Revert "Revert "Merge pull request rails#7661 from ernie/build-join-r…

…ecords-on-unsaved-hmt""

This reverts commit 18b9187.

activerecord/CHANGELOG.md

@@ -113,12 +113,6 @@
 
     *Andrew White*
 
-*   Revert creation of through association models when using `collection=[]`
-    on a `has_many :through` association from an unsaved model.
-    Fix #7661, #8269.
-
-    *Ernie Miller*
-
 *   Fix undefined method `to_i` when calling `new` on a scope that uses an
     Array; Fix FloatDomainError when setting integer column to NaN.
     Fixes #8718, #8734, #8757.

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -38,6 +38,20 @@ def concat(*records)
         super
       end
 
+      def concat_records(records)
+        ensure_not_nested
+
+        records = super
+
+        if owner.new_record? && records
+          records.flatten.each do |record|
+            build_through_record(record)
+          end
+        end
+
+        records
+      end
+
       def insert_record(record, validate = true, raise = false)
         ensure_not_nested
 

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -862,6 +862,11 @@ def test_save_should_not_raise_exception_when_join_record_has_errors
     end
   end
 
+  def test_assign_array_to_new_record_builds_join_records
+    c = Category.new(:name => 'Fishing', :authors => [Author.first])
+    assert_equal 1, c.categorizations.size
+  end
+
   def test_create_bang_should_raise_exception_when_join_record_has_errors
     repair_validations(Categorization) do
       Categorization.validate { |r| r.errors[:base] << 'Invalid Categorization' }