From f4fd7d1d20dfbc4389435a8f0f153f7960bb1544 Mon Sep 17 00:00:00 2001
From: Rob Trame <Redapted@users.noreply.github.com>
Date: Wed, 1 May 2019 13:53:35 -0700
Subject: Make scope arity check consistent (#36134)
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* Make scope arity check consistent

* Add test for arity change

[Rob Trame + Rafael Mendonça França]
---
 activerecord/lib/active_record/reflection.rb       |  2 +-
 activerecord/test/cases/associations/eager_test.rb | 18 ++++++++++++++++++
 activerecord/test/models/author.rb                 |  1 +
 3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 1312bf6f91..eefda2b8f4 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -477,7 +477,7 @@ module ActiveRecord
       def check_preloadable!
         return unless scope
 
-        if scope.arity > 0
+        unless scope.arity == 0
           raise ArgumentError, <<-MSG.squish
             The association scope '#{name}' is instance dependent (the scope
             block takes an argument). Preloading instance dependent scopes is
diff --git a/activerecord/test/cases/associations/eager_test.rb b/activerecord/test/cases/associations/eager_test.rb
index b865002f03..49bcb1d010 100644
--- a/activerecord/test/cases/associations/eager_test.rb
+++ b/activerecord/test/cases/associations/eager_test.rb
@@ -1516,6 +1516,24 @@ class EagerAssociationTest < ActiveRecord::TestCase
     assert_match message, error.message
   end
 
+  test "preloading and eager loading of optional instance dependent associations is not supported" do
+    message = "association scope 'posts_mentioning_author' is"
+    error = assert_raises(ArgumentError) do
+      Author.includes(:posts_mentioning_author).to_a
+    end
+    assert_match message, error.message
+
+    error = assert_raises(ArgumentError) do
+      Author.preload(:posts_mentioning_author).to_a
+    end
+    assert_match message, error.message
+
+    error = assert_raises(ArgumentError) do
+      Author.eager_load(:posts_mentioning_author).to_a
+    end
+    assert_match message, error.message
+  end
+
   test "preload with invalid argument" do
     exception = assert_raises(ArgumentError) do
       Author.preload(10).to_a
diff --git a/activerecord/test/models/author.rb b/activerecord/test/models/author.rb
index 67be59a1fe..b52b643ad7 100644
--- a/activerecord/test/models/author.rb
+++ b/activerecord/test/models/author.rb
@@ -153,6 +153,7 @@ class Author < ActiveRecord::Base
   has_many :comments_on_posts_with_default_include, through: :posts_with_default_include, source: :comments
 
   has_many :posts_with_signature, ->(record) { where("posts.title LIKE ?", "%by #{record.name.downcase}%") }, class_name: "Post"
+  has_many :posts_mentioning_author, ->(record = nil) { where("posts.body LIKE ?", "%#{record&.name&.downcase}%") }, class_name: "Post"
 
   has_many :posts_with_extension, -> { order(:title) }, class_name: "Post" do
     def extension_method; end
-- 
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