validating user input using flask_wtf and sqlalchemy - python-3.x

Question

Noob question. I am building a login/registration pages, on the registration page, I am using flask_wtf to verify certain things like length of password, email format and whether the two password a user supplies match. Here is the flask_wtf code I am using to do that.
# import statements omitted for brevity
class RegistrationForm(FlaskForm):
username = StringField('Username',
validators=[DataRequired(), Length(min=2, max=20)])
email = StringField('Email',validators=[DataRequired(), Email()])
password = PasswordField('Password', validators=[DataRequired()])
confirm_password = PasswordField('Confirm Password',
validators=[DataRequired(),Length(min=4, max=20), EqualTo('password')])
submit = SubmitField('Sign Up')
After checking the input, I am using sqlalchemy to check if the username and email already exists in my DB. The problem I am facing right now is I cant get flask_wtf to verify the form. I can type whatever I want and it will be converted to a sql query. Here are my two flask routes that handle registration and user input validation.
#app.route('/register',methods=['GET','POST'])
def register():
form = RegistrationForm()
if form.validate_on_submit():
return redirect(url_for('check_user_input'))
return render_template('register.html',form=form)
#app.route('/status',methods=['POST'])
def check_user_input():
name = request.form.get("username")
email = request.form.get("email")
password = request.form.get("password")
if db.execute("SELECT * FROM DB WHERE username= :username",{"username":name}).rowcount==1:
return render_template("404.html", message="Sorry username already exists")
elif db.execute("SELECT * FROM DB WHERE email= :email",
{"email":email}).rowcount==1:
return render_template("404.html", message="Sorry email already exists")
else:
db.execute("INSERT INTO DB (username,email,password) VALUES
(:username,:email,:password)",
{"username":name, "email":email,"password":password})
db.commit()
return render_template("success.html")
How can I get flask_wtf form to do its verification first and then hand the input to check_user_input() function?
My register.html contains the following line.
<form class="form-signin" method="POST" action="{{url_for('check_user_input')}}">
Any help would be greatly appreciated.

Answer 1

A way is to add a custom validation to the form :
class RegistrationForm(FlaskForm):
username = StringField('Username',
validators=[DataRequired(), Length(min=2, max=20)])
email = StringField('Email',validators=[DataRequired(), Email()])
password = PasswordField('Password', validators=[DataRequired()])
confirm_password = PasswordField('Confirm Password',
validators=[DataRequired(),Length(min=4, max=20), EqualTo('password')])
submit = SubmitField('Sign Up')
def validate(self):
rv = FlaskForm.validate(self)
if not rv:
return False
if db.execute("SELECT * FROM DB WHERE username= :username",{"username":self.username.data}).rowcount>0:
self.username.errors.append('Sorry username already exists')
return False
if db.execute("SELECT * FROM DB WHERE email= :email", {"email":self.email.data}).rowcount>0:
self.email.errors.append('Sorry email already exists')
return False
return True