Add missing hash, eql?, == to various node classes

Some of the nodes classes are missing either one or many of the common comparison methods #hash, #eql? and #==.

This makes comparision and working with the ast sometimes painful, as equality or operations likes array differences (which uses a hash behind the scene) produces unexpected results.

A test has been added that ensures that every descendants of Node:
* have all 3 methods
* that all 3 methods were defined from the same class
* that the class defining all 3 is also a descendant of Node, to avoid the default ones that rely on identity only

1 files changed, 34 insertions, 0 deletions

diff --git a/test/test_nodes.rb b/test/test_nodes.rb
new file mode 100644
index 0000000000..bf0082396d
--- /dev/null
+++ b/test/test_nodes.rb
@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+require 'helper'
+
+module Arel
+  module Nodes
+    class TestNodes < Minitest::Test
+      def test_every_arel_nodes_have_hash_eql_eqeq_from_same_class
+        # #descendants code from activesupport
+        node_descendants = []
+        ObjectSpace.each_object(Arel::Nodes::Node.singleton_class) do |k|
+          next if k.respond_to?(:singleton_class?) && k.singleton_class?
+          node_descendants.unshift k unless k == self
+        end
+        node_descendants.delete(Arel::Nodes::Node)
+
+        bad_node_descendants = node_descendants.reject do |subnode|
+          eqeq_owner = subnode.instance_method(:==).owner
+          eql_owner = subnode.instance_method(:eql?).owner
+          hash_owner = subnode.instance_method(:hash).owner
+
+          eqeq_owner < Arel::Nodes::Node &&
+              eqeq_owner == eql_owner &&
+              eqeq_owner == hash_owner
+        end
+
+        problem_msg = 'Some subclasses of Arel::Nodes::Node do not have a' \
+            ' #== or #eql? or #hash defined from the same class as the others'
+        assert_empty bad_node_descendants, problem_msg
+      end
+
+
+    end
+  end
+end