I have this test data:
[
  {
    id: 1,
    l: 'a',
    sub: [
      ]
  },
  {
    id: 2,
    l: 'b',
    sub: [
      {
        id: 4,
        l: 'd'
      },
      {
        id: 5,
        l: 'e'
      },
      {
        id: 6,
        l: 'f',
        sub: [
          {
            id: 7,
            l: 'g'
          }
        ]
      }
    ]
  },
  {
    id: 3,
    l: 'c',
    sub: []
  }
];
And I'm trying to get the path of the object with id: 7. I  tried quite some JSONPath queries, but I just can't seem to fiind out how to make JSONPath iterate over all sub keys and search in there.
How can I match the object with id: 7?
Here is my testing plunkr: http://plnkr.co/edit/RoSeRo0L1B2oH3wC5LdU?p=preview
This query should work for what you are doing:
$..[?(@.id==7)]
You need to remove the id just after the $.. as you want to select the whole object, not just the id.  You were also missing the square brackets around the query.
This query brings back the following result set:
[
    {
        "id": 7,
        "l": "g"
    }
]
If you just want to retrieve the value of the l property (since you already know the id), you can easily do that as well. Just add .l at the end of the query:
$..[?(@.id==7)].l
This brings back the following result set:
[
    "g"
]
I tested the first query out here using this online json path tester tool and using your plunker: http://www.jsonquerytool.com/sample/jsonpathfilterallbypropertyvalue
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