Flask-SQLAlchemy 多个数据库组成一个查询

Question

I am new to using Flask SQLAlchemy and I have a few questions on how can how can I link separate databases together to form a query.

I have 3 databases: User , Homework , Questions , where 1 user can have many homework and 1 homework can have many questions. I have made 3 separate.py files for the 3 classes and each file calls simple functions like query.all(), and some simple filtering.

I have previously learned MySQL and I am wondering how can I create an SQLAlchemy equivalent of the following query:

SELECT * FROM user, homework, questions 
WHERE user.user_id = homework.user_id 
AND homework.homework_id = questions.homework_id

My question is how do I go about achieving this? Do I make a new file and find a way to bind these databases together or is there a more optimal way of doing it?

Also, how do I go about retrieving aggregated function values? For instance, the average marks one would get for each homework.

User.py

class User(db.Model):
    __tablename__ = 'user'
    user_id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    username = db.Column(db.String(20), nullable=False)

Homework.py

class Homework(db.Model):
    __tablename__ = 'homework'
    homework_id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    user_id= db.Column(db.Integer, nullable=False)
    subject = db.Column(db.String(20), nullable=False)

Question.py

class Question(db.Model):
    __tablename__ = 'question'
    question_id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    homework_id = db.Column(db.Integer, nullable=False)
    marks = db.Column(db.Integer, nullable=False)

Answer 1

You can create relationships amongst your tables using the following

class User(db.Model):
    __tablename__ = 'user'
    user_id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    username = db.Column(db.String(20), nullable=False)

class Homework(db.Model):
    __tablename__ = 'homework'
    homework_id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    user_id = db.Column(db.Integer, db.ForeignKey('user.user_id'), nullable=False)
    subject = db.Column(db.String(20), nullable=False)


class Question(db.Model):
    __tablename__ = 'question'
    question_id = db.Column(db.Integer, primary_key=True, autoincrement=True)
    homework_id = db.Column(db.Integer, db.ForeignKey('homework.homework_id'), nullable=False)
    marks = db.Column(db.Integer, nullable=False)

Then you can access info from a given table using the following insert your desired user_id in replace for xxx

user_object = User.query.filter_by(user_id=xxx).first()
username = user_object.username
user_id_value = user_object.user_id

homework_object = Homework.query.filter_by(user_id=user_id_value).first()
subject = homework_object.subject
homework_id_value = homework_object.homework_id

question_object = Question.query.filter_by(homework_id=homework_id_value).first()
marks = question_object.marks