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

activerecord/CHANGELOG.md

@@ -1,3 +1,22 @@
+*   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.
+
+    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
+
+    *Bigxiang*
+
 *   Fix accessing of fixtures having non-string labels like Fixnum.
 
     *Prathamesh Sonpatki*

activerecord/lib/active_record/associations/join_dependency.rb

@@ -150,7 +150,8 @@ def instantiate(result_set, aliases)
 
         message_bus.instrument('instantiation.active_record', payload) do
           result_set.each { |row_hash|
-            parent = parents[row_hash[primary_key]] ||= join_root.instantiate(row_hash, column_aliases)
+            parent_key = primary_key ? row_hash[primary_key] : row_hash
+            parent = parents[parent_key] ||= join_root.instantiate(row_hash, column_aliases)
             construct(parent, join_root, row_hash, result_set, seen, model_cache, aliases)
           }
         end
@@ -232,7 +233,7 @@ def build(associations, base_klass)
       end
 
       def construct(ar_parent, parent, row, rs, seen, model_cache, aliases)
-        primary_id  = ar_parent.id
+        primary_id  = ar_parent.id || row
 
         parent.children.each do |node|
           if node.reflection.collection?

activerecord/test/cases/associations/has_and_belongs_to_many_associations_test.rb

@@ -902,4 +902,23 @@ def test_with_symbol_class_name
       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