Using One Database for Two Apps: Should You Copy Your Models or Import Them?

When you work on a project that has one backend but multiple clients (like a web app + a mobile app), you eventually face this question:

“I already have SQLAlchemy models in my Flask application. Now I’m building a FastAPI backend for mobile — should I copy the models into the FastAPI project or import them from my Flask project?”

This sounds like a small decision.
But it affects your entire project architecture, maintainability, and database safety.

Let’s break it down in simple words.

✅ The Two Options

Option 1 — Copy the model files into the FastAPI app

Just take the models.py files from your Flask project and paste them inside your FastAPI folder.

Option 2 — Import the same models directly from the Flask app

Your FastAPI app uses:

from flask_app.models import User, Vendor

so both apps share one source of truth.

🔍 What Happens If You Copy the Models?

Copying feels simple:

Paste the models

Make some modifications

Done?

Not really.

❌ 1. You’ll be maintaining two versions of every model

If you add a new column in Flask:

User.address = db.Column(…)

you must also manually add it in FastAPI.

Developers forget.
Then two apps start thinking the database looks different.

That’s where problems start.

❌ 2. Two separate migration systems

Flask uses Flask-Migrate.
FastAPI uses Alembic directly.

If you copy models, you now have:

two Alembic folders,

two migration histories,

two places to run migrations.

One mistake?
Inconsistent schemas → broken API.

❌ 3. Risk of database mismatch

Imagine Flask thinks:

name = String(100)

but FastAPI thinks:

name = String(50)

or one app adds a column and the other doesn’t know about it.

Results?

• Foreign key failures
• Missing column errors
• Mismatched constraints
• Strange bugs you cannot debug quickly

❌ 4. More work today + even more work tomorrow

Copying models looks easy in the beginning, but when your project grows, you’ll suffer from:

• duplicated code
• confusion
• extra debugging
• unpredictable errors

🌟 Why Importing the Models Is the Better Solution

Instead of copying, your FastAPI app can simply load the same models:

from flask_app.models import User, Vendor

This means:

✔ Only one version of models (no duplication)
✔ Only one migration folder (no conflicts)
✔ Both apps use the same Aiven database cleanly
✔ You update models in one place
✔ FastAPI and Flask stay in sync forever
✔ No surprises or schema mismatches

This approach is cleaner, scalable, and maintainable.

🔧 How to Make This Work

It’s surprisingly simple:

Ensure your Flask project is structured like a real Python package.
(it already is, if it has init.py)

Add the Flask project to Python path from FastAPI.

Import models:

from vendora_app.models import User

Use the same database connection (Aiven, PostgreSQL, etc.)

Your FastAPI app now uses the same models, same tables, same engine — with zero duplication.

🎯 So What Should You Do?

If your project will grow, if you care about maintainability, or if multiple developers will work on it:

Always import the models. Do not copy them.

Copying seems easy today but causes long-term technical debt.

Importing is the clean, scalable, professional approach.

📝 Conclusion

When building multiple apps that share the same database:

Copying models creates more problems than it solves.

Importing models keeps everything consistent and future-proof.

Always aim to maintain a single source of truth for your database schema.

Your future self (and your teammates) will thank you