```python
from sqlalchemy import Column, ForeignKey, Integer, String, Text
from sqlalchemy.orm import relationship
from sqlalchemy.sql.sqltypes import TIMESTAMP
from sqlalchemy.sql import func

from app.db import Base

class Comment(Base):
    __tablename__ = "comments"

    id = Column(Integer, primary_key=True, index=True)
    content = Column(Text, nullable=False)
    created_at = Column(TIMESTAMP, server_default=func.now())
    updated_at = Column(TIMESTAMP, server_default=func.now(), onupdate=func.now())
    post_id = Column(Integer, ForeignKey("posts.id"), nullable=False)

    post = relationship("Post", back_populates="comments")

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

Explanation:

1. We import the necessary modules from SQLAlchemy, including `Column`, `ForeignKey`, `Integer`, `String`, `Text`, `relationship`, `TIMESTAMP`, `func`, and `Base` from `app.db`.

2. The `Comment` class is defined, which inherits from `Base`. This class represents the `comments` table in the database.

   - `id`: An integer primary key column with an index.
   - `content`: A text column that cannot be null, representing the content of the comment.
   - `created_at`: A timestamp column with the server's default time as the value when a new row is inserted.
   - `updated_at`: A timestamp column with the server's default time as the value when a new row is inserted, and updated with the current time whenever the row is updated.
   - `post_id`: An integer column that is a foreign key referencing the `id` column of the `posts` table. This establishes a one-to-many relationship between posts and comments.

4. We define a `relationship` between the `Comment` and `Post` models using the `relationship` function from SQLAlchemy. This allows us to access the related post from a comment instance, and vice versa.

5. The `__repr__` method is defined to provide a string representation of a `Comment` instance, which can be useful for debugging purposes.