models.py

from flask_sqlalchemy import SQLAlchemy
from flask_login import UserMixin
from datetime import datetime

db = SQLAlchemy()

# Association table for followers
followers = db.Table('followers',
    db.Column('follower_id', db.Integer, db.ForeignKey('user.id')),
    db.Column('followed_id', db.Integer, db.ForeignKey('user.id'))
)

class User(UserMixin, db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(150), unique=True, nullable=False)
    password = db.Column(db.String(150), nullable=False)
    email = db.Column(db.String(150), unique=True, nullable=False)
    first_name = db.Column(db.String(150), nullable=False)
    last_name = db.Column(db.String(150), nullable=False)
    posts = db.relationship('Post', backref='author', lazy=True)
    followed = db.relationship(
        'User', secondary=followers,
        primaryjoin=(followers.c.follower_id == id),
        secondaryjoin=(followers.c.followed_id == id),
        backref=db.backref('followers', lazy='dynamic'), lazy='dynamic'
    )

    def follow(self, user):
        if not self.is_following(user):
            self.followed.append(user)

    def unfollow(self, user):
        if self.is_following(user):
            self.followed.remove(user)

    def is_following(self, user):
        return self.followed.filter(followers.c.followed_id == user.id).count() > 0

    def followed_posts(self):
        return Post.query.join(
            followers, (followers.c.followed_id == Post.user_id)
        ).filter(followers.c.follower_id == self.id).order_by(
            Post.date_posted.desc()
        )

class Post(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    image_file = db.Column(db.String(20), nullable=False)
    date_posted = db.Column(db.DateTime, nullable=False, default=datetime.utcnow)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)