Setting #parent_id with nested attributes can result in corrupted #ancestry

[astyagun]

Given:

• Tree model
• Node model
• Tree has_many :nodes
• Tree accepts_nested_attributes_for :nodes
• Node has_ancestry
• Nested attributes for nodes modify nodes' ancestry using #parent_id attributes

The problem occurs, for example, when:

• A tree has 3 sibling nodes (let's assume, that they have #id attributes respectively '1', '2' and '3')
• And user nests node two under node one and node three under node two resulting in two-level nesting

Then:

• Each node is assigned a new #parent_id attribute
• When assigning to child node, parent node is loaded from DB and its #ancestry attribute is used to assign #ancestry of a child node.
  • Node two gets assigned #ancestry of #id of node one, '1', which is correct
  • But node three gets assigned #ancestry of #id of node two only, '2'. Which is incorrect as it should have IDs of all chain of parent nodes in its #ancestry attribute. What it should be assigned is '1/2'.
• All nodes are saved to DB

And this results in corrupted #ancestry attribute of node three and it disappears from the nodes tree.

[kbrock]

@astyagun Have you created a test for this?

node1.parent_id = node2.id
node2.parent_id = node3.id
node1.save
node2.save

1. Using parent instead of parent_id fixes this.
2. Save order fixes this. i.e.: node2.save ; node1.save

Wonder if there is a way to enforce a good save order.

[astyagun]

@kbrock To reproduce the problem a test like this could be used:

  describe '#update nodes using nested attributes' do
    subject(:update) { instance.update nodes_attributes: nodes_attributes }

    let(:instance) { create :tree }

    context 'when nesting nodes several levels deep' do
      let!(:node_one) { create :node, tree: instance }
      let!(:node_two) { create :node, tree: instance }
      let!(:node_three) { create :node, tree: instance }
      let :nodes_attributes do
        # Order of hashes is significant
        [
          { id: node_two.id, parent_id: node_one.id },
          { id: node_three.id, parent_id: node_two.id }
        ]
      end

      it 'saves #ancestry of the most nested node correctly' do
        update
        expect(node_three.reload.ancestry).to eq "#{node_one.id}/#{node_two.id}"
      end
    end
  end

[astyagun]

1. Using parent instead of parent_id fixes this.
2. Save order fixes this. i.e.: node2.save ; node1.save

Reordering works indeed, I've tried it in the test above. But using parent instead of parent_id didn't work.

[astyagun]

More about #parent attribute. To ensure, that instances, that receive attributes, are the same as instances, that are being assigned to #parent attributes, changed the test this way:

describe '#update nodes using nested attributes' do
  subject(:update) { instance.update nodes_attributes: nodes_attributes }

  let(:instance) { create :tree }

  context 'when nesting nodes several levels deep' do
    let!(:node_one) { create :node, tree: instance }
    let!(:node_two) { create :node, tree: instance }
    let!(:node_three) { create :node, tree: instance }
    let(:tree_nodes_association) { instance.association :nodes }
    let(:tree_nodes) { tree_nodes_association.target }
    let(:node_one_from_association) { tree_nodes.detect { |node| node.id == node_one.id } }
    let(:node_two_from_association) { tree_nodes.detect { |node| node.id == node_two.id } }
    let :nodes_attributes do
      # Order of hashes is significant
      [
        { id: node_two.id, parent: node_one_from_association },
        { id: node_three.id, parent: node_two_from_association }
      ]
    end

    before { tree_nodes_association.load_target }

    it 'saves ancestry of the most nested node correctly' do
      update
      expect(node_three.reload.ancestry).to eq "#{node_one.id}/#{node_two.id}"
    end
  end
end

No luck, still fails. This is not the way to get same instances everywhere, probably.

[kbrock]

As I'm sure you already know,

The problem is when you set a parent_id into the nodes, it needs to lookup the parent record so it can properly determine the path.

There are then 2 copies of the record in memory, and when you update one, the other is wrong / stale.

A working algorithm would be to set the parent nodes instead of the parent_id nodes.
Nested attributes will be a problem with this.

There may be a way to change ancestry to delay the lookup of the parent record from the parent_id but I can't think of how to do this. And I don't even know if this is a good solution.

[astyagun]

Another option is to sort nodes before they are submitted into update as nested attributes