Add new Rails/IndexNames cop

This cop enforces the standard index naming provided by Rails, allowing developers to enjoy a standardized schema.

A bonus impact of following this convention is that fully duplicated indexes are prevented by default.

Custom index names were once a required workaround for default names that were too long for various database implementations. That could happen because of long table names, long field names, or long lists of compound index keys.

However, that problem has been fully resolved. The current automated naming approach will deterministically abbreviate indexes that would have been too long before.

changelog/new_merge_pull_request_1378_from.md

@@ -0,0 +1 @@
+* [#1378](https://github.com/rubocop/rubocop-rails/pull/1378): Add new `Rails/IndexNames` cop. ([@corsonknowles][])

config/default.yml

@@ -624,6 +624,15 @@ Rails/IndexBy:
   VersionAdded: '2.5'
   VersionChanged: '2.8'
 
+Rails/IndexNames:
+  Description: 'Checks for and removes custom index names. Since 7.1, Rails can ensure unique index names without exceeding the length limit.'
+  Enabled: pending
+  StartAfterMigrationVersion: null
+  VersionAdded: <<next>>
+  VersionChanged: <<next>>
+  Include:
+    - db/**/*.rb
+
 Rails/IndexWith:
   Description: 'Prefer `index_with` over `each_with_object`, `to_h`, or `map`.'
   Enabled: true

lib/rubocop/cop/mixin/migrations_helper.rb

@@ -12,7 +12,7 @@ module MigrationsHelper
           (send
             (const (const {nil? cbase} :ActiveRecord) :Migration)
             :[]
-            (float _))
+            (float $_))
           _)
       PATTERN
 
@@ -21,6 +21,14 @@ def in_migration?(node)
           migration_class?(class_node)
         end
       end
+
+      def in_supported_migration?(node, minimum_version)
+        node.each_ancestor(:class).any? do |class_node|
+          migration_class?(class_node) do |version|
+            version >= minimum_version
+          end
+        end
+      end
     end
   end
 end

lib/rubocop/cop/rails/index_names.rb

@@ -0,0 +1,106 @@
+# typed: false
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Checks for custom index names in migrations.
+      #
+      # @example
+      #   # bad
+      #   class ExampleMigration < ActiveRecord::Migration[7.1]
+      #     def change
+      #       change_table :users do |t|
+      #         t.index [:email], name: 'index_custom_name'
+      #       end
+      #     end
+      #   end
+      #
+      #   # good
+      #   class ExampleMigration < ActiveRecord::Migration[7.1]
+      #     def change
+      #       create_table :users do |t|
+      #         t.index [:email]
+      #       end
+      #     end
+      #   end
+      class IndexNames < Base
+        include MigrationsHelper
+        include RangeHelp
+        extend AutoCorrector
+        extend TargetRailsVersion
+
+        minimum_target_rails_version 7.1
+
+        MSG = 'Avoid specifying a custom name for indexes. Let Rails handle the index name automatically.'
+        RESTRICT_ON_SEND = %i[index add_index].freeze
+
+        def on_send(node)
+          return unless node.last_argument&.hash_type?
+          return unless in_supported_migration?(node, 7.1)
+
+          name_pair = node.last_argument.pairs.find { |pair| pair.key.value == :name || pair.key.value == 'name' }
+          return unless name_pair
+
+          add_offense(node) do |corrector|
+            remove_name_argument(corrector, name_pair, node)
+          end
+        end
+
+        private
+
+        def remove_name_argument(corrector, name_pair, node)
+          range = name_argument_range(name_pair, node)
+          corrector.remove(range)
+          remove_extra_comma_and_space(corrector, range, node)
+        end
+
+        def name_argument_range(name_pair, node)
+          hash_node = name_pair.parent
+          if hash_node.pairs.size == 1
+            # If name: is the only argument, remove the entire hash
+            range_between(node.arguments[-2].source_range.end_pos, node.source_range.end_pos)
+          else
+            # Remove the name: argument and any preceding comma and space
+            start_pos = previous_comma_pos(name_pair) || name_pair.source_range.begin_pos
+            range_between(start_pos, name_pair.source_range.end_pos)
+          end
+        end
+
+        def previous_comma_pos(pair)
+          source = pair.parent.source
+          pair_start = pair.source_range.begin_pos - pair.parent.source_range.begin_pos
+          previous_content = source[0...pair_start]
+          comma_index = previous_content.rindex(',')
+          comma_index ? pair.parent.source_range.begin_pos + comma_index : nil
+        end
+
+        def remove_extra_comma_and_space(corrector, removed_range, node)
+          remaining_source = remaining_source(node, removed_range)
+          next_relevant_content = remaining_source.lstrip
+          range_to_remove = if next_relevant_content.start_with?(',')
+                              space_after_comma(removed_range, next_relevant_content)
+                            else
+                              leading_space(remaining_source, removed_range)
+                            end
+
+          corrector.remove(range_to_remove)
+        end
+
+        def remaining_source(node, removed_range)
+          node.source[removed_range.end_pos - node.source_range.begin_pos..]
+        end
+
+        def space_after_comma(removed_range, next_relevant_content)
+          space_after_comma = next_relevant_content[1..].match(/\A\s*/)[0]
+          range_between(removed_range.end_pos, removed_range.end_pos + 1 + space_after_comma.length)
+        end
+
+        def leading_space(remaining_source, removed_range)
+          leading_space = remaining_source.match(/\A\s*/)[0]
+          range_between(removed_range.end_pos, removed_range.end_pos + leading_space.length)
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -69,6 +69,7 @@
 require_relative 'rails/ignored_columns_assignment'
 require_relative 'rails/ignored_skip_action_filter_option'
 require_relative 'rails/index_by'
+require_relative 'rails/index_names'
 require_relative 'rails/index_with'
 require_relative 'rails/inquiry'
 require_relative 'rails/inverse_of'