monkey patch for mysql 5.7

config/initializers/monkey_patch.rb

+## This patch is from rails 4.2-stable. Remove it when 4.2.6 is released
+## https://github.com/rails/rails/issues/21108
+
+module ActiveRecord
+  module ConnectionAdapters
+    class AbstractMysqlAdapter < AbstractAdapter
+      # SHOW VARIABLES LIKE 'name'
+      def show_variable(name)
+        variables = select_all("select @@#{name} as 'Value'", 'SCHEMA')
+        variables.first['Value'] unless variables.empty?
+      rescue ActiveRecord::StatementInvalid
+        nil
+      end
+
+      
+      # MySQL is too stupid to create a temporary table for use subquery, so we have
+      # to give it some prompting in the form of a subsubquery. Ugh!
+      def subquery_for(key, select)
+        subsubselect = select.clone
+        subsubselect.projections = [key]
+
+        subselect = Arel::SelectManager.new(select.engine)
+        subselect.project Arel.sql(key.name)
+        # Materialized subquery by adding distinct
+        # to work with MySQL 5.7.6 which sets optimizer_switch='derived_merge=on'
+        subselect.from subsubselect.distinct.as('__active_record_temp')
+      end
+    end
+  end
+end
+
+module ActiveRecord
+  module ConnectionAdapters
+    class MysqlAdapter < AbstractMysqlAdapter
+      ADAPTER_NAME = 'MySQL'.freeze
+
+      # Get the client encoding for this database
+      def client_encoding
+        return @client_encoding if @client_encoding
+
+        result = exec_query(
+          "select @@character_set_client",
+          'SCHEMA')
+        @client_encoding = ENCODINGS[result.rows.last.last]
+      end
+    end
+  end
+end