Here i provide the code worked sample as per the guideline given below and seems to get null values.

Here is my complete JSON Data,

```
some: {
  "success": true,
  "data": 
[
  {
    "15-10-2020": [
      {
        "id": 100,
        "details": {
          "_id": 1,
          "_title": "My Title"
        },
        "created_at": "2020-10-15"
      },
      {
        "snf_id": 101,
        "details": {
          "_id": 1,
          "_title": "My Title"
        },
        "created_at": "2020-10-15"
      },
      {
         "snf_id": 102,
        "details": {
          "_id": 1,
          "_title": "My Title"
        },
        "created_at": "2020-10-15"
      }
    ],
    "30-09-2020": [
           {
         "snf_id": 301,
        "details": {
          "_id": 8,
          "_title": "My Title"
        },
        "created_at": "2020-09-30"
      }
  ]
  }
],
  "message": "Successfully Retrieved"
}

struct Response : Codable {
    var success : Bool?
    var data : [Data]?
    var message : String?
}

struct Data: Codable {
    var snf_id: Int?
    var details: Details?
    var created_at: String?
}

// MARK: - Details
struct Details: Codable {
    var _id: Int?
    var _title: String?
}

let Response = try JSONDecoder().decode(Response.self, from: data)

```

Returns null value for data,

▿ Response
▿ success : Optional
- some : true
▿ data : Optional<Array>
▿ some : 1 element
▿ 0 : Data
- snf\_id : nil
- details : nil
- created\_at : nil
▿ message : Optional
- some : "Successfully Retrieved"