Treat `Set` as an `Array` in `Relation#where`

I do not want to set the expectation that any enumerable object should
behave this way, but this case in particular comes up frequently enough
that I'm caving on this one.

Fixes #30684.

3 files changed, 19 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index c34236d4be..7cae105ddb 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Passing a `Set` to `Relation#where` now behaves the same as passing an
+    array.
+
+    *Sean Griffin*
+
 *   Use given algorithm while removing index from database.
 
     Fixes #24190.
diff --git a/activerecord/lib/active_record/relation/predicate_builder.rb b/activerecord/lib/active_record/relation/predicate_builder.rb
index 5c42414072..be4b169f67 100644
--- a/activerecord/lib/active_record/relation/predicate_builder.rb
+++ b/activerecord/lib/active_record/relation/predicate_builder.rb
@@ -13,6 +13,7 @@ module ActiveRecord
       register_handler(Range, RangeHandler.new(self))
       register_handler(Relation, RelationHandler.new)
       register_handler(Array, ArrayHandler.new(self))
+      register_handler(Set, ArrayHandler.new(self))
     end
 
     def build_from_hash(attributes)
diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index ae1dc35bff..6270533dc2 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -1912,6 +1912,19 @@ class RelationTest < ActiveRecord::TestCase
     end
   end
 
+  test "#where with set" do
+    david = authors(:david)
+    mary = authors(:mary)
+
+    authors = Author.where(name: ["David", "Mary"].to_set)
+    assert_equal [david, mary], authors
+  end
+
+  test "#where with empty set" do
+    authors = Author.where(name: Set.new)
+    assert_empty authors
+  end
+
   private
     def custom_post_relation
       table_alias = Post.arel_table.alias("omg_posts")