Python Flask SQLalchemy JSON POST Error

Question

I'm trying to post the following JSON and save to a MySQL database on a Flask server and Python 2.7, restless framework and SQLAlchemy with curl:

curl -i -H "Accept: application/json" -X POST  -d '{"attribute_id": "1", "product_id": "44","text":"Something","language":"1"}' http://seroney-pc:5000/api/attributes

{
    "attribute_id": "1",
    "product_id": "44",
    "text": "Something",
    "language": "1"
}

My code is as follows:

from flask import Flask,request,jsonify, abort
from flask_sqlalchemy import SQLAlchemy
import flask_restless


app = Flask(__name__)
db = SQLAlchemy(app)
manager = flask_restless.APIManager(app)

app.config['SQLALCHEMY_DATABASE_URI'] = 'mysql://root:seroney@localhost:3306/test'


class Attributes(db.Model):
    __tablename__ = 'oc_product_attribute'
    product_id = db.Column(db.Integer,primary_key=True)
    attribute_id = db.Column(db.Integer,primary_key=True)
    language_id = db.Column(db.Integer,primary_key=True)
    text=db.Column(db.String)

@app.route('/api/attributes/',methods=['GET'])
def getProductAttributes():
    if request.method =='GET':
        results = Attributes.query.limit(10).offset(0).all()
        json_results = []
        for result in results:
            d = {
                'product_id':result.product_id,
                'attribute_id':result.attribute_id,
                'language_id':result.language_id,
                'text':result.text
            }
            json_results.append(d)

        return jsonify(items = json_results)

@app.route('/api/attributes/', methods=['POST'])

def postProductAttributes():
    product_id = request.json['product_id']
    attribute_id = request.json['attribute_id']
    language_id = request.json['language_id']
    text = request.json['text']
    if product_id is None or attribute_id is None or language_id is None or text is None:
        return jsonify({"message": "Error."}), 400
    new_attrib = (product_id,attribute_id,language_id,text)
    db.session.add(new_attrib)
    db.session.commit()
    return jsonify({'message' :'Attribute Created successfully'}), 200

if __name__ == '__main__':
    app.run(debug=True)

When I POST I keep I getting an Internal Server Error. Any help is highly appreciated.

The traceback is:

seroney-pc - - [23/Dec/2014 20:48:40] "POST /api/attributes HTTP/1.1" 500 -
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1453, in dispatch_request
    self.raise_routing_exception(req)
  File "/usr/local/lib/python2.7/dist-packages/flask/app.py", line 1439, in raise_routing_exception
    raise FormDataRoutingRedirect(request)

Note: this exception is only raised in debug mode

Answer 1

You are posting to the URL without a / at the end, but you specified your route with a trailing /. When you do that, Flask issues a redirect to the 'canonical' URL, with the /.

Because you are using POST, the post data will be lost, so in debug mode an exception is raised instead to inform you that you should use the trailing slash in your post instead.

Had you looked at the body of the error message, you'd have seen something like:

FormDataRoutingRedirect: A request was sent to this URL (http://seroney-pc:5000/api/attributes) but a redirect was issued automatically by the routing system to "http://seroney-pc:5000/api/attributes/". The URL was defined with a trailing slash so Flask will automatically redirect to the URL with the trailing slash if it was accessed without one. Make sure to directly send your POST-request to this URL since we can't make browsers or HTTP clients redirect with form data reliably or without user interaction.

Note: this exception is only raised in debug mode

See the Rule Format documentation:

URL rules that end with a slash are branch URLs, others are leaves. If you have strict_slashes enabled (which is the default), all branch URLs that are visited without a trailing slash will trigger a redirect to the same URL with that slash appended.

Note that your curl POST uses the wrong header; you need to set the Content-Type header. Your view is looking for the language_id key, but your post contains only a language key, you need to correct that too:

curl -i -H "Content-Type: application/json" -X POST \
     -d '{"attribute_id": "1", "product_id": "44","text":"Something","language_id":"1"}' http://seroney-pc:5000/api/attributes/

The Accept header may be useful too, but it is used for negotiating the response content type, and you have your views hardcoded to return JSON.

Your code creating the database object is also incorrect, you need to call the model and pass in the arguments as separate arguments, then pass in the resulting to session.add():

new_attrib = Attributes(*new_attrib)
db.session.add(new_attrib)

but just reusing the JSON object would be easier here:

db.session.add(Attributes(**request.json))