Revert "Use flat_map { } instead of map {}.flatten"

This reverts commit abf8de85519141496a6773310964ec03f6106f3f.
We should take a deeper look to those cases flat_map doesn't do deep
flattening.

irb(main):002:0> [[[1,3], [1,2]]].map{|i| i}.flatten
=> [1, 3, 1, 2]
irb(main):003:0> [[[1,3], [1,2]]].flat_map{|i| i}
=> [[1, 3], [1, 2]]

1 files changed, 2 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index dcb89178f7..8c83c4f5db 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -448,7 +448,7 @@ module ActiveRecord
       end
 
       def bulk_change_table(table_name, operations) #:nodoc:
-        sqls = operations.flat_map do |command, args|
+        sqls = operations.map do |command, args|
           table, arguments = args.shift, args
           method = :"#{command}_sql"
 
@@ -457,7 +457,7 @@ module ActiveRecord
           else
             raise "Unknown method called : #{method}(#{arguments.inspect})"
           end
-        end.join(", ")
+        end.flatten.join(", ")
 
         execute("ALTER TABLE #{quote_table_name(table_name)} #{sqls}")
       end