Implement hash equality for WhereClause

Co-authored-by: Ryuta Kamizono <kamipo@gmail.com>

activerecord/lib/active_record/relation/where_clause.rb

@@ -76,6 +76,11 @@ module ActiveRecord
         other.is_a?(WhereClause) &&
           predicates == other.predicates
       end
+      alias :eql? :==
+
+      def hash
+        [self.class, predicates].hash
+      end
 
       def invert
         if predicates.size == 1

activerecord/test/cases/relation/where_clause_test.rb

@@ -245,6 +245,19 @@ class ActiveRecord::Relation
       assert_equal only_common, common_with_extra.or(only_common)
     end
 
+    test "supports hash equality" do
+      h = Hash.new(0)
+      h[WhereClause.new(["a"])] += 1
+      h[WhereClause.new(["a"])] += 1
+      h[WhereClause.new(["b"])] += 1
+
+      expected = {
+        WhereClause.new(["a"]) => 2,
+        WhereClause.new(["b"]) => 1
+      }
+      assert_equal expected, h
+    end
+
     private
       def table
         Arel::Table.new("table")