Add new Rails/RootJoinChain cop

Instead of chaining `#join` on `Rails.root` or `Rails.public_path`

```ruby
Rails.root.join('db').join('schema.rb')
Rails.root.join('db').join(migrate).join('migration.rb')
Rails.public_path.join('path').join('file.pdf')
Rails.public_path.join('path').join(to).join('file.pdf')
```

we can combine all arguments into a single join

```ruby
Rails.root.join('db', 'schema.rb')
Rails.root.join('db', migrate, 'migration.rb')
Rails.public_path.join('path', 'file.pdf')
Rails.public_path.join('path', to, 'file.pdf')
```

CHANGELOG.md

@@ -484,3 +484,4 @@
 [@theunraveler]: https://github.com/theunraveler
 [@pirj]: https://github.com/pirj
 [@vitormd]: https://github.com/vitormd
+[@leoarnold]: https://github.com/leoarnold

changelog/new_add_new_railsrootjoinchain_cop.md

@@ -0,0 +1 @@
+* [#586](https://github.com/rubocop/rubocop-rails/pull/586): Add new `Rails/RootJoinChain` cop. ([@leoarnold][])

config/default.yml

@@ -701,6 +701,11 @@ Rails/ReversibleMigrationMethodDefinition:
   Include:
     - db/migrate/*.rb
 
+Rails/RootJoinChain:
+  Description: 'Use a single `#join` instead of chaining on `Rails.root` or `Rails.public_path`.'
+  Enabled: pending
+  VersionAdded: '2.13'
+
 Rails/SafeNavigation:
   Description: "Use Ruby's safe navigation operator (`&.`) instead of `try!`."
   Enabled: true

lib/rubocop/cop/rails/root_join_chain.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Use a single `#join` instead of chaining on `Rails.root` or `Rails.public_path`.
+      #
+      # @example
+      #   # bad
+      #   Rails.root.join('db').join('schema.rb')
+      #   Rails.root.join('db').join(migrate).join('migration.rb')
+      #   Rails.public_path.join('path').join('file.pdf')
+      #   Rails.public_path.join('path').join(to).join('file.pdf')
+      #
+      #   # good
+      #   Rails.root.join('db', 'schema.rb')
+      #   Rails.root.join('db', migrate, 'migration.rb')
+      #   Rails.public_path.join('path', 'file.pdf')
+      #   Rails.public_path.join('path', to, 'file.pdf')
+      #
+      class RootJoinChain < Base
+        extend AutoCorrector
+
+        MSG = 'Use `Rails.%<root>s.join(...)` instead of chaining `#join` calls.'
+
+        RESTRICT_ON_SEND = %i[join].to_set.freeze
+
+        def_node_matcher :rails_root_join_chain, <<~PATTERN
+          (send
+            (send
+              $(send
+                (const {nil? cbase} :Rails) ${:root :public_path}) :join $...) :join $...)
+        PATTERN
+
+        def on_send(node)
+          rails_root_join_chain(node) do |rails_root, root, args1, args2|
+            add_offense(node, message: format(MSG, root: root)) do |corrector|
+              args = (args1 + args2).map(&:source).join(', ')
+
+              corrector.replace(node, "#{rails_root.source}.join(#{args})")
+            end
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -11,6 +11,7 @@
 require_relative 'rails/active_record_callbacks_order'
 require_relative 'rails/active_record_override'
 require_relative 'rails/active_support_aliases'
+require_relative 'rails/root_join_chain'
 require_relative 'rails/schema_comment'
 require_relative 'rails/add_column_index'
 require_relative 'rails/after_commit_override'

spec/rubocop/cop/rails/root_join_chain_spec.rb

@@ -0,0 +1,70 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::RootJoinChain, :config do
+  it 'does not register an offense for `Rails.root.join(...)`' do
+    expect_no_offenses(<<~RUBY)
+      Rails.root.join('db', 'schema.rb')
+    RUBY
+  end
+
+  it 'registers and offense and corrects for `::Rails.root.join(...).join(...)`' do
+    expect_offense(<<~RUBY)
+      ::Rails.root.join('db').join('sch' + 'ema.rb')
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Use `Rails.root.join(...)` instead of chaining `#join` calls.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      ::Rails.root.join('db', 'sch' + 'ema.rb')
+    RUBY
+  end
+
+  it 'registers and offense and corrects for `Rails.root.join(...).join(...)`' do
+    expect_offense(<<~RUBY)
+      Rails.root.join('db').join('sch' + 'ema.rb')
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Use `Rails.root.join(...)` instead of chaining `#join` calls.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Rails.root.join('db', 'sch' + 'ema.rb')
+    RUBY
+  end
+
+  it 'registers and offense and corrects for `Rails.root` with any number of joins greater one' do
+    expect_offense(<<~RUBY)
+      Rails.root.join.join.join('db').join(migrate).join.join("migration.\#{rb}")
+      ^^^^^^^^^^^^^^^^^^^^ Use `Rails.root.join(...)` instead of chaining `#join` calls.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Rails.root.join('db', migrate, "migration.\#{rb}")
+    RUBY
+  end
+
+  it 'does not register an offense for `Rails.public_path.join(...)`' do
+    expect_no_offenses(<<~RUBY)
+      Rails.public_path.join('path', 'file.pdf')
+    RUBY
+  end
+
+  it 'registers and offense and corrects for `Rails.public_path.join(...).join(...)`' do
+    expect_offense(<<~RUBY)
+      Rails.public_path.join('path').join('fi' + 'le.pdf')
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Use `Rails.public_path.join(...)` instead of chaining `#join` calls.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Rails.public_path.join('path', 'fi' + 'le.pdf')
+    RUBY
+  end
+
+  it 'registers and offense and corrects for `Rails.public_path` with any number of joins greater one' do
+    expect_offense(<<~RUBY)
+      Rails.public_path.join.join.join('path').join(to).join.join("file.\#{pdf}")
+      ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Use `Rails.public_path.join(...)` instead of chaining `#join` calls.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      Rails.public_path.join('path', to, "file.\#{pdf}")
+    RUBY
+  end
+end