Add reversible syntax for change_column_default

Passing `:from` and `:to` to `change_column_default` makes this command
reversible as user has defined its previous state.

So, instead of having the migration command as:

    change_column_default(:posts, :state, "draft")

They can write it as:

    change_column_default(:posts, :state, from: nil, to: "draft")

1 files changed, 2 insertions, 1 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 00e3d2965b..2027492f29 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -629,7 +629,8 @@ module ActiveRecord
         end
       end
 
-      def change_column_default(table_name, column_name, default) #:nodoc:
+      def change_column_default(table_name, column_name, default_or_changes) #:nodoc:
+        default = extract_new_default_value(default_or_changes)
         column = column_for(table_name, column_name)
         change_column table_name, column_name, column.sql_type, :default => default
       end