Deprecate `where.not` working as NOR and will be changed to NAND in Rails 6.1

`where.not` with polymorphic association is partly fixed incidentally at
213796f (refer #33493, #26207, #17010, #16983, #14161), and I've added
test case e9ba12f to avoid lose that fix accidentally in the future.

In Rails 5.2, `where.not(polymorphic: object)` works as expected as
NAND, but `where.not(polymorphic_type: object.class.polymorphic_name,
polymorphic_id: object.id)` still unexpectedly works as NOR.

To will make `where.not` working desiredly as NAND in Rails 6.1, this
deprecates `where.not` working as NOR. If people want to continue NOR
conditions, we'd encourage to them to `where.not` each conditions
manually.

```ruby
all = [treasures(:diamond), treasures(:sapphire), cars(:honda), treasures(:sapphire)]
assert_equal all, PriceEstimate.all.map(&:estimate_of)
```

In Rails 6.0:

```ruby
sapphire = treasures(:sapphire)

nor = all.reject { |e|
  e.estimate_of_type == sapphire.class.polymorphic_name
}.reject { |e|
  e.estimate_of_id == sapphire.id
}
assert_equal [cars(:honda)], nor

without_sapphire = PriceEstimate.where.not(
  estimate_of_type: sapphire.class.polymorphic_name, estimate_of_id: sapphire.id
)
assert_equal nor, without_sapphire.map(&:estimate_of)
```

In Rails 6.1:

```ruby
sapphire = treasures(:sapphire)

nand = all - [sapphire]
assert_equal [treasures(:diamond), cars(:honda)], nand

without_sapphire = PriceEstimate.where.not(
  estimate_of_type: sapphire.class.polymorphic_name, estimate_of_id: sapphire.id
)
assert_equal nand, without_sapphire.map(&:estimate_of)
```

Resolves #31209.

2 files changed, 26 insertions, 9 deletions

diff --git a/activerecord/lib/active_record/relation/query_methods.rb b/activerecord/lib/active_record/relation/query_methods.rb
index 90b5e9a118..0a481dcbdc 100644
--- a/activerecord/lib/active_record/relation/query_methods.rb
+++ b/activerecord/lib/active_record/relation/query_methods.rb
@@ -41,18 +41,31 @@ module ActiveRecord
       #
       #    User.where.not(name: %w(Ko1 Nobu))
       #    # SELECT * FROM users WHERE name NOT IN ('Ko1', 'Nobu')
-      #
-      #    User.where.not(name: "Jon", role: "admin")
-      #    # SELECT * FROM users WHERE name != 'Jon' AND role != 'admin'
       def not(opts, *rest)
         opts = sanitize_forbidden_attributes(opts)
 
         where_clause = @scope.send(:where_clause_factory).build(opts, rest)
 
         @scope.references!(PredicateBuilder.references(opts)) if Hash === opts
-        @scope.where_clause += where_clause.invert
+
+        if not_behaves_as_nor?(opts)
+          ActiveSupport::Deprecation.warn(<<~MSG.squish)
+            NOT conditions will no longer behave as NOR in Rails 6.1.
+            To continue using NOR conditions, NOT each conditions manually
+            (`#{ opts.keys.map { |key| ".where.not(#{key.inspect} => ...)" }.join }`).
+          MSG
+          @scope.where_clause += where_clause.invert(:nor)
+        else
+          @scope.where_clause += where_clause.invert
+        end
+
         @scope
       end
+
+      private
+        def not_behaves_as_nor?(opts)
+          opts.is_a?(Hash) && opts.size > 1
+        end
     end
 
     FROZEN_EMPTY_ARRAY = [].freeze
diff --git a/activerecord/lib/active_record/relation/where_clause.rb b/activerecord/lib/active_record/relation/where_clause.rb
index 47728aac30..b91b135867 100644
--- a/activerecord/lib/active_record/relation/where_clause.rb
+++ b/activerecord/lib/active_record/relation/where_clause.rb
@@ -70,7 +70,15 @@ module ActiveRecord
           predicates == other.predicates
       end
 
-      def invert
+      def invert(as = :nand)
+        if predicates.size == 1
+          inverted_predicates = [ invert_predicate(predicates.first) ]
+        elsif as == :nor
+          inverted_predicates = predicates.map { |node| invert_predicate(node) }
+        else
+          inverted_predicates = [ Arel::Nodes::Not.new(ast) ]
+        end
+
         WhereClause.new(inverted_predicates)
       end
 
@@ -115,10 +123,6 @@ module ActiveRecord
           node.respond_to?(:operator) && node.operator == :==
         end
 
-        def inverted_predicates
-          predicates.map { |node| invert_predicate(node) }
-        end
-
         def invert_predicate(node)
           case node
           when NilClass