这个错误可能是因为使用不正确的查询，以及尝试在 Int 实例上设置实例状态。解决方法是确保你正在正确查询对象，并在选择多个字段中使用字符串而不是整数作为值。或者，如果你真的想使用整数，请首先将其转换为字符串。下面是使用 Flask，SQLAlchemy 和 WTF SelectMultipleField 的示例代码，可以帮助避免这个错误：

from flask import Flask, render_template, request
from flask_sqlalchemy import SQLAlchemy
from wtforms import SelectMultipleField, widgets

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///test.db'
db = SQLAlchemy(app)

association_table = db.Table('association', db.Model.metadata,
    db.Column('student_id', db.Integer, db.ForeignKey('student.id')),
    db.Column('class_id', db.Integer, db.ForeignKey('class.id'))
)

class Student(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    classes = db.relationship('Class', secondary=association_table, backref=db.backref('students', lazy='dynamic'))

class Class(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))

class MultiCheckboxField(SelectMultipleField):
    widget = widgets.ListWidget(prefix_label=False)
    option_widget = widgets.CheckboxInput()

@app.route('/', methods=['GET', 'POST'])
def index():
    form = StudentForm(request.form)
    form.classes.choices = [(str(c.id), c.name) for c in Class.query.all()]
    if request.method == 'POST' and form.validate():
        student = Student(name=form.name.data)
        for class_id in form.classes.data:
            student.classes.append(Class.query.get(int(class_id)))
        db.session.add(student)
        db.session.commit()
    students = Student.query.all()
    return render_template('index.html', form=form, students=students)

class StudentForm(FlaskForm):
    name = StringField('Name')
    classes = MultiCheckboxField('Classes')

if __name__ ==