MySQL: Specify utf8mb4 charset for versions table

Fixes #651

lib/generators/paper_trail/templates/create_versions.rb

@@ -1,5 +1,13 @@
 class CreateVersions < ActiveRecord::Migration
 
+  # Class names of MySQL adapters.
+  # - `MysqlAdapter` - Used by gems: `mysql`, `activerecord-jdbcmysql-adapter`.
+  # - `Mysql2Adapter` - Used by `mysql2` gem.
+  MYSQL_ADAPTERS = [
+    "ActiveRecord::ConnectionAdapters::MysqlAdapter",
+    "ActiveRecord::ConnectionAdapters::Mysql2Adapter"
+  ]
+
   # The largest text column available in all supported RDBMS is
   # 1024^3 - 1 bytes, roughly one gibibyte.  We specify a size
   # so that MySQL will use `longtext` instead of `text`.  Otherwise,
@@ -7,7 +15,7 @@ class CreateVersions < ActiveRecord::Migration
   TEXT_BYTES = 1_073_741_823
 
   def change
-    create_table :versions do |t|
+    create_table :versions, versions_table_options do |t|
       t.string   :item_type, :null => false
       t.integer  :item_id,   :null => false
       t.string   :event,     :null => false
@@ -31,4 +39,29 @@ class CreateVersions < ActiveRecord::Migration
     end
     add_index :versions, [:item_type, :item_id]
   end
+
+  private
+
+  # Even modern versions of MySQL still use `latin1` as the default character
+  # encoding. Many users are not aware of this, and run into trouble when they
+  # try to use PaperTrail in apps that otherwise tend to use UTF-8. Postgres, by
+  # comparison, uses UTF-8 except in the unusual case where the OS is configured
+  # with a custom locale.
+  #
+  # - https://dev.mysql.com/doc/refman/5.7/en/charset-applications.html
+  # - http://www.postgresql.org/docs/9.4/static/multibyte.html
+  #
+  # Furthermore, MySQL's original implementation of UTF-8 was flawed, and had
+  # to be fixed later by introducing a new charset, `utf8mb4`.
+  #
+  # - https://mathiasbynens.be/notes/mysql-utf8mb4
+  # - https://dev.mysql.com/doc/refman/5.5/en/charset-unicode-utf8mb4.html
+  #
+  def versions_table_options
+    if MYSQL_ADAPTERS.include?(connection.class.name)
+      { options: 'ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_col' }
+    else
+      {}
+    end
+  end
 end

spec/generators/install_generator_spec.rb

@@ -13,7 +13,7 @@ describe PaperTrail::InstallGenerator, :type => :generator do
       prepare_destination
       run_generator
     end
-  
+
     it "generates a migration for creating the 'versions' table" do
       expect(destination_root).to have_structure {
         directory 'db' do
@@ -21,7 +21,7 @@ describe PaperTrail::InstallGenerator, :type => :generator do
             migration 'create_versions' do
               contains 'class CreateVersions'
               contains 'def change'
-              contains 'create_table :versions do |t|'
+              contains 'create_table :versions'
             end
           end
         end
@@ -42,7 +42,7 @@ describe PaperTrail::InstallGenerator, :type => :generator do
             migration 'create_versions' do
               contains 'class CreateVersions'
               contains 'def change'
-              contains 'create_table :versions do |t|'
+              contains 'create_table :versions'
             end
           end
         end
@@ -63,5 +63,4 @@ describe PaperTrail::InstallGenerator, :type => :generator do
       }
     end
   end
-
 end

spec/generators/paper_trail/templates/create_versions_spec.rb

@@ -0,0 +1,51 @@
+require "rails_helper"
+require "generators/paper_trail/templates/create_versions"
+
+RSpec.describe CreateVersions do
+  describe "#change", verify_stubs: false do
+    let(:migration) { described_class.new }
+
+    before do
+      allow(migration).to receive(:add_index)
+      allow(migration).to receive(:create_table)
+    end
+
+    it "creates the versions table" do
+      migration.change
+      expect(migration).to have_received(:create_table) do |arg1|
+        expect(arg1).to eq(:versions)
+      end
+    end
+
+    case ENV["DB"]
+    when "mysql"
+      it "uses InnoDB engine" do
+        migration.change
+        expect(migration).to have_received(:create_table) do |_, arg2|
+          expect(arg2[:options]).to match(/ENGINE=InnoDB/)
+        end
+      end
+
+      it "uses utf8mb4 character set" do
+        migration.change
+        expect(migration).to have_received(:create_table) do |_, arg2|
+          expect(arg2[:options]).to match(/DEFAULT CHARSET=utf8mb4/)
+        end
+      end
+
+      it "uses utf8mb4_col collation" do
+        migration.change
+        expect(migration).to have_received(:create_table) do |_, arg2|
+          expect(arg2[:options]).to match(/COLLATE=utf8mb4_col/)
+        end
+      end
+    else
+      it "passes an empty options hash to create_table" do
+        migration.change
+        expect(migration).to have_received(:create_table) do |_, arg2|
+          expect(arg2).to eq({})
+        end
+      end
+    end
+  end
+end

spec/spec_helper.rb

@@ -44,7 +44,16 @@ RSpec.configure do |config|
     mocks.verify_partial_doubles = true
   end
 
-# The settings below are suggested to provide a good initial experience
+  # Support for disabling `verify_partial_doubles` on specific examples.
+  config.around(:each, verify_stubs: false) do |ex|
+    config.mock_with :rspec do |mocks|
+      mocks.verify_partial_doubles = false
+      ex.run
+      mocks.verify_partial_doubles = true
+    end
+  end
+
+  # The settings below are suggested to provide a good initial experience
 # with RSpec, but feel free to customize to your heart's content.
 =begin
   # These two settings work together to allow you to limit a spec run