When I configure a tree...

Code:
xtype: 'treepanel',
rootVisible: false,
store: {
    proxy: {
        type: 'direct',
        directFn: tree.get_lines,
        paramOrder: ['node'], // nodeParam is "node" by default
        reader: {
            type: 'json',
            root: 'children', // redundant
            idProperty: 'line_id',

...
...and load the root node from this JSON...

Code:
{
    "children":[
        {"line_id":1, "name":"foo", "id":"dont_look_here"},
        {"line_id":2, "name":"bar", "id":"really, no"},
    ]
}
...when I then expand the foo node, get_lines() is called with "dont_look_here" as parameter. If I omit the id property in the JSON, get_lines() is called with "" as parameter.