How to ignore specific field by name?

[criscola]

Hello, I hope you can help me. I would like to know if it's possible to ignore a specific field during comparison. I had a look at this https://github.com/google/go-cmp/issues/156 and tried to modify it a bit to make it work for my use case, but I don't understand how to do it. For more context: my objective is to compare two JSON strings, ignoring a specific field by name present in various levels of the hierarchy. Thank you!

Test samples:

{
"name": "foo",
"archived": false,
"birth_date": "2016-05-01T00:00:00Z",
"edges": {
  "firstEdge": {
    "id": 1438,
    "edges": {
      "arrayOfStuff": [
        {
          "id": 98,
          "create_time": "2023-08-22T22:32:31.681874237+02:00",
          "update_time": "2023-08-22T22:32:31.681874317+02:00",
          "edges": {
            "yetAnotherEdge": {
              "id": 98,
              "some_date": "2020-11-02T00:00:00Z",
              "create_time": "2023-08-22T22:32:31.680396095+02:00",
              "update_time": "2023-08-22T22:32:31.680396165+02:00"
            }
          }
        }]
      }
    }
  }
}

compared to this (only create and update times are changed):

{
"name": "foo",
"archived": false,
"birth_date": "2016-05-01T00:00:00Z",
"edges": {
  "firstEdge": {
    "id": 1438,
    "edges": {
      "arrayOfStuff": [
        {
          "id": 98,
          "create_time": "1999-08-22T22:32:31.681874237+02:00",
          "update_time": "1999-08-22T22:32:31.681874317+02:00",
          "edges": {
            "yetAnotherEdge": {
              "id": 98,
              "some_date": "1999-11-02T00:00:00Z",
              "create_time": "1999-08-22T22:32:31.680396095+02:00",
              "update_time": "1999-08-22T22:32:31.680396165+02:00"
            }
          }
        }]
      }
    }
  }
}

should return a match. I know that I can use some trick like reflection to zero-out the fields, or unmarshal into a struct omitting the values, but if I can implement what I found with go-cmp it would definitely lead to more clear, concise and maintenable code.

[criscola]

Turns out, it is pretty simple! Here FilterPath is necessary because FilterValues is used to ignore based on values, for example to compare float values etc. but in my case I needed to ignore a path which includes the left-hand side of the "map entry" in the predicate. Simple get all the leaves with p.Last() and you're set:

opt := cmp.FilterPath(func(p cmp.Path) bool {
    vx := p.Last().String()

    if vx == `["create_time"]` || vx == `["update_time"]` {
        return true
    }
    return false
}, cmp.Ignore())

note also that if you know your struct in advance, you can use IgnoreFields as in the docs (in my case, I don't know, hence I had to use the jsonTransformer as in #156).