How to add a text field to a symbol mark - vega

Question

I'm trying to add a text mark to the nodes in the force-directed-layout example on the github page of vega.
I converted the symbol mark to a group mark and moved the "encode" part to a child "symbol" mark. In this new group-mark I now also add a text mark which I give the name of the node.
This is the nodes mark, the "on" and "transform" parts are left out because these are unchanged.
{
"name": "nodes",
"type": "group",
"zindex": 1,
"from": {
"data": "node-data"
},
"marks": [
{
"type": "symbol",
"encode": {
"enter": {
"fill": {
"scale": "color",
"field": "group"
},
"stroke": {
"value": "white"
}
},
"update": {
"size": {
"signal": "2 * nodeRadius * nodeRadius"
},
"cursor": {
"value": "pointer"
}
}
}
},
{
"type": "text",
"interactive": false,
"encode": {
"enter": {
"fill": {
"value": "black"
},
"fontSize": {
"value": 12
},
"align": {
"value": "center"
},
"text": {
"field": "name"
},
"y": {
"value": -5
}
}
}
}
],
"on": [...],
"transform": [...]
}
Expected was to see the name of the node above each node, but the text AND the symbol become invisible.
Steps to Reproduce
Make the type "group", add a "marks" array, add a mark object with type "symbol" to this marks array
-> the nodes still show
Move the encode part to the nested symbol group
-> the nodes dissapear
Adding the text mark also does nothing
I have a hunch that its the "field" property that borks when you try to nest it while keeping the data in the parent because the text mark does show in the group when I give it a fixed value, but I can't seem to figured out how to get it working.
(I thought this was the way because the docs say that a child mark gets the parent data automatically when ommiting a data field)

Answer 1

Within a group, you have to use {"parent": <field_name>} to refer to a field. For example, to refer to the field "group" in the parent use:
"field": {"parent": "group"}}
See an example here.

Answer 2

Adding the following mark also works, without breaking dragging (example link):
{
"type": "text",
"from": {"data": "nodes"},
"interactive": false,
"enter": {
"fill": {"value": "black"},
"fontSize": {"value": 10}
},
"encode": {
"update": {
"y": {"field": "y", "offset": {"signal": "nodeRadius * -0.9"}},
"x": {"field": "x"},
"text": {"field": "datum.name"},
"align": {"value": "center"}
}
}
}
This text mark uses the symbol mark (called nodes) as its backing data source. See https://vega.github.io/vega/docs/marks/#reactive-geometry.