title: SWIFT 4 nested JSON Struct - Codable
body: <p>I'm having issues creating a struct to parse JSON in Swift 4. I'm able to parse small JSONs and JSONDecoder seems to work fine. Just need help to create a struct to parse JSON like that:</p>
  
  <pre><code>{
      "main": {
          "solutions": [
                      {
                      "exersises": [
                                       {
                                       "book_title": "test",
                                       "release_date": "2015-01-12T11:00",
                                       "price": 100,
                                       "additional": [
                                                     {
                                                     "item1": "test",
                                                     "item2": "test",
                                                     "number": 1
                                                     },
                                                     {
                                                      "item1": "test2",
                                                      "item2": "test2",
                                                      "number": 2
                                                     }
                                                     ],
                                       "availability": "Yes",
                                       "item_id": 43534
                                       }
                                       ]
  
  
                      }
  
                      ]
      }
  }
  </code></pre>
  
  <p>What kind of struct do I need to get to value of book_title for example? </p>