FastAPI Schema Diff — Compare Python Database Schemas Online

FastAPI Schema Diff.
Compare Alembic migrations.
Zero setup.

Paste two CREATE TABLE dumps from your FastAPI app — whether from alembic upgrade head output, SQLModel sync, SQLAlchemy reflect, or raw SQL. Spot every table, column, index, and constraint change instantly. Get ready-to-run ALTER TABLE scripts for PostgreSQL or MySQL — all in your browser.

No account required. No data leaves your device.

Built for FastAPI & Python workflows

SchemaLens understands Python-specific patterns that generic diff tools miss.

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Alembic Migration Review

Compare alembic revision --autogenerate outputs before and after model changes. Verify that SQLAlchemy migrations produced exactly the schema you expected.

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SQLModel & Pydantic Sync

Diff schemas generated from SQLModel tables with Pydantic-validated columns. Detects Optional[int] → int nullability changes and Field default modifications.

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SQLAlchemy Column Changes

Detects String(50) → Text(), Integer → BigInteger, and Enum value changes. Warns when length reductions could truncate data.

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Relationship & Foreign Key

Validates relationship() and ForeignKey constraints. Detects missing or changed foreign keys that could break FastAPI's dependency-injected database sessions.

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Enum & Check Constraint

Compares PostgreSQL ENUM types and CheckConstraint declarations. Catches schema drift that breaks Pydantic model validation or OpenAPI schema generation.

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Privacy First

Your schema never leaves the browser. Compare production FastAPI schemas safely without uploading sensitive table structures to a third-party server.

How it works with FastAPI

Dump your schemas

Run alembic upgrade head --sql or use pg_dump --schema-only on both environments (local vs staging, or staging vs production).

Paste into SchemaLens

Copy the SQL into the two editor panes. Dialect auto-detects PostgreSQL or MySQL based on your DATABASE_URL in .env.

Review the diff

See added tables, dropped columns, type changes, and constraint modifications highlighted in color. Check for breaking changes before your next deploy.

Copy the migration

Export ALTER TABLE scripts, save as Markdown for PR descriptions, or generate rollback SQL. Run in staging, then production.

FastAPI migration examples

SchemaLens generates production-ready ALTER TABLE scripts for every change it detects.

Add a new column

ALTER TABLE users
  ADD COLUMN is_active BOOLEAN DEFAULT TRUE;

Extend enum values

ALTER TYPE status_enum ADD VALUE 'archived';
ALTER TABLE posts
  ALTER COLUMN status TYPE status_enum;

Add a foreign key

ALTER TABLE comments
  ADD CONSTRAINT comments_post_id_fkey
  FOREIGN KEY (post_id) REFERENCES posts(id)
  ON DELETE CASCADE;

Add a check constraint

ALTER TABLE products
  ADD CONSTRAINT chk_price_positive
  CHECK (price >= 0);

Related guides

Python