Use full precision for `updated_at` in `insert_all`/`upsert_all`

`CURRENT_TIMESTAMP` provides differing precision depending on the database,
and not all databases support explicitly specifying additional precision.

Instead, we delegate to the new `connection.high_precision_current_timestamp`
for the SQL to produce a high precision timestamp on the current database.

activerecord/CHANGELOG.md

@@ -1,3 +1,15 @@
+*   Use full precision for `updated_at` in `insert_all`/`upsert_all`
+
+    `CURRENT_TIMESTAMP` provides differing precision depending on the database,
+    and not all databases support explicitly specifying additional precision.
+
+    Instead, we delegate to the new `connection.high_precision_current_timestamp`
+    for the SQL to produce a high precision timestamp on the current database.
+
+    Fixes #42992
+
+    *Sam Bostock*
+
 *   Add config option for ignoring tables when dumping the schema cache.
 
     Applications can now be configured to ignore certain tables when dumping the schema cache.

activerecord/lib/active_record/insert_all.rb

@@ -198,7 +198,7 @@ module ActiveRecord
         def touch_model_timestamps_unless(&block)
           model.timestamp_attributes_for_update_in_model.filter_map do |column_name|
             if touch_timestamp_attribute?(column_name)
-              "#{column_name}=(CASE WHEN (#{updatable_columns.map(&block).join(" AND ")}) THEN #{model.quoted_table_name}.#{column_name} ELSE CURRENT_TIMESTAMP END),"
+              "#{column_name}=(CASE WHEN (#{updatable_columns.map(&block).join(" AND ")}) THEN #{model.quoted_table_name}.#{column_name} ELSE #{connection.high_precision_current_timestamp} END),"
             end
           end.join
         end

activerecord/test/cases/insert_all_test.rb

@@ -364,6 +364,15 @@ class InsertAllTest < ActiveRecord::TestCase
     assert_equal Time.now.year, Book.find(101).updated_on.year
   end
 
+  def test_upsert_all_respects_updated_at_precision_when_touched_implicitly
+    skip unless supports_insert_on_duplicate_update? && supports_datetime_with_precision?
+
+    Book.insert_all [{ id: 101, name: "Out of the Silent Planet", published_on: Date.new(1938, 4, 1), updated_at: 5.years.ago, updated_on: 5.years.ago }]
+    Book.upsert_all [{ id: 101, name: "Out of the Silent Planet", published_on: Date.new(1938, 4, 8) }]
+
+    assert_not_predicate Book.find(101).updated_at.usec, :zero?, "updated_at should have sub-second precision"
+  end
+
   def test_upsert_all_uses_given_updated_at_over_implicit_updated_at
     skip unless supports_insert_on_duplicate_update?