AttributeError: 'NoneType' object has no attribute 'time_recorded' in Flask, SQLAlchemy

问题

I have an api endpoint that passes a variable which is used to make a call in the database. For some reason it cannot run the query yet the syntax is correct. My code is below.

@app.route('/api/update/<lastqnid>')
def check_new_entries(lastqnid):
    result = Trades.query.filter_by(id=lastqnid).first()
    new_entries = Trades.query.filter(Trades.time_recorded > result.time_recorded).all()

The id field is:

id = db.Column(db.String,default=lambda: str(uuid4().hex), primary_key=True)

I have tried filter instead of filter_by and it does not work. When I remove the filter_by(id=lastqnid) it works. What could be the reason it is not running the query?

The trades table am querying from is

class Trades(db.Model):
    id = db.Column(db.String,default=lambda: str(uuid4().hex), primary_key=True)
    amount = db.Column(db.Integer, unique=False)
    time_recorded = db.Column(db.DateTime, unique=False)

回答1:

The issue you seem to be having is not checking if you found anything before using your result

@app.route('/api/update/<lastqnid>')
def check_new_entries(lastqnid):
    result = Trades.query.filter_by(id=lastqnid).first()
    # Here result may very well be None, so we can make an escape here
    if result == None:
        # You may not want to do exactly this, but this is an example
        print("No Trades found with id=%s" % lastqnid)
        return redirect(request.referrer)
    new_entries = Trades.query.filter(Trades.time_recorded > result.time_recorded).all()

来源：https://stackoverflow.com/questions/51077280/attributeerror-nonetype-object-has-no-attribute-time-recorded-in-flask-sql