Ensure that `ids_reader` respects dirty target whether target is loaded or not

Currently `ids_reader` doesn't respect dirty target when the target is
not loaded yet unlike `collection.size`. I believe the inconsistency is
a bug, fixes the `ids_reader` to behave consistently regardless of
whether target is loaded or not.

2 files changed, 5 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/associations/collection_association.rb b/activerecord/lib/active_record/associations/collection_association.rb
index f233947fbd..d61d105544 100644
--- a/activerecord/lib/active_record/associations/collection_association.rb
+++ b/activerecord/lib/active_record/associations/collection_association.rb
@@ -45,6 +45,8 @@ module ActiveRecord
       def ids_reader
         if loaded?
           target.pluck(reflection.association_primary_key)
+        elsif !target.empty?
+          load_target.pluck(reflection.association_primary_key)
         else
           @association_ids ||= scope.pluck(reflection.association_primary_key)
         end
@@ -212,7 +214,7 @@ module ActiveRecord
       def size
         if !find_target? || loaded?
           target.size
-        elsif @association_ids && target.empty?
+        elsif @association_ids
           @association_ids.size
         elsif !association_scope.group_values.empty?
           load_target.size
diff --git a/activerecord/test/cases/associations/has_many_associations_test.rb b/activerecord/test/cases/associations/has_many_associations_test.rb
index feb7390e2c..2e0a7ab1ab 100644
--- a/activerecord/test/cases/associations/has_many_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_associations_test.rb
@@ -989,7 +989,7 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     assert_equal 0, post.readers.size
     post.readers.reset
     post.readers.build
-    assert_equal [], post.reader_ids
+    assert_equal [nil], post.reader_ids
     assert_equal 1, post.readers.size
   end
 
@@ -999,7 +999,7 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     assert_empty post.readers
     post.readers.reset
     post.readers.build
-    assert_equal [], post.reader_ids
+    assert_equal [nil], post.reader_ids
     assert_not_empty post.readers
   end