MONGOID-4160 Don't persist document if associated document is invalid with optional: false (#5303)

* MONGOID-4160 what happens if you use validations on autosave?

* Update lib/mongoid/association/referenced/auto_save.rb

* MONGOID-4160 attempt to reject invalid foreign keys

* MONGOID-4160 if it's not persisted

* MONGOID-4160 undo changes

* MONGOID-4160 don't persist document if associated document is invalid

* MONGOID-4160 release notes

* MONGOID-4160 update docs

Author: neilshweky

docs/release-notes/mongoid-8.0.txt

@@ -352,3 +352,59 @@ Removed ``Array#update_values`` and ``Hash#update_values`` methods
 
 The previously deprecated ``Array#update_values`` and ``Hash#update_values``
 methods have been removed in Mongoid 8.
+
+
+No Longer Persisting Documents with Invalid ``belongs_to`` Associations
+-----------------------------------------------------------------------
+
+In Mongoid 8, if an invalid document is assigned to a ``belongs_to`` association,
+and the base document is saved, if the ``belongs_to`` association had the
+option ``optional: false`` or ``optional`` is unset and the global flag
+``require_belongs_to_by_default`` is true, neither the document nor the
+associated document will be persisted. For example, given the following
+models:
+
+.. code::
+
+  class Parent
+    include Mongoid::Document
+    has_one :child
+    field :name
+    validates :name, presence: true
+  end
+
+  class Child
+    include Mongoid::Document
+
+    belongs_to :parent, autosave: true, validate: false, optional: false
+  end
+
+  child = Child.new
+  parent = Parent.new
+  child.parent = parent # parent is invalid, it does not have a name
+  child.save
+
+In this case, both the child and the parent will not be persisted.
+
+.. note::
+  If ``save!`` were called, a validation error would be raised.
+
+If optional is false, then the Child will be persisted with a parent_id, but the
+parent will not be persisted:
+
+.. code::
+
+  child = Child.new
+  parent = Parent.new
+  child.parent = parent # parent is invalid, it does not have a name
+  child.save
+
+  p Child.first
+  # => <Child _id: 629a50b0d1327aad89d214d2, parent_id: BSON::ObjectId('629a50b0d1327aad89d214d3')>
+  p Parent.first
+  # => nil
+
+If you want the functionality of neither document being persisted in Mongoid 7 or 8
+and earlier, the ``validate: true`` option can be set on the association. If
+you want the functionality of only the Child being persisted, the ``validate:
+false`` option can be set on the association.

lib/mongoid/association/referenced/belongs_to.rb

@@ -153,7 +153,7 @@ def setup_instance_methods!
           create_foreign_key_field!
           setup_index!
           define_touchable!
-          @owner_class.validates_associated(name) if validate?
+          @owner_class.validates_associated(name) if validate? || require_association?
           @owner_class.validates(name, presence: true) if require_association?
         end
 

spec/mongoid/association/referenced/has_one/proxy_spec.rb

@@ -1148,4 +1148,101 @@
       expect(Vehicle.aliased_fields["driver"]).to eq("driver_id")
     end
   end
+
+  context "when the document is not persisted and the association is invalid" do
+
+    before do
+      class HomParent
+        include Mongoid::Document
+        has_one :child, class_name: "HomChild"
+        field :name
+        validates :name, presence: true
+      end
+
+      class HomChild; include Mongoid::Document; end
+
+      belongs_to
+      child.parent = parent
+      child.save
+    end
+
+    after do
+      Object.send(:remove_const, :HomParent)
+      Object.send(:remove_const, :HomChild)
+    end
+
+    let(:belongs_to) do
+      HomChild.belongs_to :parent, autosave: true, validate: false, optional: optional
+    end
+
+    let(:child) { HomChild.new }
+    let(:parent) { HomParent.new }
+
+
+    context "when belongs_to_required_by_default is true" do
+      config_override :belongs_to_required_by_default, true
+
+      context "when optional is true" do
+        let(:optional) { true }
+
+        it "persists the child with the parent_id" do
+          expect(HomChild.first.parent_id).to eq(parent._id)
+          expect(HomParent.count).to eq(0)
+        end
+      end
+
+      context "when optional is false" do
+        let(:optional) { false }
+
+        it "doesn't persist the parent or the child" do
+          expect(HomChild.count).to eq(0)
+          expect(HomParent.count).to eq(0)
+        end
+      end
+
+      context "when optional is not set" do
+        let(:belongs_to) do
+          HomChild.belongs_to :parent, autosave: true, validate: false
+        end
+
+        it "doesn't persist the parent or the child" do
+          expect(HomChild.count).to eq(0)
+          expect(HomParent.count).to eq(0)
+        end
+      end
+    end
+
+    context "when belongs_to_required_by_default is false" do
+      config_override :belongs_to_required_by_default, false
+
+      context "when optional is true" do
+        let(:optional) { true }
+
+        it "persists the child with the parent_id" do
+          expect(HomChild.first.parent_id).to eq(parent._id)
+          expect(HomParent.count).to eq(0)
+        end
+      end
+
+      context "when optional is false" do
+        let(:optional) { false }
+
+        it "doesn't persist the parent or the child" do
+          expect(HomChild.count).to eq(0)
+          expect(HomParent.count).to eq(0)
+        end
+      end
+
+      context "when optional is not set" do
+        let(:belongs_to) do
+          HomChild.belongs_to :parent, autosave: true, validate: false
+        end
+
+        it "persists the child with the parent_id" do
+          expect(HomChild.first.parent_id).to eq(parent._id)
+          expect(HomParent.count).to eq(0)
+        end
+      end
+    end
+  end
 end