Includes HABTM returns correct size now. It's caused by the join dependency

only instantiates one HABTM object because the join table hasn't a primary key.
Updated commit from @bigxiang commit dbaa837

Fixes #16032.

Examples:

    before:

    Project.first.salaried_developers.size # => 3
    Project.includes(:salaried_developers).first.salaried_developers.size # => 1

    after:

    Project.first.salaried_developers.size # => 3
    Project.includes(:salaried_developers).first.salaried_developers.size # => 3

1 files changed, 19 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb b/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb
index aea9207bfe..fb7630fc4a 100644
--- a/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb
+++ b/activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb
@@ -902,4 +902,23 @@ class HasAndBelongsToManyAssociationsTest < ActiveRecord::TestCase
       DeveloperWithSymbolClassName.new
     end
   end
+
+  def test_preloaded_associations_size
+    assert_equal Project.first.salaried_developers.size,
+      Project.preload(:salaried_developers).first.salaried_developers.size
+
+    assert_equal Project.includes(:salaried_developers).references(:salaried_developers).first.salaried_developers.size,
+      Project.preload(:salaried_developers).first.salaried_developers.size
+
+    # Nested HATBM
+    first_project = Developer.first.projects.first
+    preloaded_first_project =
+      Developer.preload(projects: :salaried_developers).
+        first.
+        projects.
+        detect { |p| p.id == first_project.id }
+
+    assert preloaded_first_project.salaried_developers.loaded?, true
+    assert_equal first_project.salaried_developers.size, preloaded_first_project.salaried_developers.size
+  end
 end