title: Flask-SQLAlchemy: How to delete row from many-to-many secondary table with the user id
body: <p>I have these two classes and a (many-to-many) secondary table linking them:</p>
  <pre><code>association_table = db.Table('association_table',
      db.Column('project_id',db.Integer, db.ForeignKey('project.id'), primary_key=True),
      db.Column('usr_id',db.Integer, db.ForeignKey('usr.id'), primary_key=True)
  )
  
  class Usr(db.Model, UserMixin):
      id = db.Column(db.Integer, primary_key=True)
      full_name = db.Column(db.String(150), unique=True, nullable=False)
      email = db.Column(db.String(150), unique=True, nullable=False)
      password = db.Column(db.String(150), nullable=False)
      projects = db.relationship('Project', secondary=association_table, lazy='subquery', backref=db.backref('usrs', lazy=True))
  
  class Project(db.Model):
      id = db.Column(db.Integer, primary_key=True)
      name = db.Column(db.String(150), unique=True, nullable=False)
      description = db.Column(db.String(1000))
      creation_date = db.Column(db.DateTime(timezone=True), server_default=func.now())
      status = db.Column(db.Boolean, nullable=False)
      tickets = db.relationship('Ticket')
      owner = db.Column(db.Integer, db.ForeignKey('usr.id'))
  </code></pre>
  <p>I want to remove one user from the association_table with his id (user no longer assigned to the project instance) without removing all the other users. How can I do that using Flask-SQLAlchemy?</p>
  <p>Example: how can I convert the following command in flask-sqlalchemy?</p>
  <pre><code>DELETE FROM association_table WHERE usr_id = 1;
  </code></pre>