Do not validate associated records that haven't changed

Fix: rails#17621
Redo of: rails#18612

You should be able to create a reference to an invalid record as
long as you don't modify it.

This was initially fixed in rails#18612
but was reverted in rails@2c02bc0
because it was causing other bugs.

Some tests were added as part of the revert to avoid future regressions.

Author: byroot

activerecord/lib/active_record/autosave_association.rb

@@ -370,8 +370,14 @@ def validate_collection_association(reflection)
       # enabled records if they're marked_for_destruction? or destroyed.
       def association_valid?(association, record)
         return true if record.destroyed? || (association.options[:autosave] && record.marked_for_destruction?)
-
-        context = validation_context if custom_validation_context?
+        if custom_validation_context?
+          context = validation_context
+        else
+          # If the associated record is unchanged we shouldn't auto validate it.
+          # Even if a record is invalid you should still be able to create new references
+          # to it.
+          return true if !record.new_record? && !record.changed?
+        end
 
         unless valid = record.valid?(context)
           if association.options[:autosave]

activerecord/test/cases/autosave_association_test.rb

@@ -1490,6 +1490,17 @@ def test_should_skip_validation_on_habtm_if_destroyed
     assert_predicate @pirate, :valid?
   end
 
+  def test_should_skip_validation_on_habtm_if_persisted_and_unchanged
+    parrot = @pirate.parrots.create!(name: "parrots_1")
+    parrot.update_column(:name, "")
+    parrot.reload
+    assert_not_predicate parrot, :valid?
+
+    new_pirate = Pirate.new(catchphrase: "Arr")
+    new_pirate.parrots = @pirate.parrots
+    new_pirate.save!
+  end
+
   def test_a_child_marked_for_destruction_should_not_be_destroyed_twice_while_saving_habtm
     @pirate.parrots.create!(name: "parrots_1")