title: flask-restplus fields.Nested() with raw Dict (not model)
body: <p>Spoiler alert: I posted my solution as an answer to this question</p>
  
  <p>I am using <a href="https://flask-restplus.readthedocs.io/en/stable/index.html" rel="noreferrer">flastk-resptlus</a> to create an API. I have to provide the data in a specific structure, which I have problems to get, see an example below:</p>
  
  <p>What I need to get is this structure:</p>
  
  <pre class="lang-py prettyprint-override"><code>{
      "metadata": {
          "files": [] 
      },
      "result" : {
          "data": [
                  {
                   "user_id": 1,
                    "user_name": "user_1",
                    "user_role": "editor"
                  },
                  {
                    "user_id": 2
                    "user_name": "user_2",
                    "user_role": "editor"
                  },
                  {
                    "user_id": 3,
                    "user_name": "user_3",
                    "user_role": "curator"
                  }
              ]
      }
  }
  </code></pre>
  
  <p>But the problem comes that I cannot manage to get the structure of <code>"result" : { "data": []}</code> without making "data" a model itself.</p>
  
  <p>What I tried to do so far (and did not work)</p>
  
  <pre class="lang-py prettyprint-override"><code># define metadata model
  metadata_model = api.model('MetadataModel', {
            "files": fields.List(fields.String(required=False, description='')),
  }
  # define user model 
  user_model = api.model('UserModel', {
            "user_id": fields.Integer(required=True, description=''),
            "user_name": fields.String(required=True, description=''),
            "user_role": fields.String(required=False, description='')
  }
  
  # here is where I have the problems
  user_list_response =  api.model('ListUserResponse', {
              'metadata': fields.Nested(metadata_model),
              'result' :  {"data" : fields.List(fields.Nested(user_model))}
               })
  </code></pre>
  
  <p>Complains that cannot get the "schema" from <code>"data"</code> (because is not a defined model), but I don't want to be a new api model, just want to append a key called "data". Any suggestions?</p>
  
  <p>This I tried and works, but is not what I want (because I miss the "data"):</p>
  
  <pre class="lang-py prettyprint-override"><code>user_list_response =  api.model('ListUserResponse', {
              'metadata': fields.Nested(metadata_model),
              'result' :  fields.List(fields.Nested(user_model))
              })
  </code></pre>
  
  <p>I don't want <code>data</code> to be a model because the common structure of the api is the following:</p>
  
  <pre class="lang-py prettyprint-override"><code>{
      "metadata": {
          "files": [] 
      },
      "result" : {
          "data": [
                  &lt;list of objects&gt; # here must be listed the single model
              ]
      }
  }
  </code></pre>
  
  <p>Then, <code>&lt;list of objects&gt;</code> can be users, addresses, jobs, whatever.. so I want to make a "general structure" in which then I can just inject the particular models (UserModel, AddressModel, JobModel, etc) without creating a special <code>data</code> model for each one.</p>