title: gorm: json of json not work
body: <p>Sample:</p>
  
  <pre><code>{
    "id": 1
    "data": {"1": 2}
  }
  </code></pre>
  
  <p>Struct definition:</p>
  
  <pre><code>type Item struct {
    id int `json:"id"`
    data interface{} `json:"data"`
  }
  </code></pre>
  
  <p>I need to parse the payload from a http post, so I used <code>interface{}</code> for <code>data</code>, <code>json.Unmarshal()</code> is successful, but gorm produces error while calling <code>db.Create(item)</code>:</p>
  
  <pre><code>(sql: converting Exec argument #5's type: unsupported type map[string]interface {}, a map)
  </code></pre>
  
  <p>Instead, I change from <code>interface{}</code> to <code>string</code>, calling <code>json.Unmarshal()</code> to parse json POST payload produces error.</p>
  
  <pre><code>unmarshal type error: expected=string, got=object
  </code></pre>
  
  <p>Basically, one requires <code>interface{}</code>, one requires <code>string</code>.</p>
  
  <p>Anyone encountered this?</p>