Database migration in production in Django - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a database migration in Django?

A database migration in Django is a way to update the database schema to match changes in your models. It helps keep the database structure in sync with your code without losing data.

beginner

Why should you be careful when running migrations in production?

Because migrations can change the database structure, running them in production can cause downtime or data loss if not done carefully. It's important to plan and test migrations to avoid breaking the live app.

beginner

What command do you use to create migrations in Django?

You use python manage.py makemigrations to create migration files based on model changes.

beginner

How do you apply migrations to the production database?

You run python manage.py migrate on the production server to apply all pending migrations safely.

intermediate

What is a common strategy to avoid downtime during production migrations?

A common strategy is to use zero-downtime deployment techniques, like running migrations in steps, using database locks carefully, or applying non-blocking migrations during low traffic times.

Which Django command creates migration files after model changes?

A python manage.py makemigrations

B python manage.py migrate

C python manage.py runserver

D python manage.py collectstatic

The command makemigrations creates migration files based on model changes.

What does python manage.py migrate do?

A Creates migration files

B Applies migrations to the database

C Starts the Django server

D Deletes old migrations

The migrate command applies all pending migrations to the database.

Why is it important to test migrations before running them in production?

A To avoid syntax errors in Python code

B To speed up the server

C To update Django version

D To prevent downtime or data loss

Testing migrations helps prevent downtime or data loss in the live environment.

Which of these is a good practice for production migrations?

A Backup the database before migrating

B Ignore migration errors

C Run migrations during peak traffic

D Skip migrations to save time

Backing up the database before migrating protects data if something goes wrong.

What can cause downtime during a migration?

A Running migrations on a test server

B Using <code>makemigrations</code> command

C Adding new fields without default values

D Writing model comments

Adding new fields without defaults can lock tables and cause downtime.

Explain the steps to safely perform a database migration in a Django production environment.

Think about preparation, testing, backup, and applying changes carefully.

Describe common challenges you might face when running migrations in production and how to handle them.

Consider risks and strategies to minimize impact.

Practice

1. What is the primary purpose of running python manage.py migrate in a Django production environment?

easy

A. To create new migration files based on model changes

B. To apply database schema changes defined in migration files

C. To start the Django development server

D. To reset the database to its initial state

Solution

1. Step 1: Understand the migrate command

   The migrate command applies changes to the database schema based on migration files already created.

2. Step 2: Differentiate from makemigrations

   makemigrations creates migration files, but migrate applies them to the database.

3. Final Answer:

   To apply database schema changes defined in migration files -> Option B

4. Quick Check:

   migrate applies changes = A [OK]

Hint: migrate applies changes, makemigrations creates files [OK]

Common Mistakes:

• Confusing migrate with makemigrations
• Thinking migrate resets the database
• Believing migrate starts the server

2. Which of the following is the correct command to create new migration files after changing Django models?

easy

A. python manage.py runserver

B. python manage.py migrate

C. python manage.py makemigrations

D. python manage.py flush

Solution

1. Step 1: Identify the command for creating migrations

   makemigrations scans model changes and creates migration files.

2. Step 2: Confirm other commands' purposes

   migrate applies migrations, runserver starts server, flush clears data.

3. Final Answer:

   python manage.py makemigrations -> Option C

4. Quick Check:

   makemigrations creates files = A [OK]

Hint: makemigrations creates files, migrate applies them [OK]

Common Mistakes:

• Using migrate instead of makemigrations to create files
• Confusing runserver with migration commands
• Using flush to manage migrations

3. Given the following sequence of commands in production:

python manage.py makemigrations
python manage.py migrate

What will happen if a model field was renamed but the migration was not created before running migrate?

medium

A. No changes will be applied to the database schema

B. An error will occur because migrate requires new migration files

C. The database schema will update correctly with the renamed field

D. The old field will be deleted automatically

Solution

1. Step 1: Understand migration dependency

   migrate applies changes only if migration files exist. Without new migration files, no schema changes happen.

2. Step 2: Effect of missing migration files

   If you rename a field but skip makemigrations, the database stays unchanged after migrate.

3. Final Answer:

   No changes will be applied to the database schema -> Option A

4. Quick Check:

   migrate needs migration files = C [OK]

Hint: Always run makemigrations before migrate to apply changes [OK]

Common Mistakes:

• Assuming migrate updates schema without migration files
• Expecting automatic field deletion
• Thinking migrate throws error without new migrations

4. You ran python manage.py migrate in production but got an error about conflicting migrations. What is the best way to fix this?

medium

A. Use python manage.py migrate --merge to resolve conflicts

B. Manually edit the database tables to match models

C. Ignore the error and restart the server

D. Delete all migration files and recreate them

Solution

1. Step 1: Identify cause of migration conflicts

   Conflicts happen when multiple migration branches exist. Django offers a merge option to fix this.

2. Step 2: Use the merge option

   migrate --merge helps combine conflicting migrations safely without deleting files or manual edits.

3. Final Answer:

   Use python manage.py migrate --merge to resolve conflicts -> Option A

4. Quick Check:

   migrate --merge resolves conflicts = D [OK]

Hint: Use migrate --merge to fix conflicts safely [OK]

Common Mistakes:

• Deleting migration files causing data loss
• Manually editing tables risking corruption
• Ignoring errors leads to bigger issues

5. In a production environment, you want to add a new non-nullable field to a large existing table without downtime. Which approach is safest?

hard

A. Add the field as non-nullable directly and run migrate

B. Skip migrations and add the field manually in the database

C. Drop the table and recreate it with the new field

D. Add the field with null=True, migrate, then update data and alter to null=False

Solution

1. Step 1: Understand downtime risks

   Adding a non-nullable field directly can lock the table and cause downtime in production.

2. Step 2: Use a two-step migration

   First add the field as nullable (null=True), migrate, then fill data, and finally alter to non-nullable (null=False).

3. Final Answer:

   Add the field with null=True, migrate, then update data and alter to null=False -> Option D

4. Quick Check:

   Two-step migration avoids downtime = B [OK]

Hint: Add nullable field first, then make non-nullable after data update [OK]

Common Mistakes:

• Adding non-nullable field directly causing downtime
• Dropping tables losing data
• Skipping migrations causing inconsistencies