Database migrations with Flask-Migrate - Step-by-Step Execution

Concept Flow - Database migrations with Flask-Migrate

Write/Change Models

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Run 'flask db migrate'

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Generate Migration Script

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Review/Edit Script

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Run 'flask db upgrade'

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Apply Changes to DB

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Database Updated

You first change your data models, then create a migration script, and finally apply it to update the database.

Execution Sample

Flask

from flask_migrate import Migrate
migrate = Migrate(app, db)

# Terminal commands:
# flask db migrate -m "Add user email"
# flask db upgrade

This sets up Flask-Migrate and runs commands to create and apply a migration.

Execution Table

Step	Action	Command/Code	Result
1	Change model code	Add 'email' field to User model	Model updated in code
2	Generate migration	flask db migrate -m "Add user email"	Migration script created in migrations/versions
3	Review migration	Check generated script for correctness	Migration script ready
4	Apply migration	flask db upgrade	Database schema updated with new 'email' column
5	Verify	Check DB schema or app behavior	New column available in DB

💡

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
User model fields	id, username	id, username, email	Migration script includes 'email' column	Database schema includes 'email' column	Database schema includes 'email' column

Key Moments - 2 Insights

Why do I need to run 'flask db migrate' after changing my models?

Because 'flask db migrate' creates a migration script that tells the database how to update its structure to match your new model changes, as shown in step 2 of the execution table.

What happens if I skip 'flask db upgrade'?

Skipping 'flask db upgrade' means the database won't apply the changes from the migration script, so your database schema stays old and mismatched with your models, as seen between steps 2 and 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the result after running 'flask db migrate'?

A Database schema updated

B Migration script created in migrations/versions

C Model code changed

D Migration script applied to DB

💡 Hint

Check step 2 in the execution table for the result of 'flask db migrate'

At which step does the database schema actually get updated?

A Step 2

B Step 1

C Step 4

D Step 3

💡 Hint

Look for when 'flask db upgrade' is run in the execution table

If you forget to add the new field to your model before migrating, what happens?

A Migration script will not include the new field

B Database will update correctly anyway

C Migration script will fail to generate

D Database schema will update with extra fields

💡 Hint

Refer to the variable tracker and step 1 where model changes happen before migration

Concept Snapshot

Flask-Migrate helps update your database schema when models change.
1. Change your models in code.
2. Run 'flask db migrate' to create migration scripts.
3. Review scripts if needed.
4. Run 'flask db upgrade' to apply changes to the database.
Always migrate before upgrading to keep DB and models in sync.

Full Transcript

Database migrations with Flask-Migrate involve changing your data models first, then generating a migration script using 'flask db migrate'. This script describes how to update the database schema. After reviewing the script, you apply the changes with 'flask db upgrade', which updates the actual database. This process keeps your database structure in sync with your application models. Skipping migration or upgrade steps can cause mismatches. The execution table shows each step and its result, while the variable tracker follows the model and database schema changes. Remember to always migrate after model changes and upgrade to apply them.