Pare back default `index` option for the migration generator

- Using `references` or `belongs_to` in migrations will always add index
  for the referenced column by default, without adding `index:true` option
  to generated migration file.
- Users can opt out of this by passing `index: false`.
- Legacy migrations won't be affected by this change. They will continue
  to run as they were before.
- Fixes #18146

1 files changed, 3 insertions, 2 deletions

diff --git a/railties/lib/rails/generators/generated_attribute.rb b/railties/lib/rails/generators/generated_attribute.rb
index 8145a26e22..7e437e7344 100644
--- a/railties/lib/rails/generators/generated_attribute.rb
+++ b/railties/lib/rails/generators/generated_attribute.rb
@@ -23,8 +23,9 @@ module Rails
           type = type.to_sym if type
 
           if type && reference?(type)
-            references_index = UNIQ_INDEX_OPTIONS.include?(has_index) ? { unique: true } : true
-            attr_options[:index] = references_index
+            if UNIQ_INDEX_OPTIONS.include?(has_index)
+              attr_options[:index] = { unique: true }
+            end
           end
 
           new(name, type, has_index, attr_options)