Fix Rails/WhereNot when using implicit receiver

Fix an exception in `Rails/WhereNot` when calling `.where` on an implicit
receiver (e.g. inside model code).

Fixes #1221

changelog/fix_where_not_with_implicit_receiver.md

@@ -0,0 +1 @@
+* [#1221](https://github.com/rubocop/rubocop-rails/issues/1221): Fix an exception in `Rails/WhereNot` when calling `.where` on an implicit receiver (e.g. inside model code). ([@bquorning][])

lib/rubocop/cop/rails/where_not.rb

@@ -46,7 +46,7 @@ def on_send(node)
             column_and_value = extract_column_and_value(template_node, value_node)
             return unless column_and_value
 
-            good_method = build_good_method(node.loc.dot.source, *column_and_value)
+            good_method = build_good_method(node.loc.dot&.source, *column_and_value)
             message = format(MSG, good_method: good_method)
 
             add_offense(range, message: message) do |corrector|
@@ -88,6 +88,7 @@ def extract_column_and_value(template_node, value_node)
         end
 
         def build_good_method(dot, column, value)
+          dot ||= '.'
           if column.include?('.')
             table, column = column.split('.')
 

spec/rubocop/cop/rails/where_not_spec.rb

@@ -111,6 +111,17 @@
     RUBY
   end
 
+  it 'registers an offense and corrects when using implicit receiver' do
+    expect_offense(<<~RUBY)
+      where('name != ?', 'Gabe')
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^ Use `where.not(name: 'Gabe')` instead of manually constructing negated SQL in `where`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      where.not(name: 'Gabe')
+    RUBY
+  end
+
   context 'with array arguments' do
     it 'registers an offense and corrects when using `!=` and anonymous placeholder' do
       expect_offense(<<~RUBY)