jsonify a SQLAlchemy result set in Flask


Question

I'm trying to jsonify a SQLAlchemy result set in Flask/Python.

The Flask mailing list suggested the following method http://librelist.com/browser//flask/2011/2/16/jsonify-sqlalchemy-pagination-collection-result/#04a0754b63387f87e59dda564bde426e :

return jsonify(json_list = qryresult)

However I'm getting the following error back:

TypeError: <flaskext.sqlalchemy.BaseQuery object at 0x102c2df90> 
is not JSON serializable

What am I overlooking here?

I have found this question: How to serialize SqlAlchemy result to JSON? which seems very similar however I didn't know whether Flask had some magic to make it easier as the mailing list post suggested.

Edit: for clarification, this is what my model looks like

class Rating(db.Model):

    __tablename__ = 'rating'

    id = db.Column(db.Integer, primary_key=True)
    fullurl = db.Column(db.String())
    url = db.Column(db.String())
    comments = db.Column(db.Text)
    overall = db.Column(db.Integer)
    shipping = db.Column(db.Integer)
    cost = db.Column(db.Integer)
    honesty = db.Column(db.Integer)
    communication = db.Column(db.Integer)
    name = db.Column(db.String())
    ipaddr = db.Column(db.String())
    date = db.Column(db.String())

    def __init__(self, fullurl, url, comments, overall, shipping, cost, honesty, communication, name, ipaddr, date):
        self.fullurl = fullurl
        self.url = url
        self.comments = comments
        self.overall = overall
        self.shipping = shipping
        self.cost = cost
        self.honesty = honesty
        self.communication = communication
        self.name = name
        self.ipaddr = ipaddr
        self.date = date
1
109
5/23/2017 12:34:30 PM

Accepted Answer

It seems that you actually haven't executed your query. Try following:

return jsonify(json_list = qryresult.all())

[Edit]: Problem with jsonify is, that usually the objects cannot be jsonified automatically. Even Python's datetime fails ;)

What I have done in the past, is adding an extra property (like serialize) to classes that need to be serialized.

def dump_datetime(value):
    """Deserialize datetime object into string form for JSON processing."""
    if value is None:
        return None
    return [value.strftime("%Y-%m-%d"), value.strftime("%H:%M:%S")]

class Foo(db.Model):
    # ... SQLAlchemy defs here..
    def __init__(self, ...):
       # self.foo = ...
       pass

    @property
    def serialize(self):
       """Return object data in easily serializable format"""
       return {
           'id'         : self.id,
           'modified_at': dump_datetime(self.modified_at),
           # This is an example how to deal with Many2Many relations
           'many2many'  : self.serialize_many2many
       }
    @property
    def serialize_many2many(self):
       """
       Return object's relations in easily serializable format.
       NB! Calls many2many's serialize property.
       """
       return [ item.serialize for item in self.many2many]

And now for views I can just do:

return jsonify(json_list=[i.serialize for i in qryresult.all()])

Hope this helps ;)

[Edit 2019]: In case you have more complex objects or circular references, use a library like marshmallow).

154
7/27/2019 3:08:35 PM

I had the same need, to serialize into json. Take a look at this question. It shows how to discover columns programmatically. So, from that I created the code below. It works for me, and I'll be using it in my web app. Happy coding!


def to_json(inst, cls):
    """
    Jsonify the sql alchemy query result.
    """
    convert = dict()
    # add your coversions for things like datetime's 
    # and what-not that aren't serializable.
    d = dict()
    for c in cls.__table__.columns:
        v = getattr(inst, c.name)
        if c.type in convert.keys() and v is not None:
            try:
                d[c.name] = convert[c.type](v)
            except:
                d[c.name] = "Error:  Failed to covert using ", str(convert[c.type])
        elif v is None:
            d[c.name] = str()
        else:
            d[c.name] = v
    return json.dumps(d)

class Person(base):
    __tablename__ = 'person'
    id = Column(Integer, Sequence('person_id_seq'), primary_key=True)
    first_name = Column(Text)
    last_name = Column(Text)
    email = Column(Text)

    @property
    def json(self):
        return to_json(self, self.__class__)

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