How to map table relationships with flask-sqlalchemy, before tables exist?

Solution
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()