Here's the `user.py` file for the `blog_app` FastAPI backend, located in the `app/api/v1/models/` directory:

from typing import Optional
from sqlalchemy import Column, ForeignKey, Integer, String, Text
from sqlalchemy.orm import relationship
from app.api.db.database import Base

class User(Base):
    __tablename__ = 'user'

    id = Column(Integer, primary_key=True, index=True)
    username = Column(String, unique=True, nullable=False)
    email = Column(String, unique=True, nullable=False)
    password = Column(String, nullable=False)
    full_name = Column(String)
    bio = Column(Text)

    posts = relationship("Post", back_populates="author")
    comments = relationship("Comment", back_populates="author")

    def __repr__(self):
        return f"User(id={self.id}, username='{self.username}', email='{self.email}')"

Explanation:

1. Imports:
   - `typing.Optional`: Used for type hints.
   - `sqlalchemy`: Imported `Column`, `ForeignKey`, `Integer`, `String`, and `Text` classes for defining table columns.
   - `sqlalchemy.orm`: Imported `relationship` for defining relationships between models.
   - `app.api.db.database`: Imported `Base` class, which is the base class for all SQLAlchemy models.

2. `User` class:
   - Inherits from the `Base` class.
   - `__tablename__` attribute specifies the name of the database table for this model.
   - Columns:
     - `id`: Integer primary key column with an index.
     - `username`: String column, unique and not nullable.
     - `email`: String column, unique and not nullable.
     - `password`: String column, not nullable.
     - `full_name`: String column.
     - `bio`: Text column.
   - Relationships:
     - `posts`: One-to-many relationship with the `Post` model, where a user can have multiple posts.
     - `comments`: One-to-many relationship with the `Comment` model, where a user can have multiple comments.
   - `__repr__` method: Provides a string representation of the `User` object for debugging purposes.

Note: The `Post` and `Comment` models are not defined in this file, but they are assumed to exist and have the appropriate relationships defined with the `User` model.