Flatten nested json in kusto column

Question

I have a kusto table with one of the columns as dynamic type with nested json, How do I flatten in kusto? mv-expand is only doing one level.

column1 : timetsamp
column2 : id
column3 : json object

timestamp   id  value
2020-10-13 22:42:05.0000000 d0  "{
  ""value"": ""0"",
  ""max"": ""0"",
  ""min"": ""0"",
  ""avg"": ""0""
}"
2020-10-13 22:42:05.0000000 d0  "{
  ""sid"": ""a0"",
  ""data"": {
    ""x"": {
      ""a"": {
        ""t1"": ""2020-10-13T22:46:50.1310000Z"",
        ""m1"": 446164,
        ""m4"": {
          ""m41"": ""abcd"",
          ""m42"": 1234
        }
      }
    }
  }
}"

#update2 : I was able to faltten keys, but not the values

let testJson = datatable(timestamp : datetime, id : string, value : dynamic )
    [datetime(2020-10-13T22:42:05Z), 'd0', dynamic({"value":"0","max":"0","min":"0","avg":"0"}),
     datetime(2020-10-13T22:42:05Z), 'd1', dynamic({"sid":"a0","data":{"x":{"a":{"t1":"2020-10-13T22:46:50.131Z","m1":446164,"m4":{"m41":"abcd","m42":1234}}}}})];
testJson
| extend key=treepath(value)
| mv-expand key
| extend value1 = value[tostring(key)]

Answer 1

You can invoke mv-expand several times:

let _data = datatable (column1:datetime , column2:string , column3:dynamic )
[
datetime(2020-10-13 22:42:05.0000000), 'd0', dynamic({
  "value": "0",
  "max": "0",
  "min": "0",
  "avg": "0"
}),
datetime(2020-10-13 22:42:05.0000000), 'd0', dynamic({
  "sid": "a0",
  "data": {
    "x": {
      "a": {
        "t1": "2020-10-13T22:46:50.1310000Z",
        "m1": 446164,
        "m4": {
          "m41": "abcd",
          "m42": 1234
        }
      }
    }
  }
})
];
_data
| mv-expand column3
| mv-expand more_data=column3.data.x.a
| mv-expand more_data_m4=more_data.m4

You can also promote dynamic fields into columns using evaluate bag_unpack(): https://learn.microsoft.com/en-us/azure/data-explorer/kusto/query/bag-unpackplugin

let _data = datatable (column1:datetime , column2:string , column3:dynamic )
[
datetime(2020-10-13 22:42:05.0000000), 'd0', dynamic({
  "value": "0",
  "max": "0",
  "min": "0",
  "avg": "0"
}),
datetime(2020-10-13 22:42:05.0000000), 'd0', dynamic({
  "sid": "a0",
  "data": {
    "x": {
      "a": {
        "t1": "2020-10-13T22:46:50.1310000Z",
        "m1": 446164,
        "m4": {
          "m41": "abcd",
          "m42": 1234
        }
      }
    }
  }
})
];
_data
| mv-expand column3
| extend expanded_data = column3.data.x.a
| evaluate bag_unpack(expanded_data)