在表存在之前，如何使用 flask-sqlalchemy 映射表关系？

Question

I have a problem creating relationships between my tables in flask-sqlalchemy. I have a table with project overview, and from there on out I want to dynamically create new experiment tables with a relationship to my project overview. However, when I try to define the relationship, sqlalchemy throws the following error:

sqlalchemy.exc.InvalidRequestError: When initializing mapper mapped class Projects->projects, expression 'Experiment_Overview' failed to locate a name ('Experiment_Overview'). If this is a c
lass name, consider adding this relationship() to the <class 'app.Projects'> class after both dependent classes have been defined.

This seems to be the case because the class Experiment_Overview(db.Model) does not exist yet , which is correct since it will be dynamically generated later on through user input. How can I mitigate this error?

import os
from flask import Flask, render_template, redirect, request, url_for
from flask_sqlalchemy import SQLAlchemy
from datetime import datetime

app = Flask(__name__)
Bootstrap(app)
app.config["SQLALCHEMY_DATABASE_URI"] = "sqlite:///DATA/DB.db"
db = SQLAlchemy(app)


def TableCreator(tablename):
  class Experiment_Overview(db.Model):
    __tablename__ = tablename
    creation_date = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
    experiment_name = db.Column(db.String(30), unique=False, nullable=False, primary_key=True)
    projectname = db.Column(db.String(150), db.ForeignKey('projects.projectname'), nullable=False, unique=True)
  return MyTable

class Projects(db.Model):
    projectname = db.Column(db.String(150), unique=True, nullable=False, primary_key=True)
    creation_date = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
    experiments = db.relationship('Experiment_Overview', backref="experiments", lazy=True, uselist=False)

    def __init__(self, owner, projectname, status, created_at):
        self.owner=owner
        self.projectname=projectname
        self.status=status
        self.created_at=created_at

db.create_all()

Answer 1

Generally speaking , you don't dynamically create tables; you usually shouldn't create or drop tables while your program is running.
Additionally, I believe it's impossible to create a true relationship that links back to an entire table. Relationships/Foreign Keys are for linking between rows in tables.

Don't worry, there are easier ways to achieve the behavior that you are looking for here.

From your question it sounds like you can have multiple Projects, and each project can have multiple Experiments within the project.
This would make the relationship between a Project and its Experiments a One-To-Many relationship .

If this is the case, you would need one Projects table (which you have already in your code), and you would also have one Experiments table.

Each row in the Projects table represents one Project. Each row in the Experiments table represents one Experiment. The Experiments table will have a column containing a foreign key linking back to the Project the Experiment is linked to.

I've modified your code according to the One-To-Many example code given in the SQLAlchemy documentation that I linked above. Note the addition of the back_populates option to the relationship() , this allows bi-directional knowledge of the relationship: the Experiment know what project it belongs to, and the Project know what Experiments it has.

import os
from flask import Flask, render_template, redirect, request, url_for
from flask_sqlalchemy import SQLAlchemy
from datetime import datetime

app = Flask(__name__)
Bootstrap(app)
app.config["SQLALCHEMY_DATABASE_URI"] = "sqlite:///DATA/DB.db"
db = SQLAlchemy(app)

class Projects(db.Model):
    __tablename__ = "projects"
    projectname = db.Column(db.String(150), unique=True, nullable=False, primary_key=True)
    creation_date = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
    experiments = db.relationship("Experiment", back_populates="project")

    def __init__(self, owner, projectname, status, created_at):
        self.owner=owner
        self.projectname=projectname
        self.status=status
        self.created_at=created_at

class Experiment(db.Model):
    __tablename__ = "experiments"
    creation_date = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
    experiment_name = db.Column(db.String(30), unique=False, nullable=False, primary_key=True)
    projectname = db.Column(db.String(150), db.ForeignKey('projects.projectname'), nullable=False)
    project = relationship("Project", back_populates="experiments")

db.create_all()