How to decode a nested JSON struct with Swift Decodable protocol?

Json Problem Overview

Here is my JSON

{
    "id": 1,
    "user": {
        "user_name": "Tester",
        "real_info": {
         	"full_name":"Jon Doe"
        }
    },
    "reviews_count": [
        {
            "count": 4
        }
    ]
}

Here is the structure I want it saved to (incomplete)

struct ServerResponse: Decodable {
	var id: String
	var username: String
	var fullName: String
	var reviewCount: Int
	
	enum CodingKeys: String, CodingKey {
       case id, 
       // How do i get nested values?
	}
}

I have looked at Apple's Documentation on decoding nested structs, but I still do not understand how to do the different levels of the JSON properly. Any help will be much appreciated.

Json Solutions

Solution 1 - Json

Another approach is to create an intermediate model that closely matches the JSON (with the help of a tool like quicktype.io), let Swift generate the methods to decode it, and then pick off the pieces that you want in your final data model:

// snake_case to match the JSON and hence no need to write CodingKey enums
fileprivate struct RawServerResponse: Decodable {
	struct User: Decodable {
		var user_name: String
		var real_info: UserRealInfo
	}

	struct UserRealInfo: Decodable {
		var full_name: String
	}

	struct Review: Decodable {
		var count: Int
	}

	var id: Int
	var user: User
	var reviews_count: [Review]
}

struct ServerResponse: Decodable {
	var id: String
	var username: String
	var fullName: String
	var reviewCount: Int

	init(from decoder: Decoder) throws {
		let rawResponse = try RawServerResponse(from: decoder)
		
		// Now you can pick items that are important to your data model,
        // conveniently decoded into a Swift structure
		id = String(rawResponse.id)
		username = rawResponse.user.user_name
		fullName = rawResponse.user.real_info.full_name
		reviewCount = rawResponse.reviews_count.first!.count
	}
}

This also allows you to easily iterate through reviews_count, should it contain more than 1 value in the future.

Solution 2 - Json

In order to solve your problem, you can split your RawServerResponse implementation into several logic parts (using Swift 5).

#1. Implement the properties and required coding keys

import Foundation

struct RawServerResponse {
    
    enum RootKeys: String, CodingKey {
        case id, user, reviewCount = "reviews_count"
    }
    
    enum UserKeys: String, CodingKey {
        case userName = "user_name", realInfo = "real_info"
    }
    
    enum RealInfoKeys: String, CodingKey {
        case fullName = "full_name"
    }
    
    enum ReviewCountKeys: String, CodingKey {
        case count
    }
    
    let id: Int
    let userName: String
    let fullName: String
    let reviewCount: Int
          
}

#2. Set the decoding strategy for `id` property

extension RawServerResponse: Decodable {
    
    init(from decoder: Decoder) throws {
        // id
        let container = try decoder.container(keyedBy: RootKeys.self)
        id = try container.decode(Int.self, forKey: .id)

        /* ... */                 
    }
    
}

#3. Set the decoding strategy for `userName` property

extension RawServerResponse: Decodable {
    
    init(from decoder: Decoder) throws {
        /* ... */
        
        // userName
        let userContainer = try container.nestedContainer(keyedBy: UserKeys.self, forKey: .user)
        userName = try userContainer.decode(String.self, forKey: .userName)
        
        /* ... */
    }
    
}

#4. Set the decoding strategy for `fullName` property

extension RawServerResponse: Decodable {

    init(from decoder: Decoder) throws {
        /* ... */
        
        // fullName
        let realInfoKeysContainer = try userContainer.nestedContainer(keyedBy: RealInfoKeys.self, forKey: .realInfo)
        fullName = try realInfoKeysContainer.decode(String.self, forKey: .fullName)

        /* ... */
    }
    
}

#5. Set the decoding strategy for `reviewCount` property

extension RawServerResponse: Decodable {

    init(from decoder: Decoder) throws {
        /* ...*/        

        // reviewCount
        var reviewUnkeyedContainer = try container.nestedUnkeyedContainer(forKey: .reviewCount)
        var reviewCountArray = [Int]()
        while !reviewUnkeyedContainer.isAtEnd {
            let reviewCountContainer = try reviewUnkeyedContainer.nestedContainer(keyedBy: ReviewCountKeys.self)
            reviewCountArray.append(try reviewCountContainer.decode(Int.self, forKey: .count))
        }
        guard let reviewCount = reviewCountArray.first else {
            throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: container.codingPath + [RootKeys.reviewCount], debugDescription: "reviews_count cannot be empty"))
        }
        self.reviewCount = reviewCount
    }
    
}

Complete implementation

import Foundation

struct RawServerResponse {
    
    enum RootKeys: String, CodingKey {
        case id, user, reviewCount = "reviews_count"
    }
    
    enum UserKeys: String, CodingKey {
        case userName = "user_name", realInfo = "real_info"
    }
    
    enum RealInfoKeys: String, CodingKey {
        case fullName = "full_name"
    }
    
    enum ReviewCountKeys: String, CodingKey {
        case count
    }
    
    let id: Int
    let userName: String
    let fullName: String
    let reviewCount: Int
    
}

extension RawServerResponse: Decodable {

    init(from decoder: Decoder) throws {
        // id
        let container = try decoder.container(keyedBy: RootKeys.self)
        id = try container.decode(Int.self, forKey: .id)
        
        // userName
        let userContainer = try container.nestedContainer(keyedBy: UserKeys.self, forKey: .user)
        userName = try userContainer.decode(String.self, forKey: .userName)
        
        // fullName
        let realInfoKeysContainer = try userContainer.nestedContainer(keyedBy: RealInfoKeys.self, forKey: .realInfo)
        fullName = try realInfoKeysContainer.decode(String.self, forKey: .fullName)

        // reviewCount
        var reviewUnkeyedContainer = try container.nestedUnkeyedContainer(forKey: .reviewCount)
        var reviewCountArray = [Int]()
        while !reviewUnkeyedContainer.isAtEnd {
            let reviewCountContainer = try reviewUnkeyedContainer.nestedContainer(keyedBy: ReviewCountKeys.self)
            reviewCountArray.append(try reviewCountContainer.decode(Int.self, forKey: .count))
        }
        guard let reviewCount = reviewCountArray.first else {
            throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: container.codingPath + [RootKeys.reviewCount], debugDescription: "reviews_count cannot be empty"))
        }
        self.reviewCount = reviewCount
    }
    
}

Usage

let jsonString = """
{
    "id": 1,
    "user": {
        "user_name": "Tester",
        "real_info": {
            "full_name":"Jon Doe"
        }
    },
    "reviews_count": [
    {
    "count": 4
    }
    ]
}
"""

let jsonData = jsonString.data(using: .utf8)!
let decoder = JSONDecoder()
let serverResponse = try! decoder.decode(RawServerResponse.self, from: jsonData)
dump(serverResponse)

/*
prints:
▿ RawServerResponse #1 in __lldb_expr_389
  - id: 1
  - user: "Tester"
  - fullName: "Jon Doe"
  - reviewCount: 4
*/

Solution 3 - Json

Rather than having one big CodingKeys enumeration with all the keys you'll need for decoding the JSON, I would advise splitting the keys up for each of your nested JSON objects, using nested enumerations to preserve the hierarchy:

// top-level JSON object keys
private enum CodingKeys : String, CodingKey {

    // using camelCase case names, with snake_case raw values where necessary.
    // the raw values are what's used as the actual keys for the JSON object,
    // and default to the case name unless otherwise specified.
    case id, user, reviewsCount = "reviews_count"
    
    // "user" JSON object keys
    enum User : String, CodingKey {
        case username = "user_name", realInfo = "real_info"
        
        // "real_info" JSON object keys
        enum RealInfo : String, CodingKey {
            case fullName = "full_name"
        }
    }
    
    // nested JSON objects in "reviews" keys
    enum ReviewsCount : String, CodingKey {
        case count
    }
}

This will make it easier to keep track of the keys at each level in your JSON.

Now, bearing in mind that:

A keyed container is used to decode a JSON object, and is decoded with a CodingKey conforming type (such as the ones we've defined above).
An unkeyed container is used to decode a JSON array, and is decoded sequentially (i.e each time you call a decode or nested container method on it, it advances to the next element in the array). See the second part of the answer for how you can iterate through one.

After getting your top-level keyed container from the decoder with container(keyedBy:) (as you have a JSON object at the top-level), you can repeatedly use the methods:

nestedContainer(keyedBy:forKey:) to get a nested object from an object for a given key
nestedUnkeyedContainer(forKey:) to get a nested array from an object for a given key
nestedContainer(keyedBy:) to get the next nested object from an array
nestedUnkeyedContainer() to get the next nested array from an array

For example:

struct ServerResponse : Decodable {
    
    var id: Int, username: String, fullName: String, reviewCount: Int
    
    private enum CodingKeys : String, CodingKey { /* see above definition in answer */ }
    
    init(from decoder: Decoder) throws {
        
        // top-level container
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.id = try container.decode(Int.self, forKey: .id)
        
        // container for { "user_name": "Tester", "real_info": { "full_name": "Jon Doe" } }
        let userContainer =
            try container.nestedContainer(keyedBy: CodingKeys.User.self, forKey: .user)
        
        self.username = try userContainer.decode(String.self, forKey: .username)
        
        // container for { "full_name": "Jon Doe" }
        let realInfoContainer =
            try userContainer.nestedContainer(keyedBy: CodingKeys.User.RealInfo.self,
                                              forKey: .realInfo)
        
        self.fullName = try realInfoContainer.decode(String.self, forKey: .fullName)
        
        // container for [{ "count": 4 }] – must be a var, as calling a nested container
        // method on it advances it to the next element.
        var reviewCountContainer =
            try container.nestedUnkeyedContainer(forKey: .reviewsCount)
        
        // container for { "count" : 4 }
        // (note that we're only considering the first element of the array)
        let firstReviewCountContainer =
            try reviewCountContainer.nestedContainer(keyedBy: CodingKeys.ReviewsCount.self)

        self.reviewCount = try firstReviewCountContainer.decode(Int.self, forKey: .count)
    }
}

Example decoding:

let jsonData = """
{
  "id": 1,
  "user": {
    "user_name": "Tester",
    "real_info": {
    "full_name":"Jon Doe"
  }
  },
  "reviews_count": [
    {
      "count": 4
    }
  ]
}
""".data(using: .utf8)!

do {
    let response = try JSONDecoder().decode(ServerResponse.self, from: jsonData)
    print(response)
} catch {
    print(error)
}

// ServerResponse(id: 1, username: "Tester", fullName: "Jon Doe", reviewCount: 4)

Iterating through an unkeyed container

Considering the case where you want reviewCount to be an [Int], where each element represents the value for the "count" key in the nested JSON:

  "reviews_count": [
    {
      "count": 4
    },
    {
      "count": 5
    }
  ]

You'll need to iterate through the nested unkeyed container, getting the nested keyed container at each iteration, and decoding the value for the "count" key. You can use the count property of the unkeyed container in order to pre-allocate the resultant array, and then the isAtEnd property to iterate through it.

For example:

struct ServerResponse : Decodable {
    
    var id: Int
    var username: String
    var fullName: String
    var reviewCounts = [Int]()
    
    // ...

    init(from decoder: Decoder) throws {
        
        // ...
        
        // container for [{ "count": 4 }, { "count": 5 }]
        var reviewCountContainer =
            try container.nestedUnkeyedContainer(forKey: .reviewsCount)
        
        // pre-allocate the reviewCounts array if we can
        if let count = reviewCountContainer.count {
            self.reviewCounts.reserveCapacity(count)
        }
        
        // iterate through each of the nested keyed containers, getting the
        // value for the "count" key, and appending to the array.
        while !reviewCountContainer.isAtEnd {
            
            // container for a single nested object in the array, e.g { "count": 4 }
            let nestedReviewCountContainer = try reviewCountContainer.nestedContainer(
                                                 keyedBy: CodingKeys.ReviewsCount.self)
            
            self.reviewCounts.append(
                try nestedReviewCountContainer.decode(Int.self, forKey: .count)
            )
        }
    }
}

Solution 4 - Json

Many good answers have already been posted, but there is a simpler method not described yet IMO.

When the JSON field names are written using snake_case_notation you can still use the camelCaseNotation in your Swift file.

You just need to set

decoder.keyDecodingStrategy = .convertFromSnakeCase

After this ☝️ line Swift will automatically match all the snake_case fields from the JSON to the camelCase fields in the Swift model.

E.g.

user_name` -> userName
reviews_count -> `reviewsCount
...

Here's the full code

Writing the Model --

struct Response: Codable {

    let id: Int
    let user: User
    let reviewsCount: [ReviewCount]
    
    struct User: Codable {
        let userName: String
        
        struct RealInfo: Codable {
            let fullName: String
        }
    }
    
    struct ReviewCount: Codable {
        let count: Int
    }
}

Setting the Decoder --

let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase

Decoding --

do {
    let response = try? decoder.decode(Response.self, from: data)
    print(response)
} catch {
    debugPrint(error)
}

Solution 5 - Json

Copy the json file to https://app.quicktype.io
Select Swift (if you use Swift 5, check the compatibility switch for Swift 5)
Use the following code to decode the file
Voila!

let file = "data.json"

guard let url = Bundle.main.url(forResource: "data", withExtension: "json") else{
    fatalError("Failed to locate \(file) in bundle.")
}

guard let data = try? Data(contentsOf: url) else{
    fatalError("Failed to locate \(file) in bundle.")
}
        
let yourObject = try? JSONDecoder().decode(YourModel.self, from: data)

Solution 6 - Json

Also you can use library KeyedCodable I prepared. It will require less code. Let me know what you think about it.

struct ServerResponse: Decodable, Keyedable {
  var id: String!
  var username: String!
  var fullName: String!
  var reviewCount: Int!

  private struct ReviewsCount: Codable {
    var count: Int
  }

  mutating func map(map: KeyMap) throws {
    var id: Int!
    try id <<- map["id"]
    self.id = String(id)

    try username <<- map["user.user_name"]
    try fullName <<- map["user.real_info.full_name"]

    var reviewCount: [ReviewsCount]!
    try reviewCount <<- map["reviews_count"]
    self.reviewCount = reviewCount[0].count
  }

  init(from decoder: Decoder) throws {
    try KeyedDecoder(with: decoder).decode(to: &self)
  }
}

Content Type	Original Author	Original Content on Stackoverflow
Question	Just a coder	View Question on Stackoverflow
Solution 1 - Json	Code Different	View Answer on Stackoverflow
Solution 2 - Json	Imanou Petit	View Answer on Stackoverflow
Solution 3 - Json	Hamish	View Answer on Stackoverflow
Solution 4 - Json	Luca Angeletti	View Answer on Stackoverflow
Solution 5 - Json	simibac	View Answer on Stackoverflow
Solution 6 - Json	decybel	View Answer on Stackoverflow

How to decode a nested JSON struct with Swift Decodable protocol?

Json Problem Overview

Json Solutions

Solution 1 - Json

Solution 2 - Json

#1. Implement the properties and required coding keys

#2. Set the decoding strategy for `id` property

#3. Set the decoding strategy for `userName` property

#4. Set the decoding strategy for `fullName` property

#5. Set the decoding strategy for `reviewCount` property

Complete implementation

Usage

Solution 3 - Json

Iterating through an unkeyed container

Solution 4 - Json

Solution 5 - Json

Solution 6 - Json

How to debug spring-boot application with IntelliJ IDEA community Edition?

How to make vscode compact multiple lines to a single line?

Attributions

Json Problem Overview

Json Solutions

Solution 1 - Json

Solution 2 - Json

#1. Implement the properties and required coding keys

#2. Set the decoding strategy for id property

#3. Set the decoding strategy for userName property

#4. Set the decoding strategy for fullName property

#5. Set the decoding strategy for reviewCount property

Complete implementation

Usage

Solution 3 - Json

Iterating through an unkeyed container

Solution 4 - Json

Solution 5 - Json

Solution 6 - Json

How to debug spring-boot application with IntelliJ IDEA community Edition?

How to make vscode compact multiple lines to a single line?

Attributions

#2. Set the decoding strategy for `id` property

#3. Set the decoding strategy for `userName` property

#4. Set the decoding strategy for `fullName` property

#5. Set the decoding strategy for `reviewCount` property