[Fix #215] Fix a false positive for Rails/UniqueValidationWithoutIndex

Fixes #215.

This PR fixes a false positive for `Rails/UniqueValidationWithoutIndex`
when using Expression Indexes for PostgreSQL.

Since it is difficult to predict a kind of string described in Expression Indexes,
this implementation judges based on whether or not an Expression Indexes include
a column name.
There is a possibility of false negative, but in most cases I expect it works.

## References

- https://www.postgresql.org/docs/12/indexes-expressional.html
- rails/rails@edc2b77

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Bug fixes
 
 * [#213](https://github.com/rubocop-hq/rubocop-rails/pull/213): Fix a false positive for `Rails/UniqueValidationWithoutIndex` when using conditions. ([@sunny][])
+* [#215](https://github.com/rubocop-hq/rubocop-rails/issues/215): Fix a false positive for `Rails/UniqueValidationWithoutIndex` when using Expression Indexes. ([@koic][])
 
 ## 2.5.0 (2020-03-24)
 

lib/rubocop/cop/rails/unique_validation_without_index.rb

@@ -32,6 +32,7 @@ class UniqueValidationWithoutIndex < Cop
         def on_send(node)
           return unless node.method?(:validates)
           return unless uniqueness_part(node)
+          return if condition_part?(node)
           return unless schema
           return if with_index?(node)
 
@@ -47,13 +48,21 @@ def with_index?(node)
           table = schema.table_by(name: table_name(klass))
           return true unless table # Skip analysis if it can't find the table
 
-          return true if condition_part?(node)
-
           names = column_names(node)
           return true unless names
 
           table.indices.any? do |index|
-            index.unique && index.columns.to_set == names
+            index.unique &&
+              (index.columns.to_set == names ||
+               include_column_names_in_expression_index?(index, names))
+          end
+        end
+
+        def include_column_names_in_expression_index?(index, column_names)
+          return false unless (expression_index = index.expression)
+
+          column_names.all? do |column_name|
+            expression_index.include?(column_name)
           end
         end
 

spec/rubocop/cop/rails/unique_validation_without_index_spec.rb

@@ -406,5 +406,46 @@ class User
         RUBY
       end
     end
+
+    context 'with expression indexes' do
+      context 'when column name is included in expression index' do
+        let(:schema) { <<~RUBY }
+          ActiveRecord::Schema.define(version: 2020_02_02_075409) do
+            create_table 'emails', force: :cascade do |t|
+              t.string 'address', null: false
+              t.index 'lower(address)', name: 'index_emails_on_lower_address', unique: true
+            end
+          end
+        RUBY
+
+        it 'does not register an offense' do
+          expect_no_offenses(<<~RUBY)
+            class Email < ApplicationRecord
+              validates :address, presence: true, uniqueness: { case_sensitive: false }, email: true
+            end
+          RUBY
+        end
+      end
+
+      context 'when column name is not included in expression index' do
+        let(:schema) { <<~RUBY }
+          ActiveRecord::Schema.define(version: 2020_02_02_075409) do
+            create_table 'emails', force: :cascade do |t|
+              t.string 'address', null: false
+              t.index 'lower(unexpected_column_name)', name: 'index_emails_on_lower_address', unique: true
+            end
+          end
+        RUBY
+
+        it 'registers an offense' do
+          expect_offense(<<~RUBY)
+            class Email < ApplicationRecord
+              validates :address, presence: true, uniqueness: { case_sensitive: false }, email: true
+              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Uniqueness validation should be with a unique index.
+            end
+          RUBY
+        end
+      end
+    end
   end
 end