let AR::FinderMethods#exists? return singletons in all cases [closes #11592]

This fixes a regression. The documentation said in its introduction
paragraph that the method returns truthy/falsy, but then below it
was said that if there were no arguments you'd get `true` or `false`.
Also when the argument is exactly `false` a singleton is documented
to be returned.

The method was not returning the singletons so it didn't conform to
those special cases.

The best solution here seems to be to just return singletons in all
cases. This solution is backwards compatible. Also, the contract
has been revised because it has no sense that the predicate varies
that way depending on the input. I bet the previous contract was just
an accident, not something mixed on purpose.

Conflicts:
	activerecord/lib/active_record/relation/finder_methods.rb
	activerecord/test/cases/finder_test.rb

1 files changed, 29 insertions, 27 deletions

diff --git a/activerecord/test/cases/finder_test.rb b/activerecord/test/cases/finder_test.rb
index 51a8a13d04..949e994f8d 100644
--- a/activerecord/test/cases/finder_test.rb
+++ b/activerecord/test/cases/finder_test.rb
@@ -45,17 +45,18 @@ class FinderTest < ActiveRecord::TestCase
   end
 
   def test_exists
-    assert Topic.exists?(1)
-    assert Topic.exists?("1")
-    assert Topic.exists?(title: "The First Topic")
-    assert Topic.exists?(heading: "The First Topic")
-    assert Topic.exists?(:author_name => "Mary", :approved => true)
-    assert Topic.exists?(["parent_id = ?", 1])
-    assert !Topic.exists?(45)
-    assert !Topic.exists?(Topic.new)
+    assert_equal true, Topic.exists?(1)
+    assert_equal true, Topic.exists?("1")
+    assert_equal true, Topic.exists?(title: "The First Topic")
+    assert_equal true, Topic.exists?(heading: "The First Topic")
+    assert_equal true, Topic.exists?(:author_name => "Mary", :approved => true)
+    assert_equal true, Topic.exists?(["parent_id = ?", 1])
+
+    assert_equal false, Topic.exists?(45)
+    assert_equal false, Topic.exists?(Topic.new)
 
     begin
-      assert !Topic.exists?("foo")
+      assert_equal false, Topic.exists?("foo")
     rescue ActiveRecord::StatementInvalid
       # PostgreSQL complains about string comparison with integer field
     rescue Exception
@@ -72,61 +73,62 @@ class FinderTest < ActiveRecord::TestCase
   end
 
   def test_exists_returns_true_with_one_record_and_no_args
-    assert Topic.exists?
+    assert_equal true, Topic.exists?
   end
 
   def test_exists_returns_false_with_false_arg
-    assert !Topic.exists?(false)
+    assert_equal false, Topic.exists?(false)
   end
 
   # exists? should handle nil for id's that come from URLs and always return false
   # (example: Topic.exists?(params[:id])) where params[:id] is nil
   def test_exists_with_nil_arg
-    assert !Topic.exists?(nil)
-    assert Topic.exists?
-    assert !Topic.first.replies.exists?(nil)
-    assert Topic.first.replies.exists?
+    assert_equal false, Topic.exists?(nil)
+    assert_equal true, Topic.exists?
+
+    assert_equal false, Topic.first.replies.exists?(nil)
+    assert_equal true, Topic.first.replies.exists?
   end
 
   # ensures +exists?+ runs valid SQL by excluding order value
   def test_exists_with_order
-    assert Topic.order(:id).distinct.exists?
+    assert_equal true, Topic.order(:id).distinct.exists?
   end
 
   def test_exists_with_includes_limit_and_empty_result
-    assert !Topic.includes(:replies).limit(0).exists?
-    assert !Topic.includes(:replies).limit(1).where('0 = 1').exists?
+    assert_equal false, Topic.includes(:replies).limit(0).exists?
+    assert_equal false, Topic.includes(:replies).limit(1).where('0 = 1').exists?
   end
 
   def test_exists_with_distinct_association_includes_and_limit
     author = Author.first
-    assert !author.unique_categorized_posts.includes(:special_comments).limit(0).exists?
-    assert author.unique_categorized_posts.includes(:special_comments).limit(1).exists?
+    assert_equal false, author.unique_categorized_posts.includes(:special_comments).limit(0).exists?
+    assert_equal true, author.unique_categorized_posts.includes(:special_comments).limit(1).exists?
   end
 
   def test_exists_with_distinct_association_includes_limit_and_order
     author = Author.first
-    assert !author.unique_categorized_posts.includes(:special_comments).order('comments.taggings_count DESC').limit(0).exists?
-    assert author.unique_categorized_posts.includes(:special_comments).order('comments.taggings_count DESC').limit(1).exists?
+    assert_equal false, author.unique_categorized_posts.includes(:special_comments).order('comments.taggings_count DESC').limit(0).exists?
+    assert_equal true, author.unique_categorized_posts.includes(:special_comments).order('comments.taggings_count DESC').limit(1).exists?
   end
 
   def test_exists_with_empty_table_and_no_args_given
     Topic.delete_all
-    assert !Topic.exists?
+    assert_equal false, Topic.exists?
   end
 
   def test_exists_with_aggregate_having_three_mappings
     existing_address = customers(:david).address
-    assert Customer.exists?(:address => existing_address)
+    assert_equal true, Customer.exists?(:address => existing_address)
   end
 
   def test_exists_with_aggregate_having_three_mappings_with_one_difference
     existing_address = customers(:david).address
-    assert !Customer.exists?(:address =>
+    assert_equal false, Customer.exists?(:address =>
       Address.new(existing_address.street, existing_address.city, existing_address.country + "1"))
-    assert !Customer.exists?(:address =>
+    assert_equal false, Customer.exists?(:address =>
       Address.new(existing_address.street, existing_address.city + "1", existing_address.country))
-    assert !Customer.exists?(:address =>
+    assert_equal false, Customer.exists?(:address =>
       Address.new(existing_address.street + "1", existing_address.city, existing_address.country))
   end