make `remove_index :table, :column` reversible.

This used to raise a `IrreversibleMigration` error (since #10437).
However since `remove_index :table, :column` is probably the most basic
use-case we should make it reversible again.

Conflicts:
	activerecord/CHANGELOG.md

1 files changed, 9 insertions, 7 deletions

diff --git a/activerecord/lib/active_record/migration/command_recorder.rb b/activerecord/lib/active_record/migration/command_recorder.rb
index ee4545ed71..b592c004aa 100644
--- a/activerecord/lib/active_record/migration/command_recorder.rb
+++ b/activerecord/lib/active_record/migration/command_recorder.rb
@@ -151,14 +151,16 @@ module ActiveRecord
       end
 
       def invert_remove_index(args)
-        table, options = *args
-
-        unless options && options.is_a?(Hash) && options[:column]
-          raise ActiveRecord::IrreversibleMigration, "remove_index is only reversible if given a :column option."
+        table, options_or_column = *args
+        if (options = options_or_column).is_a?(Hash)
+          unless options[:column]
+            raise ActiveRecord::IrreversibleMigration, "remove_index is only reversible if given a :column option."
+          end
+          options = options.dup
+          [:add_index, [table, options.delete(:column), options]]
+        elsif (column = options_or_column).present?
+          [:add_index, [table, column]]
         end
-
-        options = options.dup
-        [:add_index, [table, options.delete(:column), options]]
       end
 
       alias :invert_add_belongs_to :invert_add_reference