Example of a monkey patch

sqlserver_adapter.rb

# frozen_string_literal: true

require "activerecord-sqlserver-adapter"

# Monkey-patch the SqlServer/ActiveRecord adapter to set
# certain connection options.
#
# --- SET ARITHABORT ON
#
# SqlServer, when a tcp/ip connection is made, sets some default
# options for that connection.  One of those options is `SET
# ARITHABORT OFF`.  This option has two known effects:
#
# * Performance for certain queries done via TCP/IP becomes very bad.
#
# * The query plan for queries made via TCP/IP differs from queries
#   made in Management Studio.
#
# Here is one such query.  This query takes a long time (more than a
# minute) when there is a great deal of data in these tables _and_
# `SET ARITHABORT OFF` is in effect:
#
#     EXEC sp_executesql N'SELECT COUNT(*) FROM [MigrationSource]
#     left outer join MigrationDest on MigrationSource.Id = MigrationSourceId
#     WHERE [MigrationSource].[FranchiseNickname] = @0
#     AND [MigrationSource].[Table] = @1
#     AND [MigrationSource].[Migrate] = @2
#     AND [MigrationDest].[MigrationSourceId] IS NULL',
#     N'@0 nvarchar(64), @1 nvarchar(64), @2 bit',
#     @0 = N'hs', @1 = N'tbl_smartpanel_vendor', @2 = 1"
#
# Turning the ARITHABORT option ON causes the query to succeed
# quickly, exactly as it does in management studio, so monkey-patch
# the SqlServer/ActiveRecord adapter accordingly.
#
# See also:
#
# * https://msdn.microsoft.com/en-us/library/ms190306.aspx
# * http://dba.stackexchange.com/q/9840/42317
# * http://www.sommarskog.se/query-plan-mysteries.html
class ActiveRecord::ConnectionAdapters::SQLServerAdapter
  orig_configure_connection = instance_method(:configure_connection)
  define_method(:configure_connection) do
    orig_configure_connection.bind(self).call
    @connection.execute('SET ARITHABORT ON').do
  end
end