from collections.abc import AsyncGenerator

import uuid # UUID = Universally Unique Identifier.

from sqlalchemy import Column , String , Text , DateTime ,  ForeignKey

from sqlalchemy.dialects.postgresql import UUID

from sqlalchemy.ext.asyncio import AsyncSession , create_async_engine , async_sessionmaker

from sqlalchemy.orm import DeclarativeBase , relationship

import datetime

DATABASE_URL = "sqlite+aiosqlite:///./test.db" 
# this will use local data base for practive later on we will use the actual data base like postgresql

class Post(DeclarativeBase):
    __tablename__ = "posts"

    id = Column(UUID(as_uuid=True) , primary_key=True , default=uuid.uuid4) 
    # as_uuid=True is Returned as Python UUID object so inside python it will be like type(post.id)(better)
    # default=uuid.uuid4 : if no id is provided, automatically generate one..
    # uuid.uuid4() generates a random UUID.

    caption = Column(Text)
    url = Column(String , nullable=False)
    file_type = Column(String , nullable=False)
    file_name = Column(String , nullable=False)
    created_at = Column(DateTime , dafault=datetime.utcnow)

Dont worry i will explain:

class Post(DeclarativeBase):
    __tablename__ = "posts"

Notice important snippet here :

class Post(DeclarativeBase):

You are creating a model class
That class represents a database table

In SQLAlchemy ORM:

• A class = a table

• An object of that class = a row in the table

class Post(DeclarativeBase):

means : Post is a table model that inherits from SQLAlchemy's base class

What is DeclarativeBas ?

In SQLAlchemy 2.0, we define a base like this:

from sqlalchemy.orm import DeclarativeBase

class Base(DeclarativeBase):
    pass

This Base is parent of all models…

class Post(Base):
    __tablename__ = "posts"

So DeclarativeBase :

• Registers the class as a database model

• Connects it to SQLAlchemy ORM system

• Keeps track of metadata

Without inheriting from it → SQLAlchemy will NOT recognize it as a table.

What is __tablename__ = “posts” :

Means:

Database table name -> posts

NOW :

id = Column(UUID(as_uuid=True) , primary_key=True , default=uuid.uuid4)

i am telling that make a Column named as id that why i wrote id = Column()
Means :

Create a column named id in the database that stores a UUID, make it the primary key, and automatically generate a new UUID when a row is created.

UUID(as_uuid=True)

This defines the data type.

What is UUID?

UUID = Universally Unique Identifier

Example:

550e8400-e29b-41d4-a716-446655440000

primary_key=True :

Primary key:

• Must be unique

• Cannot be NULL

• Identifies each row

So every post will have a unique UUID.

default=uuid.uuid4

This is VERY important.

It means:

"If no id is provided, automatically generate one."

uuid.uuid4() generates a random UUID.

and rest are just so easy :

    caption = Column(Text)
# create a column named as caption it will contain Text
    url = Column(String , nullable=False)
# url column will contain String and can't be null so user should always put some string 
    file_type = Column(String , nullable=False)
# same apply here
    file_name = Column(String , nullable=False)
# same also 
    created_at = Column(DateTime , dafault=datetime.utcnow)
# DateTime

DateTime :

This is a SQLAlchemy database column type.

It tells the database:

"Store date and time in this column."

It defines the SQL data type, not a Python value.

datetime :

his is a Python built-in class.

It creates actual date-time values in Python.

Example:

datetime.utcnow()