0

I have the following data structures in place

struct Symbol : Codable {
    let id:Int
    let type:String
    let properties:[String:String]?
}

struct Event : Codable {
    let event:String
    let timestamp:Int
    let symbol:Symbol?
}

struct LogRow : Codable {
    let id:Int
    let user_id:String
    let question_id:String
    let actions:[Event]
    let timestamp:Double
}

and the following JSON array

[
  {
    "id": 26535754,
    "user_id": "qhv1i39wsmbkzhjiffk1rrsg",
    "question_id": "\"trapdoor|186752c1-948e-4c15-b3df-7d39a99fe9d6\"",
    "actions": "[{\"event\": \"OPEN\", \"timestamp\": 1499802241640}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 15, \"type\": \"Fn\", \"properties\": {\"name\": \"cos\", \"allowSubscript\": true, \"innerSuperscript\": true}}, \"timestamp\": 1499802243567}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 14, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 15, \"type\": \"Fn\", \"properties\": {\"name\": \"cos\", \"allowSubscript\": true, \"innerSuperscript\": true}}, \"timestamp\": 1499802243567, \"dockingPoint\": \"right\"}, {\"event\": \"DOCK_SYMBOL\", \"parent\": {\"id\": 13, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 15, \"type\": \"Fn\", \"properties\": {\"name\": \"cos\", \"allowSubscript\": true, \"innerSuperscript\": true}}, \"timestamp\": 1499802243699, \"dockingPoint\": \"denominator\"}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 16, \"type\": \"Num\", \"properties\": {\"significand\": \"60\"}}, \"timestamp\": 1499802244570}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 15, \"type\": \"Fn\", \"properties\": {\"name\": \"cos\", \"allowSubscript\": true, \"innerSuperscript\": true}}, \"symbol\": {\"id\": 16, \"type\": \"Num\", \"properties\": {\"significand\": \"60\"}}, \"timestamp\": 1499802244570, \"dockingPoint\": \"argument\"}, {\"event\": \"DROP_SYMBOL\", \"symbol\": {\"id\": 16, \"type\": \"Num\", \"properties\": {\"significand\": \"60\"}}, \"timestamp\": 1499802245281}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 13, \"type\": \"Fraction\"}, \"timestamp\": 1499802245845}, {\"event\": \"TRASH_SYMBOL\", \"symbol\": {\"id\": 13, \"type\": \"Fraction\"}, \"timestamp\": 1499802246826}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 16, \"type\": \"Num\", \"properties\": {\"significand\": \"60\"}}, \"timestamp\": 1499802247468}, {\"event\": \"TRASH_SYMBOL\", \"symbol\": {\"id\": 16, \"type\": \"Num\", \"properties\": {\"significand\": \"60\"}}, \"timestamp\": 1499802248360}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802249161}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 14, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802249161, \"dockingPoint\": \"denominator\"}, {\"event\": \"DOCK_SYMBOL\", \"parent\": {\"id\": 14, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802249289, \"dockingPoint\": \"denominator\"}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802249797}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 14, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802249797, \"dockingPoint\": \"denominator\"}, {\"event\": \"DOCK_SYMBOL\", \"parent\": {\"id\": 14, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802249906, \"dockingPoint\": \"denominator\"}, {\"event\": \"DRAG_POTENTIAL_SYMBOL\", \"symbol\": {\"id\": 20, \"type\": \"Num\", \"properties\": {\"significand\": \"4\"}}, \"timestamp\": 1499802251451}, {\"event\": \"DROP_POTENTIAL_SYMBOL\", \"symbol\": {\"id\": 20, \"type\": \"Num\", \"properties\": {\"significand\": \"4\"}}, \"timestamp\": 1499802254229}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802255231}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 14, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802255231, \"dockingPoint\": \"denominator\"}, {\"event\": \"TRASH_SYMBOL\", \"symbol\": {\"id\": 19, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802256593}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 20, \"type\": \"Num\", \"properties\": {\"significand\": \"4\"}}, \"timestamp\": 1499802257376}, {\"event\": \"DOCK_SYMBOL\", \"parent\": {\"id\": 14, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 20, \"type\": \"Num\", \"properties\": {\"significand\": \"4\"}}, \"timestamp\": 1499802258062, \"dockingPoint\": \"denominator\"}, {\"event\": \"CLOSE\", \"timestamp\": 1499802259093}]",
    "timestamp": 1.499802259277E9
  },
  {
    "id": 26535718,
    "user_id": "qhv1i39wsmbkzhjiffk1rrsg",
    "question_id": "\"trapdoor|186752c1-948e-4c15-b3df-7d39a99fe9d6\"",
    "actions": "[{\"event\": \"OPEN\", \"timestamp\": 1499802175061}, {\"event\": \"DRAG_POTENTIAL_SYMBOL\", \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802178936}, {\"event\": \"DOCK_POTENTIAL_SYMBOL\", \"parent\": {\"id\": 7, \"type\": \"Symbol\", \"properties\": {\"letter\": \"g\"}}, \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802183785, \"dockingPoint\": \"subscript\"}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802184864}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 7, \"type\": \"Symbol\", \"properties\": {\"letter\": \"g\"}}, \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802184865, \"dockingPoint\": \"subscript\"}, {\"event\": \"DOCK_SYMBOL\", \"parent\": {\"id\": 6, \"type\": \"Symbol\", \"properties\": {\"letter\": \"m\"}}, \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802185857, \"dockingPoint\": \"subscript\"}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802186710}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 6, \"type\": \"Symbol\", \"properties\": {\"letter\": \"m\"}}, \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802186710, \"dockingPoint\": \"subscript\"}, {\"event\": \"DROP_SYMBOL\", \"symbol\": {\"id\": 10, \"type\": \"Fraction\"}, \"timestamp\": 1499802188430}, {\"event\": \"DRAG_POTENTIAL_SYMBOL\", \"symbol\": {\"id\": 11, \"type\": \"Fraction\"}, \"timestamp\": 1499802194665}, {\"event\": \"DROP_POTENTIAL_SYMBOL\", \"symbol\": {\"id\": 11, \"type\": \"Fraction\"}, \"timestamp\": 1499802195427}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 6, \"type\": \"Symbol\", \"properties\": {\"letter\": \"m\"}}, \"timestamp\": 1499802196167}, {\"event\": \"DOCK_SYMBOL\", \"parent\": {\"id\": 11, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 6, \"type\": \"Symbol\", \"properties\": {\"letter\": \"m\"}}, \"timestamp\": 1499802197167, \"dockingPoint\": \"numerator\"}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 8, \"type\": \"Fn\", \"properties\": {\"name\": \"cos\", \"allowSubscript\": true, \"innerSuperscript\": true}}, \"timestamp\": 1499802198247}, {\"event\": \"UNDOCK_SYMBOL\", \"parent\": {\"id\": 7, \"type\": \"Symbol\", \"properties\": {\"letter\": \"g\"}}, \"symbol\": {\"id\": 8, \"type\": \"Fn\", \"properties\": {\"name\": \"cos\", \"allowSubscript\": true, \"innerSuperscript\": true}}, \"timestamp\": 1499802198247, \"dockingPoint\": \"right\"}, {\"event\": \"DOCK_SYMBOL\", \"parent\": {\"id\": 11, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 8, \"type\": \"Fn\", \"properties\": {\"name\": \"cos\", \"allowSubscript\": true, \"innerSuperscript\": true}}, \"timestamp\": 1499802199971, \"dockingPoint\": \"right\"}, {\"event\": \"DRAG_START\", \"symbol\": {\"id\": 11, \"type\": \"Fraction\"}, \"timestamp\": 1499802201551}, {\"event\": \"DROP_SYMBOL\", \"symbol\": {\"id\": 11, \"type\": \"Fraction\"}, \"timestamp\": 1499802202638}, {\"event\": \"DRAG_POTENTIAL_SYMBOL\", \"symbol\": {\"id\": 12, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802209025}, {\"event\": \"DOCK_POTENTIAL_SYMBOL\", \"parent\": {\"id\": 11, \"type\": \"Fraction\"}, \"symbol\": {\"id\": 12, \"type\": \"Num\", \"properties\": {\"significand\": \"2\"}}, \"timestamp\": 1499802210771, \"dockingPoint\": \"denominator\"}, {\"event\": \"CLOSE\", \"timestamp\": 1499802212398}]",
    "timestamp": 1.49980221259E9
  }
]

The JSON data comes from a database, exported via DataGrip's JSON export. The database field is of type JSON Blob (it's Postgres, so it's OK). The query is something like:

SELECT
  ...
  event_details->>'actions' AS actions,
  ...
FROM yadda_yadda...;

in case it's useful (yes, I also tried the single arrow version, no difference).

My question is how do I make Swift parse the actions string as a JSON object into an [Event]? Alternatively, is there a way of making DataGrip (or postgres) export that field as an object like its container rather than as a serialised object?

EDIT I changed LogRow like this

struct LogRow : Codable {
    let id:Int
    let user_id:String
    let question_id:String
    let actions:[Event]
    let timestamp:Double

    enum CodingKeys: String, CodingKey {
        case id
        case user_id
        case question_id
        case actions
        case timestamp
    }

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        id = try values.decode(Int.self, forKey: .id)
        user_id = try values.decode(String.self, forKey: .user_id)
        question_id = try values.decode(String.self, forKey: .question_id)
        timestamp = try values.decode(Double.self, forKey: .timestamp)

        let actions_string = try values.decode(String.self, forKey: .actions)
        let actions_data = actions_string.data(using: .utf8)!
        actions = try JSONDecoder().decode([Event].self, from: actions_data)
    }
}

and now I'm getting a similar error to before, but apparently on the Event, not on the LogRow anymore.

fatal error: Error raised at top level:
Swift.DecodingError.typeMismatch(Swift.String,
Swift.DecodingError.Context(codingPath: [Foundation.(_JSONKey in
_12768CA107A31EF2DCE034FD75B541C9)(stringValue: "Index 1", intValue:
Optional(1)), tree_builder.Event.(CodingKeys in
_D5964B2C6A943A986EE24818C2C63D9B).symbol, tree_builder.Symbol.
(CodingKeys in _D5964B2C6A943A986EE24818C2C63D9B).properties,
Swift._DictionaryCodingKey(stringValue: "allowSubscript", intValue:
nil)], debugDescription: "Expected to decode String but found a number
instead.", underlyingError: nil)): file
/Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang-
900.0.65/src/swift/stdlib/public/core/ErrorType.swift, line 187
Improve this question
17
  • First you should get rid of all this implicitly unwrapped optionals. this answer shows you how to accomplish it stackoverflow.com/a/43121890/2303865 Commented Sep 26, 2017 at 12:41
  • I normally would, but in this case I am absolutely certain I can implicitly unwrap my optionals, as I completely control the environment, including the data set. If I were to release this in the wild, I would definitely get rid of that. Commented Sep 26, 2017 at 12:46
  • 2
    The point it is not if you can or can't. You don't need it. Commented Sep 26, 2017 at 12:52
  • Huh… is this something that changed with Swift 4? I started working on this code a few days ago on 3.1, and I was pretty sure it was complaining about optionals not being unwrapped. Commented Sep 26, 2017 at 12:54
  • 1
    Makes sense. Thanks for pointing it out, I wouldn't have checked for a while :) Commented Sep 26, 2017 at 13:00

2 Answers 2

Reset to default
1

You can use optional chaining, this is the solution given by Apple : https://developer.apple.com/swift/blog/?id=37

There's also library that does it for you, like https://github.com/Hearst-DD/ObjectMapper, or https://github.com/tristanhimmelman/AlamofireObjectMapper

This is a basic example on how to do it, but you should refactor and take a look at the links I gave :

guard let event = jsonObject["event"] as? String, let timestamp = jsonObject["timestamp"] as? Int else {return}
myEvent.event = event
myEvent.timestamp = timestamp 
if let symbol = jsonObject["symbol"] as? [String:Any] {
    guard let symbolId = symbol["id"] as? Int, let type = symbol["type"] else {return}
    let mySymbol = symbol()
    mySymbol.id = id
    mySymbol.type = type
}

EDIT

If you are using swift 4 and Codable protocol it should be really easy, you just have to make your nested objects as Codable too ! Then just : 

try decoder.decode(yourType.self, from: yourJsonString)

If you use CodingKeys correctly it should do it.

Improve this answer
Sign up to request clarification or add additional context in comments.

2 Comments

Uuuh, my question was not how to decode the json string into an object. That works if I feed a test string to a decoder and ask it to spit out an Event object. My question was: given that the serialised LogRow contains a string containing a serialised representation of an array of Events in its actions key, how do I make the decoder deserialise that string and turn it into an array of Events?
developer.apple.com/documentation/foundation/…
0

The problem was the declaration of properties, as the JSON object does not encode a [String:String] dictionary, since some of the values may not be Strings. So, this is the "answer" to this question, but that does not solve the nesting problem, so I created a new question: How do I parse a JSON object that has type-dependent sub-objects, in Swift?

Improve this answer

Comments

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.