create_table 'articles', temporal: true do |t|
+      t.string :title
+      t.references :section
+    end
+
+  class ::Section < ActiveRecord::Base
+    include ChronoModel::TimeMachine
+
+    has_many :articles
+  end
+
+  class ::Article < ActiveRecord::Base
+    include ChronoModel::TimeMachine
+
+    belongs_to :section, counter_cache: true
+  end
+
+  describe 'are not subject to race condition if no_journal is set on the counter cache column' do
+    specify do
+      section = Section.create!
+      expect(section.articles_count).to eq(0)
+
+      Article.create!(section_id: section.id)
+
+      expect(section.reload.articles_count).to eq(1)
+
+      num_threads = 10
+
+      expect do
+        Array.new(num_threads).map do
+          Thread.new do
+            # sleep(rand) # With the sleep statement everything works
+            Article.create!(section_id: section.id)
+          end
+        end.each(&:join)
+      end.to_not raise_error
+    end
+  end
+end

spec/chrono_model/time_machine_spec.rb

@@ -86,10 +86,12 @@
     subject { ChronoModel.history_models }
 
     it { is_expected.to eq(
+      'articles' => Article::History,
       'foos'     => Foo::History,
       'defoos'   => Defoo::History,
       'bars'     => Bar::History,
       'elements' => Element::History,
+      'sections' => Section::History,
       'sub_bars' => SubBar::History,
     ) }
   end

spec/config.travis.yml

@@ -2,3 +2,4 @@ hostname: localhost
 username: postgres
 password: ""
 database: chronomodel
+pool: 11

spec/config.yml.example

@@ -5,4 +5,5 @@ hostname: localhost
 username: chronomodel
 password: chronomodel
 database: chronomodel
+pool: 11
 #