Exists shouldn't error when used with `includes`

Currently `exists?` does some hackery where it assumes that we can join
onto anything that we passed to `eager_load` or `includes`, which
doesn't work if we are joining onto a polymorphic association.

Actually figuring out if we want to include something would require
knowledge deep within the join dependency module, which is hard to pull
up. The simplest solution is just to pass a flag down that says we're
not actually going to try to eager load any of the data. It's not the
solution I'd like, but that code really needs to be untangled before we
can do much with it.

This is another attempt at 6d5b1fd which should address the concerns
that led to reverting it in 4ecabed.

4 files changed, 9 insertions, 11 deletions

diff --git a/activerecord/lib/active_record/associations/join_dependency.rb b/activerecord/lib/active_record/associations/join_dependency.rb
index b94feeff12..491152bbcb 100644
--- a/activerecord/lib/active_record/associations/join_dependency.rb
+++ b/activerecord/lib/active_record/associations/join_dependency.rb
@@ -92,8 +92,9 @@ module ActiveRecord
       #    associations # => [:appointments]
       #    joins # =>  []
       #
-      def initialize(base, associations, joins)
+      def initialize(base, associations, joins, eager_loading: true)
         @alias_tracker = AliasTracker.create_with_joins(base.connection, base.table_name, joins, base.type_caster)
+        @eager_loading = eager_loading
         tree = self.class.make_tree associations
         @join_root = JoinBase.new base, build(tree, base)
         @join_root.children.each { |child| construct_tables! @join_root, child }
@@ -238,11 +239,12 @@ module ActiveRecord
           reflection.check_eager_loadable!
 
           if reflection.polymorphic?
+            next unless @eager_loading
             raise EagerLoadPolymorphicError.new(reflection)
           end
 
           JoinAssociation.new reflection, build(right, reflection.klass)
-        end
+        end.compact
       end
 
       def construct(ar_parent, parent, row, rs, seen, model_cache, aliases)
diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index d042fe5f8b..7a1552856b 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -279,12 +279,7 @@ 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 e7e331f88d..87eea8277a 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -318,7 +318,7 @@ module ActiveRecord
 
       return false if !conditions
 
-      relation = apply_join_dependency(self, construct_join_dependency)
+      relation = apply_join_dependency(self, construct_join_dependency(eager_loading: false))
       return false if ActiveRecord::NullRelation === relation
 
       relation = relation.except(:select, :order).select(ONE_AS_ONE).limit(1)
@@ -392,9 +392,9 @@ module ActiveRecord
       end
     end
 
-    def construct_join_dependency(joins = [])
+    def construct_join_dependency(joins = [], eager_loading: true)
       including = eager_load_values + includes_values
-      ActiveRecord::Associations::JoinDependency.new(@klass, including, joins)
+      ActiveRecord::Associations::JoinDependency.new(@klass, including, joins, eager_loading: eager_loading)
     end
 
     def construct_relation_for_association_calculations
diff --git a/activerecord/test/cases/associations/eager_test.rb b/activerecord/test/cases/associations/eager_test.rb
index 7f2a2229ee..f7bb3e54d5 100644
--- a/activerecord/test/cases/associations/eager_test.rb
+++ b/activerecord/test/cases/associations/eager_test.rb
@@ -1341,6 +1341,7 @@ class EagerAssociationTest < ActiveRecord::TestCase
 
     assert_nothing_raised do
       authors(:david).essays.includes(:writer).any?
+      authors(:david).essays.includes(:writer).exists?
     end
   end