Yes. Your understanding is already quite good. The only missing piece is why each object exists and how they all connect together.

Think of it as building a house before people start living in it.

BIG PICTURE

                Step 1
        Tell Python where database is

DATABASE_URL
      │
      ▼
sqlite:///./users.db

                Step 2
      Build connection engine

create_engine()

      │
      ▼
engine

                Step 3
      Build session factory

sessionmaker(bind=engine)

      │
      ▼
SessionLocal

                Step 4
    Create ORM Base class

Base = declarative_base()

      │
      ▼

class User(Base)

      │
      ▼

Table Definition

                Step 5
Tell SQLAlchemy to create tables

Base.metadata.create_all(bind=engine)

      │
      ▼

users.db
      │
      ▼

users Table

                Step 6
Whenever request comes

db = SessionLocal()

      │
      ▼

Read / Write Database

Notice that Steps 1–5 happen only once (during application startup), while Step 6 happens for every request.

STEP 1 — Database URL

DATABASE_URL = "sqlite:///./users.db"

This is simply the database's address.

Imagine you're telling a delivery person:

"My house is at 25 Green Street."

Similarly:

sqlite:///./users.db

means

My database is stored in the file users.db.

Nothing is created yet.

You're only specifying where the database is located.

STEP 2 — Create Engine

engine = create_engine(DATABASE_URL)

The engine is not the database.

It is a connection manager.

Think of it like this:

Python
   │
   ▼
Engine
   │
   ▼
users.db

The engine knows:

• where the database is

• how to connect

• how to send SQL

• how to receive results

Without the engine,

Python cannot talk to SQLite.

Real-life analogy

Imagine your database is another country.

The engine is the airplane.

Without the airplane,

you cannot travel there.

STEP 3 — Session Factory

SessionLocal = sessionmaker(bind=engine)

This is one of the biggest concepts beginners struggle with.

Notice carefully:

This does NOT create a database session.

It creates a factory that knows how to create sessions.

Think of a cookie factory.

Cookie Factory

      │

Makes Cookies

Similarly,

Session Factory

      │

Makes Sessions

So

SessionLocal

is NOT a session.

It is a machine that creates sessions.

Later,

db = SessionLocal()

creates an actual session.

Exactly like

CookieFactory()

↓

Cookie

STEP 4 — Create Base

Base = declarative_base()

This tells SQLAlchemy:

"Every class inheriting from Base is a database table."

Without Base,

class User:

is just a normal Python class.

With Base,

class User(Base):

SQLAlchemy immediately understands:

This represents a database table.

STEP 5 — Define Table

class User(Base):

Now you describe the table.

__tablename__ = "users"

means

Database table name:

users

Then

id = Column(...)

means

id column

Then

username = Column(...)

means

username column

Then

hashed_password = Column(...)

means

hashed_password column

At this point,

the table still doesn't exist.

You're only describing it.

Think of drawing a house blueprint.

The house hasn't been built yet.

STEP 6 — Create Tables

Base.metadata.create_all(bind=engine)

This is where SQLAlchemy says

"Okay, I've seen all the blueprints."

Now it asks:

Does users table exist?

If NO,

it creates it.

If YES,

it skips it.

Equivalent SQL:

CREATE TABLE users(
id INTEGER PRIMARY KEY,
username TEXT,
hashed_password TEXT
);

Now your database becomes

users.db

│

└── users table

IMPORTANT

Notice something.

Until now,

no user exists.

Only the table exists.

users

-----------------------

(empty)

STEP 7 — Request Comes

Suppose someone signs up.

FastAPI calls

get_db()

Inside

db = SessionLocal()

Remember?

SessionLocal is the factory.

Session Factory

↓

Creates Session

Now

db

is your active conversation with the database.

Imagine

Python

│

db

│

Engine

│

users.db

STEP 8 — Add User

new_user = User(...)

creates a Python object.

Not a database row.

Just a Python object.

Memory

↓

User Object

Then

db.add(new_user)

tells SQLAlchemy

"Prepare to insert this."

Still not saved.

Then

db.commit()

actually writes it into

users.db

STEP 9 — Close Session

Finally,

db.close()

ends the conversation.

Exactly like hanging up a phone call.

Why do we need get_db()?

Instead of writing

db = SessionLocal()

everywhere,

we create one reusable function.

def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

Every request gets:

Open Session

↓

Use Database

↓

Close Session

Automatically.

Complete Lifecycle

Developer Starts FastAPI

        │

        ▼

DATABASE_URL

        │

        ▼

create_engine()

        │

        ▼

engine

        │

        ▼

sessionmaker()

        │

        ▼

SessionLocal (Factory)

        │

        ▼

declarative_base()

        │

        ▼

class User(Base)

        │

        ▼

create_all()

        │

        ▼

users.db

        │

        ▼

users table created

═══════════════════════════════════════

User sends Signup Request

        │

        ▼

get_db()

        │

        ▼

db = SessionLocal()

        │

        ▼

Create User Object

        │

        ▼

db.add()

        │

        ▼

db.commit()

        │

        ▼

Row Stored in users.db

        │

        ▼

db.close()

The key mental model

There are three different layers, and keeping them separate removes most confusion:

• Database configuration (created once): DATABASE_URL → engine → SessionLocal

• Database schema (created once): Base → User model → Base.metadata.create_all(bind=engine)

• Database operations (every request): db = SessionLocal() → query/add/update/delete → db.commit() (if needed) → db.close()

Once you see these as Configuration → Schema → Runtime Operations, the entire SQLAlchemy workflow becomes much easier to reason about.