## User Model

```python
class User(UserMixin, db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), index=True, unique=True, nullable=False)
    uuid = db.Column(db.String(64), index=True, default=uuid4().__str__, unique=True)
    email = db.Column(db.String(120), index=True, unique=True)
    password_hash = db.Column(db.String(128), nullable=False)
  ```

### Issue Description
The server keeps generating this same UUID, which, when using uuid4 should be next to impossible. The uuid value takes the default, which in python, must be a callable, i.e. type=function, so I was passing it
```python
default=uuid4().__str__
```
so it would convert the UUID object to string before persisting to the db. I didn't notice the bug, but the issue is that I wanted a new uuid to be created each time a user is created, but instead a new string
```python
__str__
```
was being created with the same uuid values.

### Solution
The solution is to wrap the uuid call in a lambda, so we can both generate a unique uuid, and call tostring() on it.
```python
default=lambda: uuid4().__str__()
```
Wrapping with ```lambda``` enables uniquely call ```uuid4()``` and tostring(), and make it callable.


### Final Result

```python
class User(UserMixin, db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), index=True, unique=True, nullable=False)
    uuid = db.Column(db.String(64), index=True, default=lambda: uuid4().__str__(), unique=True)
    email = db.Column(db.String(120), index=True, unique=True)
    password_hash = db.Column(db.String(128), nullable=False)
  ```

keywords=Flask, SQLAlchemy, uuid, error, collision, uuid4, uuid collision