Perform a more efficient query in `Relation#any?`

This was changed in 421c81b, as `exists?` blows up if you are eager
loading a polymorphic association, as it'll try to construct a join to
that table. The previous change decided to execute a `count` instead,
which wouldn't join.

Of course, the only time we actually need to perform a join on the eager
loaded values (which would perform a left outer join) is if they're
being referenced in the where clause. This doesn't affect inner joins.

2 files changed, 8 insertions, 8 deletions

diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index 2cf19c76c5..6728443251 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -271,12 +271,7 @@ module ActiveRecord
     def empty?
       return @records.empty? if loaded?
 
-      if limit_value == 0
-        true
-      else
-        c = count(:all)
-        c.respond_to?(:zero?) ? c.zero? : c.empty?
-      end
+      limit_value == 0 || !exists?
     end
 
     # Returns true if there are no records.
diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index 435cef901b..b83032e2a0 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -280,8 +280,8 @@ module ActiveRecord
 
       return false if !conditions
 
-      relation = apply_join_dependency(self, construct_join_dependency)
-      return false if ActiveRecord::NullRelation === relation
+      relation = apply_join_dependency(self, construct_join_dependency_for_exists)
+      return false if ActiveRecord::NullRelation === relation || limit_value == 0
 
       relation = relation.except(:select, :order).select(ONE_AS_ONE).limit(1)
 
@@ -359,6 +359,11 @@ module ActiveRecord
       ActiveRecord::Associations::JoinDependency.new(@klass, including, joins)
     end
 
+    def construct_join_dependency_for_exists
+      including = (eager_load_values + includes_values) & references_values.map(&:to_sym)
+      ActiveRecord::Associations::JoinDependency.new(@klass, including, [])
+    end
+
     def construct_relation_for_association_calculations
       from = arel.froms.first
       if Arel::Table === from