From bf2bc4ccb43baf3f6e2f7af651708dd8f468286c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?J=C3=B4natas=20Davi=20Paganini?=
 <jonatas.paganini@toptal.com>
Date: Thu, 13 Dec 2018 16:14:00 -0200
Subject: [PATCH] Skip offense when defining blank method

---
 CHANGELOG.md                         |  1 +
 lib/rubocop/cop/rails/blank.rb       | 10 ++++++++++
 manual/cops_rails.md                 |  5 +++++
 spec/rubocop/cop/rails/blank_spec.rb |  4 ++++
 4 files changed, 20 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 993809e4e34..1405c12cd15 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@
 
 * [#4229](https://github.com/rubocop-hq/rubocop/issues/4229): Fix unexpected Style/HashSyntax consistency offence. ([@timon][])
 * [#6500](https://github.com/rubocop-hq/rubocop/issues/6500): Add offense to use `in_time_zone` instead of deprecated `to_time_in_current_zone`. ([@nadiyaka][])
+* [#6577](https://github.com/rubocop-hq/rubocop/pull/6577): Prevent Rails/Blank cop from adding offense when define the blank method. ([@jonatas][])
 * [#6554](https://github.com/rubocop-hq/rubocop/issues/6554): Prevent Layout/RescueEnsureAlignment cop from breaking on block assignment when assignment is on a separate line. ([@timmcanty][])
 * [#6343](https://github.com/rubocop-hq/rubocop/pull/6343): Optimise `--auto-gen-config` when `Metrics/LineLength` cop is disabled. ([@tom-lord][])
 * [#6389](https://github.com/rubocop-hq/rubocop/pull/6389): Fix false negative for `Style/TrailingCommaInHashLitera`/`Style/TrailingCommaInArrayLiteral` when there is a comment in the last line. ([@bayandin][])
diff --git a/lib/rubocop/cop/rails/blank.rb b/lib/rubocop/cop/rails/blank.rb
index 7f1f2f6536a..28c6c4f6d3f 100644
--- a/lib/rubocop/cop/rails/blank.rb
+++ b/lib/rubocop/cop/rails/blank.rb
@@ -43,6 +43,11 @@ module Rails
       #   if foo.blank?
       #     something
       #   end
+      #
+      #   # good
+      #   def blank?
+      #     !present?
+      #   end
       class Blank < Cop
         MSG_NIL_OR_EMPTY = 'Use `%<prefer>s` instead of `%<current>s`.'.freeze
         MSG_NOT_PRESENT = 'Use `%<prefer>s` instead of `%<current>s`.'.freeze
@@ -70,6 +75,8 @@ class Blank < Cop
 
         def_node_matcher :not_present?, '(send (send $_ :present?) :!)'
 
+        def_node_matcher :defining_blank?, '(def :blank? (args) ...)'
+
         def_node_matcher :unless_present?, <<-PATTERN
           (:if $(send $_ :present?) {nil? (...)} ...)
         PATTERN
@@ -78,6 +85,9 @@ def on_send(node)
           return unless cop_config['NotPresent']
 
           not_present?(node) do |receiver|
+            # accepts !present? if its in the body of a `blank?` method
+            next if defining_blank?(node.parent)
+
             add_offense(node,
                         message: format(MSG_NOT_PRESENT,
                                         prefer: replacement(receiver),
diff --git a/manual/cops_rails.md b/manual/cops_rails.md
index e6b9c642b8f..39bddfd390b 100644
--- a/manual/cops_rails.md
+++ b/manual/cops_rails.md
@@ -216,6 +216,11 @@ end
 if foo.blank?
   something
 end
+
+# good
+def blank?
+  !present?
+end
 ```
 
 ### Configurable attributes
diff --git a/spec/rubocop/cop/rails/blank_spec.rb b/spec/rubocop/cop/rails/blank_spec.rb
index 16a1477fab3..ae98e7780d5 100644
--- a/spec/rubocop/cop/rails/blank_spec.rb
+++ b/spec/rubocop/cop/rails/blank_spec.rb
@@ -129,6 +129,10 @@
     it_behaves_like 'offense', '!present?',
                     'blank?',
                     'Use `blank?` instead of `!present?`.'
+
+    it 'accepts !present? if its in the body of a `blank?` method' do
+      expect_no_offenses('def blank?; !present? end')
+    end
   end
 
   context 'UnlessPresent set to true' do