title: TypeError: 'Transaction' object is not iterable in Flask on Python 3.6
body: <p>I am trying to use a serializer to store my JSON data into DB. I have created an API and using in postman. Like :</p>
  
  <p>POST endpoint: <code>API/transaction</code> and data with <code>JSON</code> format </p>
  
  <p>Where I load into serializer it returns an error. I tested in the shell too. I am not getting an exact error in the debug tool. </p>
  
  <p>my serializer is:</p>
  
  <pre><code>class TransactionSerializer(ma.ModelSchema):
      """transaction validation"""
      partner_client_id = fields.String(required=True)
      agent_id = fields.String(required=True)
      agent_msisdn = fields.String(required=True)
      code = fields.Integer(required=True)
      title = fields.String(required=False)
      price = fields.String(required=True)
      currency = fields.String(required=False)
      user_msisdn = fields.String(required=True)
      requested_ip = fields.String(required=False)
      platform = fields.String(required=False)
      is_recurring = fields.Boolean(required=False)
  
      class Meta:
          """Meta class."""
          model = Transaction
  
  </code></pre>
  
  <p>Here is python SHELL</p>
  
  <pre><code>&gt;&gt;&gt; from app.serializer.transaction_serializer import TransactionSerializer
  &gt;&gt;&gt; transaction_serializer = TransactionSerializer()
  
  &gt;&gt;&gt; trn_json ={
  ...     "partner_client_id": "CLIENTID009",
  ...     "agent_id": "agent222",
  ...     "agent_msisdn": "8801831803255",
  ...     "code": "10",
  ...     "title": "10 Days Unlimited ",
  ...     "price": "10.00",
  ...     "currency": "BDT",
  ...     "user_msisdn": "8801925533362",
  ...     "requested_ip": "127.0.0.1",
  ...     "platform": "universal",
  ...     "is_recurring": True
  ... }
  
  
  &gt;&gt;&gt; trn_json
  {'partner_client_id': 'CLIENTID009', 'agent_id': 'agent222', 'agent_msisdn': '8801831803255', 'code': '10', 'title': '10 Days Unlimited ', 'price': '10.00', 'currency': 'BDT', 'user_msisdn': '8801925533362', 'requested_ip': '127.0.0.1', 'platform': 'universal', 'is_recurring': True}
  
  &gt;&gt;&gt; transaction, errors = transaction_serializer.load(trn_json)
  Traceback (most recent call last):
    File "&lt;console&gt;", line 1, in &lt;module&gt;
  
  TypeError: 'Transaction' object is not iterable
  </code></pre>