Alembic migrations in FastAPI - Step-by-Step Execution

Concept Flow - Alembic migrations

Create migration script

↓

Edit migration script

↓

Run alembic upgrade

↓

Database schema updated

↓

Check migration history

↓

Repeat for new changes

This flow shows how you create, edit, and apply database schema changes step-by-step using Alembic migrations.

Execution Sample

FastAPI

alembic revision -m "add users table"
# edit migration script
alembic upgrade head

Create a new migration script, edit it to define schema changes, then apply those changes to the database.

Execution Table

Step	Command/Action	Effect	Database State	Notes
1	alembic revision -m "add users table"	Creates new migration script file	Schema unchanged	Migration script ready for edits
2	Edit migration script	Defines new table 'users'	Schema unchanged	Script now contains SQL commands for new table
3	alembic upgrade head	Runs migration script	Users table added	Database schema updated
4	alembic history	Shows applied migrations	Users table present	Confirms migration applied
5	alembic revision -m "add email column"	Creates new migration script	Users table present	Prepare next schema change
6	Edit migration script	Adds 'email' column to users	Users table present	Script ready for next upgrade
7	alembic upgrade head	Runs migration script	Users table with email column	Schema updated again
8	alembic downgrade -1	Reverts last migration	Users table without email column	Schema rolled back one step
9	alembic upgrade head	Reapplies last migration	Users table with email column	Schema restored
10	Exit	No more commands	Final schema state	Migration process complete

💡 No more migration commands; database schema is up to date

Variable Tracker

Variable	Start	After Step 3	After Step 7	After Step 8	After Step 9
Database Schema	Empty	Users table added	Users table + email column	Users table without email column	Users table + email column

Key Moments - 3 Insights

Why does the database schema not change immediately after creating a migration script?

Creating a migration script only prepares the changes; the schema updates only happen after running 'alembic upgrade' as shown in steps 1-3.

What happens when you run 'alembic downgrade -1'?

It reverts the last applied migration, rolling back the schema to the previous state, as seen in step 8.

Why do we need to edit the migration script after creating it?

Alembic generates an empty script; you must add the specific schema changes manually before applying them, shown in steps 1-2 and 5-6.

Visual Quiz - 3 Questions

Look at the execution table, what is the database state after step 3?

A Users table with email column

B Empty schema

C Users table added

D Users table without email column

💡 Hint

Check the 'Database State' column in row for step 3

At which step does the 'email' column get removed from the users table?

A Step 8

B Step 9

C Step 7

D Step 5

💡 Hint

Look for the step where the schema rolls back the last migration

If you skip editing the migration script after creating it, what happens when you run 'alembic upgrade head'?

A Schema updates with default changes

B No schema changes applied

C Error stops migration

D Alembic creates a new migration script automatically

💡 Hint

Refer to steps 1-3 and the note about editing migration scripts

Concept Snapshot

Alembic migrations manage database schema changes.
Create a migration script with 'alembic revision -m'.
Edit the script to define schema updates.
Apply changes using 'alembic upgrade head'.
Use 'alembic downgrade' to revert changes.
Check migration history with 'alembic history'.

Full Transcript

Alembic migrations help update your database schema step-by-step. First, you create a migration script using 'alembic revision -m' with a message describing the change. This script is empty initially, so you edit it to add the specific schema changes like creating tables or adding columns. Then, you apply these changes to the database by running 'alembic upgrade head'. This updates the database schema. You can check which migrations have been applied with 'alembic history'. If needed, you can undo the last migration using 'alembic downgrade -1'. This process allows you to safely and clearly manage database changes over time.