Add `config.active_record.warn_on_records_fetched_greater_than` option

When set to an integer, a warning will be logged whenever a result set
larger than the specified size is returned by a query. Fixes #16463

The warning is outputed a module which is prepended in an initializer,
so there will be no performance impact if
`config.active_record.warn_on_records_fetched_greater_than` is not set.

1 files changed, 28 insertions, 0 deletions

diff --git a/activerecord/test/cases/relation/record_fetch_warning_test.rb b/activerecord/test/cases/relation/record_fetch_warning_test.rb
new file mode 100644
index 0000000000..62f0a7cc49
--- /dev/null
+++ b/activerecord/test/cases/relation/record_fetch_warning_test.rb
@@ -0,0 +1,28 @@
+require 'cases/helper'
+require 'models/post'
+
+module ActiveRecord
+  class RecordFetchWarningTest < ActiveRecord::TestCase
+    fixtures :posts
+
+    def test_warn_on_records_fetched_greater_than
+      original_logger = ActiveRecord::Base.logger
+      orginal_warn_on_records_fetched_greater_than = ActiveRecord::Base.warn_on_records_fetched_greater_than
+
+      log = StringIO.new
+      ActiveRecord::Base.logger = ActiveSupport::Logger.new(log)
+      ActiveRecord::Base.logger.level = Logger::WARN
+
+      require 'active_record/relation/record_fetch_warning'
+
+      ActiveRecord::Base.warn_on_records_fetched_greater_than = 1
+
+      Post.all.to_a
+
+      assert_match(/Query fetched/, log.string)
+    ensure
+      ActiveRecord::Base.logger = original_logger
+      ActiveRecord::Base.warn_on_records_fetched_greater_than = orginal_warn_on_records_fetched_greater_than
+    end
+  end
+end