Here's a `comments.py` file for the `app/api/v1/models/` directory of the `blog_app_x0iat` FastAPI backend:

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 Comments(Base):
    __tablename__ = "comments"

    id = Column(Integer, primary_key=True, index=True)
    content = Column(Text, nullable=False)
    post_id = Column(Integer, ForeignKey("posts.id"), nullable=False)
    user_id = Column(Integer, ForeignKey("users.id"), nullable=False)
    parent_comment_id = Column(Integer, ForeignKey("comments.id"), nullable=True)

    post = relationship("Posts", back_populates="comments")
    user = relationship("Users", back_populates="comments")
    parent_comment = relationship("Comments", remote_side=[id], back_populates="replies")
    replies = relationship("Comments", back_populates="parent_comment")

    def __repr__(self):
        return f"Comment(id={self.id}, content='{self.content[:20]}...', post_id={self.post_id}, user_id={self.user_id})"

Explanation:

4. Defined columns for the `Comments` table:
   - `id`: Integer primary key with index
   - `content`: Text column for the comment content (required)
   - `post_id`: Integer foreign key referencing the `posts` table (required)
   - `user_id`: Integer foreign key referencing the `users` table (required)
   - `parent_comment_id`: Integer foreign key referencing the `comments` table itself (nullable, for nested comments)
5. Defined relationships with other models:
   - `post`: One-to-many relationship with the `Posts` model
   - `user`: One-to-many relationship with the `Users` model
   - `parent_comment`: One-to-many self-referential relationship for nested comments
   - `replies`: One-to-many self-referential relationship for nested comments (reverse of `parent_comment`)

Note: This implementation assumes the existence of `Posts` and `Users` models in the same directory. You may need to adjust the relationship definitions based on the actual structure of your project.