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, 7 insertions, 7 deletions

diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index 2d3bd563ac..0132a02f83 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -171,21 +171,21 @@ module ActiveRecord
       last or raise RecordNotFound
     end
 
-    # Returns truthy if a record exists in the table that matches the +id+ or
-    # conditions given, or falsy otherwise. The argument can take six forms:
+    # Returns +true+ if a record exists in the table that matches the +id+ or
+    # conditions given, or +false+ otherwise. The argument can take six forms:
     #
     # * Integer - Finds the record with this primary key.
     # * String - Finds the record with a primary key corresponding to this
     #   string (such as <tt>'5'</tt>).
     # * Array - Finds the record that matches these +find+-style conditions
-    #   (such as <tt>['color = ?', 'red']</tt>).
+    #   (such as <tt>['name LIKE ?', "%#{query}%"]</tt>).
     # * Hash - Finds the record that matches these +find+-style conditions
-    #   (such as <tt>{color: 'red'}</tt>).
+    #   (such as <tt>{name: 'David'}</tt>).
     # * +false+ - Returns always +false+.
     # * No args - Returns +false+ if the table is empty, +true+ otherwise.
     #
-    # For more information about specifying conditions as a Hash or Array,
-    # see the Conditions section in the introduction to ActiveRecord::Base.
+    # For more information about specifying conditions as a hash or array,
+    # see the Conditions section in the introduction to <tt>ActiveRecord::Base</tt>.
     #
     # Note: You can't pass in a condition as a string (like <tt>name =
     # 'Jamie'</tt>), since it would be sanitized and then queried against
@@ -213,7 +213,7 @@ module ActiveRecord
         relation = relation.where(table[primary_key].eq(conditions)) if conditions != :none
       end
 
-      connection.select_value(relation.arel, "#{name} Exists", relation.bind_values)
+      connection.select_value(relation, "#{name} Exists", relation.bind_values) ? true : false
     end
 
     # This method is called whenever no records are found with either a single