Add experimental server-side notification grouping (#29889)

lib/arel/union_parenthesizing.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+# Fix an issue with `LIMIT` ocurring on the left side of a `UNION` causing syntax errors.
+# See https://github.com/rails/rails/issues/40181
+
+# The fix has been merged in ActiveRecord: https://github.com/rails/rails/pull/51549
+# TODO: drop this when available in ActiveRecord
+
+# rubocop:disable all -- This is a mostly vendored file
+
+module Arel
+  module Visitors
+    class ToSql
+      private
+
+        def infix_value_with_paren(o, collector, value, suppress_parens = false)
+          collector << "( " unless suppress_parens
+          collector = if o.left.class == o.class
+            infix_value_with_paren(o.left, collector, value, true)
+          else
+            select_parentheses o.left, collector, false # Changed from `visit o.left, collector`
+          end
+          collector << value
+          collector = if o.right.class == o.class
+            infix_value_with_paren(o.right, collector, value, true)
+          else
+            select_parentheses o.right, collector, false # Changed from `visit o.right, collector`
+          end
+          collector << " )" unless suppress_parens
+          collector
+        end
+
+        def select_parentheses(o, collector, always_wrap_selects = true)
+          if o.is_a?(Nodes::SelectStatement) && (always_wrap_selects || require_parentheses?(o))
+            collector << "("
+            visit o, collector
+            collector << ")"
+            collector
+          else
+            visit o, collector
+          end
+        end
+
+        def require_parentheses?(o)
+          !o.orders.empty? || o.limit || o.offset
+        end
+    end
+  end
+end
+
+# rubocop:enable all