Apply monkey patches for SQL Server connections only

Recent versions of Rails support multiple database connections within
the same app. It is possible for these connections to use different
adapters. For example, one adapter may use SQL Server, and another uses
PostgreSQL.

This gem applies some monkey patches to ActiveRecord for SQL Server
compatibility. These patches could break other adapters, though, in a
multiple-database scenario.

This commit modifies the patches so that they are applied only if the
connection is SQL Server. If not, the original ActiveRecord
implementation (`super`) is used instead.

Fixes #929

Author: mattbrictson

lib/active_record/connection_adapters/sqlserver/core_ext/attribute_methods.rb

@@ -10,6 +10,8 @@ module AttributeMethods
           private
 
           def attributes_for_update(attribute_names)
+            return super unless self.class.connection.is_a?(SQLServerAdapter)
+
             super.reject do |name|
               column = self.class.columns_hash[name]
               column && column.respond_to?(:is_identity?) && column.is_identity?

lib/active_record/connection_adapters/sqlserver/core_ext/calculations.rb

@@ -10,6 +10,8 @@ module CoreExt
         module Calculations
           # Same as original except we don't perform PostgreSQL hack that removes ordering.
           def calculate(operation, column_name)
+            return super unless klass.connection.is_a?(SQLServerAdapter)
+
             if has_include?(column_name)
               relation = apply_join_dependency
 
@@ -29,6 +31,8 @@ def calculate(operation, column_name)
           private
 
           def build_count_subquery(relation, column_name, distinct)
+            return super unless klass.connection.is_a?(SQLServerAdapter)
+
             super(relation.unscope(:order), column_name, distinct)
           end
         end

lib/active_record/connection_adapters/sqlserver/core_ext/explain.rb

@@ -9,6 +9,8 @@ module Explain
           SQLSERVER_STATEMENT_REGEXP = /N'(.+)', N'(.+)', (.+)/
 
           def exec_explain(queries)
+            return super unless connection.is_a?(SQLServerAdapter)
+
             unprepared_queries = queries.map do |(sql, binds)|
               [unprepare_sqlserver_statement(sql, binds), binds]
             end

lib/active_record/connection_adapters/sqlserver/core_ext/finder_methods.rb

@@ -12,6 +12,8 @@ module FinderMethods
 
           # Same as original except we order by values in distinct select if present.
           def construct_relation_for_exists(conditions)
+            return super unless klass.connection.is_a?(SQLServerAdapter)
+
             conditions = sanitize_forbidden_attributes(conditions)
 
             if distinct_value && offset_value

lib/active_record/connection_adapters/sqlserver/core_ext/preloader.rb

@@ -10,6 +10,8 @@ module Preloader
           private
 
           def records_for(ids)
+            return super unless klass.connection.is_a?(SQLServerAdapter)
+
             ids.each_slice(in_clause_length).flat_map do |slice|
               scope.where(association_key_name => slice).load do |record|
                 # Processing only the first owner