Fix for has_and_belongs_to_many & has_many_through associations while partial_writes is false

This will fix #19663

Also with this fix, active record does not fire unnecassary update queries while partial_writes is true

3 files changed, 24 insertions, 2 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index f13a80195a..f7fdb6755d 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Improved partial writes with HABTM and has many through associations
+    to fire database query only if relation has been changed.
+
+    Fixes #19663.
+
+    *Mehmet Emin İNAÇ*
+
 *   Rework `ActiveRecord::Relation#last` 
     
     1. Never perform additional SQL on loaded relation
@@ -18,7 +25,6 @@
 
     *Bogdan Gusiev*
 
-
 *   Allow `joins` to be unscoped.
 
     Closes #13775.
diff --git a/activerecord/lib/active_record/associations/has_many_through_association.rb b/activerecord/lib/active_record/associations/has_many_through_association.rb
index 36fc381343..7ddb96db13 100644
--- a/activerecord/lib/active_record/associations/has_many_through_association.rb
+++ b/activerecord/lib/active_record/associations/has_many_through_association.rb
@@ -86,7 +86,10 @@ module ActiveRecord
         end
 
         def save_through_record(record)
-          build_through_record(record).save!
+          association = build_through_record(record)
+          if association.changed?
+            association.save!
+          end
         ensure
           @through_records.delete(record.object_id)
         end
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 5c4586da19..cab9dff6ca 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
@@ -982,4 +982,17 @@ class HasAndBelongsToManyAssociationsTest < ActiveRecord::TestCase
       Project.first.developers_required_by_default.create!(name: "Sean", salary: 50000)
     end
   end
+
+  def test_has_and_belongs_to_many_while_partial_writes_false
+    begin
+      original_partial_writes = ActiveRecord::Base.partial_writes
+      ActiveRecord::Base.partial_writes = false
+      developer = Developer.new(name: "Mehmet Emin İNAÇ")
+      developer.projects << Project.new(name: "Bounty")
+
+      assert developer.save
+    ensure
+      ActiveRecord::Base.partial_writes = original_partial_writes
+    end
+  end
 end