Fix issue with expression index in insert_all

Based on the docs which state unique_by option of insert_all can use the
index name if desired, one would expect the method to work normally and
use the `unique_by` option to determine duplicates.

However, there's an issue where the insert_all expects a Set instead
of the string representing the index expression it is given. This causes
an error. Returning the string expression instead of attempting to
format it works perfectly though.

activerecord/CHANGELOG.md

@@ -1,3 +1,24 @@
+*   Resolve issue with insert_all unique_by option when used with expression index.
+
+    When the `:unique_by` option of `ActiveRecord::Persistence.insert_all` and
+    `ActiveRecord::Persistence.upsert_all` was used with the name of an expression index, an error
+    was raised. Adding a guard around the formatting behavior for the `:unique_by` corrects this.
+
+    Usage:
+
+    ```ruby
+    create_table :books, id: :integer, force: true do |t|
+      t.column :name, :string
+      t.index "lower(name)", unique: true
+    end
+
+    Book.insert_all [{ name: "MyTest" }], unique_by: :index_books_on_lower_name
+    ```
+
+    Fixes #39516.
+
+    *Austen Madden*
+
 *   Add basic support for CHECK constraints to database migrations.
 
     Usage:

activerecord/lib/active_record/insert_all.rb

@@ -195,7 +195,7 @@ module ActiveRecord
           end
 
           def format_columns(columns)
-            quote_columns(columns).join(",")
+            columns.respond_to?(:map) ? quote_columns(columns).join(",") : columns
           end
 
           def quote_columns(columns)

activerecord/test/cases/helper.rb

@@ -58,6 +58,7 @@ end
   supports_savepoints?
   supports_partial_index?
   supports_partitioned_indexes?
+  supports_expression_index?
   supports_insert_returning?
   supports_insert_on_duplicate_skip?
   supports_insert_on_duplicate_update?

activerecord/test/cases/insert_all_test.rb

@@ -178,6 +178,20 @@ class InsertAllTest < ActiveRecord::TestCase
     end
   end
 
+  def test_insert_all_and_upsert_all_with_expression_index
+    skip unless supports_expression_index? && supports_insert_conflict_target?
+
+    book = Book.create!(external_id: "abc")
+
+    assert_no_difference "Book.count" do
+      Book.insert_all [{ external_id: "ABC" }], unique_by: :index_books_on_lower_external_id
+    end
+
+    Book.upsert_all [{ external_id: "Abc" }], unique_by: :index_books_on_lower_external_id
+
+    assert_equal "Abc", book.reload.external_id
+  end
+
   def test_insert_all_and_upsert_all_raises_when_index_is_missing
     skip unless supports_insert_conflict_target?
 

activerecord/test/schema/schema.rb

@@ -116,10 +116,12 @@ ActiveRecord::Schema.define do
     t.column :difficulty, :integer, **default_zero
     t.column :cover, :string, default: "hard"
     t.string :isbn, **case_sensitive_options
+    t.string :external_id
     t.datetime :published_on
     t.boolean :boolean_status
     t.index [:author_id, :name], unique: true
     t.index :isbn, where: "published_on IS NOT NULL", unique: true
+    t.index "(lower(external_id))", unique: true if supports_expression_index?
 
     t.datetime :created_at
     t.datetime :updated_at