Rails 6.1: Add support for if_not_exists to indexes (#891)

* add create index if exists support

See
rails/rails@7ba0803

* add changelog item

Author: mgrunberg

CHANGELOG.md

@@ -12,6 +12,7 @@
 - [#861](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/861) Fix Rails 6.1 database config
 - [#890](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/890) Fix removal of invalid ordering from select statements
 - [#881](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/881) Dump column collation to schema.rb and allow collation changes using column_change
+- [#891](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/891) Add support for if_not_exists to indexes
 - [#892](https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/pull/892) Add support for if_exists on remove_column
 
 #### Changed

lib/active_record/connection_adapters/sqlserver/schema_creation.rb

@@ -33,6 +33,20 @@ def visit_TableDefinition(o)
           sql
         end
 
+        def visit_CreateIndexDefinition(o)
+          if_not_exists = o.if_not_exists
+
+          o.if_not_exists = false
+
+          sql = super
+
+          if if_not_exists
+            sql = "IF NOT EXISTS (SELECT name FROM sysindexes WHERE name = '#{o.index.name}') #{sql}"
+          end
+
+          sql
+        end
+
         def add_column_options!(sql, options)
           sql << " DEFAULT #{quote_default_expression(options[:default], options[:column])}" if options_include_default?(options)
           if options[:null] == false