How to implement recursion in YANG model using leafref?

I have the following XML that I want to model with YANG. The XML contains a list of nodes and each node contains a list of neighbor nodes.

<nodes>
  <node>
    <id>1</id>
    <node>
      <id>2</id>
    </node>
  </node>
  <node>
    <id>3</id>
    <node>
      <id>4</id>
    </node>
  </node>
</nodes>

Please find below the YANG model I tried to create. Unfortunately, Yang does not support circular references in grouping.

grouping node {
  list node {
    leaf id {
      type int32;
    }
    uses node;
  }
}

container nodes {
   uses node;
}

I saw in draft-ietf-netmod-routing-cfg-16 and on ietf mail archive that a way to emulate recursion is to use leafref. How can the above xml be modeled with grouping and leafref?

As you said, recursion using groupings is not supported. The simplest approach would be to have a flat list of nodes, where each node has a parent node, referenced as a leafref. Something like:

container nodes {
  list node {
    key id;
    leaf id { type int32; }
    leaf parent-id {
      type leafref {
        path "../../node/id";
      }
    }
  }
}

The equivalent XML would be:

<nodes>
  <node>
    <id>1</id>
  <node>
  <node>
    <id>2</id>
    <parent-id>1</parent-id>
  </node>
  <node>
    <id>3</id>
  <node>
  <node>
    <id>4</id>
    <parent-id>3</parent-id>
  </node>
</nodes>

You could do the opposite, where a node refers its children (via a leafref leaf-list) instead of its parents.

---

Of course, using the same data node directly recursively does work:

container nodes {
  list node {
    key id;
    leaf id { type int32; }

    list node {
      key id;
      leaf id { type int32; }

      list node {
        key id;
        leaf id { type int32; }

        // and so on
      }
    }
  }
}

but this does not allow indefinitely deep hierarchies. I would avoid this pattern, though, not a clean model design.