KeyError with the class Relationship(Model)

import datetime

from flask_bcrypt import generate_password_hash
from flask_bcrypt import check_password_hash
from flask_login import UserMixin
from peewee import *

DATABASE = SqliteDatabase('social.db')

class User(UserMixin, Model):
    username = CharField(unique=True)
    email = CharField(unique=True)
    password = CharField(max_length=100)
    joined_at = DateTimeField(default=datetime.datetime.now)
    is_admin = BooleanField(default=False)

    class Meta:
        database = DATABASE
        order_by = ('-joined_at',)

    def get_posts(self):
        return Post.select().where(Post.user == self)

    def get_stream(self):
        return Post.select().where(
            (Post.user == self)
        )

    def following(self):
        """The users that we are following."""
        return (
            User.select().join(
                Relationship, on=Relationship.to_user
            ).where(
                Relationship.from_user == self
            )
        )

    def followers(self):
        """Get Users following the current user"""
        return (
            User.select().join(
                Relationship, on=Relationship.from_user
            ).where(
                Relationship.to_user == self
            )
        )

    @classmethod
    def create_user(cls, username, email, password, admin=False):
        try:
            with DATABASE.transaction():
                cls.create(
                    username = username,
                    email = email,
                    password = generate_password_hash(password),
                    is_admin = admin
                )
        except IntegrityError:
            raise ValueError ("User already exists")

class Post(Model):
    timestamp = DateTimeField(default=datetime.datetime.now)
    user = ForeignKeyField(
        model=User,
        backref='posts'
    )
    content = TextField()

    class Meta:
        database = DATABASE
        order_by = ('-timestamp',)

class Relationship(Model):
    from_user = ForeignKeyField(User, backref='relationships')
    to_user = ForeignKeyField(User, backref='related_to')

    class Meta:
        database = DATABASE
        indexes = (
            (('from_user', 'to_user'), True)
        )

def initialize():
    DATABASE.connect()
    DATABASE.create_tables([User, Post, Relationship], safe=True)
    DATABASE.close()

from flask import (Flask, g, render_template, flash,
                   redirect, url_for)
from flask_bcrypt import check_password_hash
from flask_login import (LoginManager, login_user, logout_user,
                         login_required, current_user)

import forms
import models

DEBUG = True
PORT = 8000
HOST = "0.0.0.0"

app = Flask(__name__)
app.secret_key = 'xVQFFPq798Z6JejNkfxHS'

login_manager = LoginManager()
login_manager.init_app(app)
login_manager.login_view = 'login'

@login_manager.user_loader
def load_user(userid):
    try:
        return models.User.get(models.User.id == userid)
    except models.DoesNotExist:
        return None

@app.before_request
def before_request():
    """Connect to DB before each request"""
    g.db = models.DATABASE
    g.db.connect()
    g.user = current_user


@app.after_request
def after_request(response):
    """Close the DB Connection after each request"""
    g.db.close()
    return response

@app.route('/register', methods=('GET', 'POST'))
def register():
    form = forms.RegisterForm()
    if form.validate_on_submit():
        flash("You have successfully registered!", 'success')
        models.User.create_user(
            username = form.username.data,
            email = form.email.data,
            password= form. password.data
        )
        return redirect(url_for('index'))
    return render_template('register.html', form=form)

@app.route('/login', methods=('GET', 'POST'))
def login():
    form = forms.LoginForm()
    if form.validate_on_submit():
        try:
            user = models.User.get(models.User.email == form.email.data)
        except models.DoesNotExist:
            flash("Your email or password doesn't match!", 'error')
        else:
            if check_password_hash(user.password, form.password.data):
                login_user(user)
                flash('You have been logged in!', 'success')
                return redirect(url_for('index'))
            else:
                flash("Your email or password doesn't match!", 'error')
    return render_template('login.html', form=form)

@app.route('/logout')
@login_required
def logout():
    logout_user()
    flash("You have been logged out!", 'success')
    return redirect(url_for('index'))

@app.route('/new_post', methods=('GET', 'POST'))
@login_required
def post():
    form = forms.PostForm()
    if form.validate_on_submit():
        models.Post.create(user=g.user._get_current_object(),
                           content = form.content.data.strip())
        flash("Message Posted! Thanks!", "success")
        return redirect(url_for('index'))
    return render_template('post.html', form=form)


@app.route('/')
def index():
    stream = models.Post.select().limit(100)
    return render_template('stream.html', stream=stream)

@app.route('/stream')
@app.route('/stream/<username>')
def stream(username=None):
    template = 'stream.html'
    if username and username != current_user.username:
        user=models.User.select().where(models.User.username**username).get()
        stream = user.posts.limit(100)
    else:
        stream = current_user.get_stream().limit(100)
        user = current_user  
    if username:
        template = 'user_stream.html'
    return render_template(template, stream=stream, user=user)

if __name__ == '__main__':
    models.initialize()
    try:
        models.User.create_user(
            username='ryan',
            email='ryan@ryan.com',
            password='password',
            admin=True
        )
    except ValueError:
        pass
    app.run(debug=DEBUG, port=PORT, host=HOST)