sets limit 191 for schema_migrations#version in mysql2 if the encodin…

…g is "utf8mb4"

Please, see rationale in the included CHANGELOG patch.

activerecord/CHANGELOG.md

@@ -1,5 +1,20 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   The length of the `version` column in the `schema_migrations` table
+    created by the `mysql2` adapter is 191 if the encoding is "utf8mb4".
+
+    The "utf8" encoding in MySQL has support for a maximum of 3 bytes per character,
+    and only contains characters from the BMP. The recently added
+    [utf8mb4](http://dev.mysql.com/doc/refman/5.5/en/charset-unicode-utf8mb4.html)
+    encoding extends the support to four bytes. As of this writing, said encoding
+    is supported in the betas of the `mysql2` gem.
+
+    Setting the encoding to "utf8mb4" has
+    [a few implications](http://dev.mysql.com/doc/refman/5.5/en/charset-unicode-upgrading.html).
+    This change addresses the max length for indexes, which is 191 instead of 255.
+
+    *Xavier Noria*
+
 *   Counter caches on associations will now stay valid when attributes are
     updated (not just when records are created or destroyed), for example,
     when calling `update_attributes`. The following code now works:

activerecord/lib/active_record/connection_adapters/mysql2_adapter.rb

@@ -38,6 +38,15 @@ def initialize(connection, logger, connection_options, config)
         configure_connection
       end
 
+      MAX_INDEX_LENGTH_FOR_UTF8MB4 = 191
+      def initialize_schema_migrations_table
+        if @config[:encoding] == 'utf8mb4'
+          ActiveRecord::SchemaMigration.create_table(MAX_INDEX_LENGTH_FOR_UTF8MB4)
+        else
+          ActiveRecord::SchemaMigration.create_table
+        end
+      end
+
       def supports_explain?
         true
       end

activerecord/lib/active_record/schema_migration.rb

@@ -13,18 +13,21 @@ def self.index_name
       "#{Base.table_name_prefix}unique_schema_migrations#{Base.table_name_suffix}"
     end
 
-    def self.create_table
+    def self.create_table(limit=nil)
       unless connection.table_exists?(table_name)
-        connection.create_table(table_name, :id => false) do |t|
-          t.column :version, :string, :null => false
+        version_options = {null: false}
+        version_options[:limit] = limit if limit
+
+        connection.create_table(table_name, id: false) do |t|
+          t.column :version, :string, version_options
         end
-        connection.add_index table_name, :version, :unique => true, :name => index_name
+        connection.add_index table_name, :version, unique: true, name: index_name
       end
     end
 
     def self.drop_table
       if connection.table_exists?(table_name)
-        connection.remove_index table_name, :name => index_name
+        connection.remove_index table_name, name: index_name
         connection.drop_table(table_name)
       end
     end

activerecord/test/cases/adapters/mysql2/schema_migrations_test.rb

@@ -0,0 +1,26 @@
+require "cases/helper"
+
+module ActiveRecord
+  module ConnectionAdapters
+    class Mysql2Adapter
+      class SchemaMigrationsTest < ActiveRecord::TestCase
+        def test_initializes_schema_migrations_for_encoding_utf8mb4
+          conn = ActiveRecord::Base.connection
+
+          smtn = ActiveRecord::Migrator.schema_migrations_table_name
+          conn.drop_table(smtn) if conn.table_exists?(smtn)
+
+          config = conn.instance_variable_get(:@config)
+          original_encoding = config[:encoding]
+
+          config[:encoding] = 'utf8mb4'
+          conn.initialize_schema_migrations_table
+
+          assert conn.column_exists?(smtn, :version, :string, limit: Mysql2Adapter::MAX_INDEX_LENGTH_FOR_UTF8MB4)
+        ensure
+          config[:encoding] = original_encoding
+        end
+      end
+    end
+  end
+end