diff --git a/changelog/new_delegate_private_cop.md b/changelog/new_delegate_private_cop.md
new file mode 100644
index 0000000000..c4f869a1a2
--- /dev/null
+++ b/changelog/new_delegate_private_cop.md
@@ -0,0 +1 @@
+* [#368](https://github.com/rubocop/rubocop-rails/issues/368): Add DelegatePrivate cop. ([@povilasjurcys][])
diff --git a/config/default.yml b/config/default.yml
index 704ba72ca2..a68f844343 100644
--- a/config/default.yml
+++ b/config/default.yml
@@ -336,6 +336,11 @@ Rails/DelegateAllowBlank:
Enabled: true
VersionAdded: '0.44'
+Rails/DelegatePrivate:
+ Description: 'Use delegate with `private: true` option in private scope.'
+ Enabled: true
+ VersionAdded: '2.20'
+
Rails/DeprecatedActiveModelErrorsMethods:
Description: 'Avoid manipulating ActiveModel errors hash directly.'
Enabled: pending
diff --git a/lib/rubocop/cop/rails/delegate_private.rb b/lib/rubocop/cop/rails/delegate_private.rb
new file mode 100644
index 0000000000..314489c382
--- /dev/null
+++ b/lib/rubocop/cop/rails/delegate_private.rb
@@ -0,0 +1,103 @@
+# frozen_string_literal: true
+
+module RuboCop
+ module Cop
+ module Rails
+ # Looks for `delegate` in private section without `private: true` option.
+ #
+ # @example
+ # # bad
+ # private
+ # delegate :baz, to: :bar
+ #
+ # # bad
+ # delegate :baz, to: :bar, private: true
+ #
+ # # good
+ # private
+ # delegate :baz, to: :bar, private: true
+ class DelegatePrivate < Base
+ extend TargetRailsVersion
+
+ MSG_MISSING_PRIVATE = '`delegate` in private section should have `private: true` option'
+ MSG_WRONG_PRIVATE = 'private `delegate` should be put in private section'
+
+ minimum_target_rails_version 6.0
+
+ def on_send(node)
+ mark_scope(node)
+ return unless delegate_node?(node)
+
+ if private_scope?(node) && !private_delegate?(node)
+ add_offense(node, message: MSG_MISSING_PRIVATE)
+ elsif public_scope?(node) && private_delegate?(node)
+ add_offense(node, message: MSG_WRONG_PRIVATE)
+ end
+ end
+
+ def on_class(node)
+ cut_from_private_range(node.location.first_line..node.location.last_line)
+ end
+
+ private
+
+ def private_delegate?(node)
+ node.arguments.select(&:hash_type?).each do |hash_node|
+ hash_node.each_pair do |key_node, value_node|
+ return true if key_node.value == :private && value_node.true_type?
+ end
+ end
+
+ false
+ end
+
+ def mark_scope(node)
+ return if node.receiver || !node.arguments.empty?
+
+ @private_ranges ||= []
+
+ scope_range = node.parent.location.first_line..node.parent.location.last_line
+ if node.method?(:private)
+ add_to_private_range(scope_range)
+ elsif node.method?(:public)
+ cut_from_private_range(scope_range)
+ end
+ end
+
+ def delegate_node?(node)
+ return false if node.receiver
+
+ node.method?(:delegate)
+ end
+
+ def private_scope?(node)
+ @private_ranges&.any? { |range| range.include?(node.location.first_line) }
+ end
+
+ def public_scope?(node)
+ !private_scope?(node)
+ end
+
+ def add_to_private_range(scope_range)
+ @private_ranges ||= []
+ @private_ranges += [scope_range]
+ end
+
+ def cut_from_private_range(scope_range)
+ @private_ranges ||= []
+
+ @private_ranges = @private_ranges.each.with_object([]) do |private_range, new_ranges|
+ next if scope_range.cover?(private_range)
+
+ if private_range.cover?(scope_range)
+ new_ranges << (private_range.begin...scope_range.begin)
+ new_ranges << ((scope_range.end + 1)..private_range.end)
+ else
+ new_ranges << private_range
+ end
+ end
+ end
+ end
+ end
+ end
+end
diff --git a/lib/rubocop/cop/rails_cops.rb b/lib/rubocop/cop/rails_cops.rb
index 5e0db983d6..3577cbf3a3 100644
--- a/lib/rubocop/cop/rails_cops.rb
+++ b/lib/rubocop/cop/rails_cops.rb
@@ -36,6 +36,7 @@
require_relative 'rails/default_scope'
require_relative 'rails/delegate'
require_relative 'rails/delegate_allow_blank'
+require_relative 'rails/delegate_private'
require_relative 'rails/deprecated_active_model_errors_methods'
require_relative 'rails/dot_separated_keys'
require_relative 'rails/duplicate_association'
diff --git a/spec/rubocop/cop/rails/delegate_private_spec.rb b/spec/rubocop/cop/rails/delegate_private_spec.rb
new file mode 100644
index 0000000000..9b22962ecf
--- /dev/null
+++ b/spec/rubocop/cop/rails/delegate_private_spec.rb
@@ -0,0 +1,131 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::DelegatePrivate, :config, :rails60 do
+ context 'when no delegate is provided' do
+ it 'registers no offense' do
+ expect_no_offenses(<<~RUBY)
+ class User
+ end
+ RUBY
+ end
+ end
+
+ context 'when delegate is provided in public scope without "private: true"' do
+ it 'registers no offense' do
+ expect_no_offenses(<<~RUBY)
+ class User
+ delegate :name, to: :user
+ end
+ RUBY
+ end
+ end
+
+ context 'when delegate is provided in public scope with "private: true"' do
+ it 'registers an offense' do
+ expect_offense(<<~RUBY)
+ class User
+ delegate :name, to: :user, private: true
+ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ private `delegate` should be put in private section
+ end
+ RUBY
+ end
+ end
+
+ context 'when delegate is provided in private scope without "private: true"' do
+ it 'registers an offense' do
+ expect_offense(<<~RUBY)
+ class User
+ private
+
+ delegate :name, to: :user
+ ^^^^^^^^^^^^^^^^^^^^^^^^^ `delegate` in private section should have `private: true` option
+ end
+ RUBY
+ end
+ end
+
+ context 'when delegate is provided in private scope with "private: true"' do
+ it 'registers no offense' do
+ expect_no_offenses(<<~RUBY)
+ class User
+ private
+
+ delegate :name, to: :user, private: true
+ end
+ RUBY
+ end
+ end
+
+ context 'when delegate is provided in public scope without "private: true" in outer class' do
+ it 'registers no offense' do
+ expect_no_offenses(<<~RUBY)
+ class User
+ class InnerUser
+ private
+ def foo; end
+ end
+
+ delegate :name, to: :user
+ end
+ RUBY
+ end
+ end
+
+ context 'when inner class is put in private scope and has delegate in public scope without "private: true"' do
+ it 'does not register an offense' do
+ expect_no_offenses(<<~RUBY)
+ class User
+ private
+
+ class InnerUser
+ delegate :name, to: :user
+ end
+
+ delegate :foo, to: :bar, private: true
+ end
+ RUBY
+ end
+ end
+
+ context 'when `private: true` is in explicit public scope' do
+ it 'registers no offense' do
+ expect_no_offenses(<<~RUBY)
+ class User
+ private
+
+ def foo
+ end
+
+ public
+
+ delegate :name, to: :user
+ end
+ RUBY
+ end
+ end
+
+ context 'when private scope is set on method, but `private: true` is used in public scope' do
+ it 'registers no offense' do
+ expect_no_offenses(<<~RUBY)
+ class User
+ private def foo
+ end
+
+ delegate :name, to: :user
+ end
+ RUBY
+ end
+ end
+
+ context 'with rails < 6.0', :rails52 do
+ it 'registers no offense' do
+ expect_no_offenses(<<~RUBY)
+ class User
+ private
+
+ delegate :name, to: :user
+ end
+ RUBY
+ end
+ end
+end