Flask Schema Diff — Compare SQLAlchemy Database Schemas Online

Flask Schema Diff.
Compare SQLAlchemy migrations.
Zero setup.

Paste two CREATE TABLE dumps from your Flask app — whether from flask db migrate (Flask-Migrate / Alembic), SQLAlchemy create_all() output, or raw SQL exports from PostgreSQL, MySQL, or SQLite. Spot every table, column, index, and constraint change instantly. Get ready-to-run ALTER TABLE scripts — all in your browser.

No account required. No data leaves your device.

Built for Flask & SQLAlchemy workflows

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

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SQLAlchemy Model Diff

Compare db.Model class changes reflected in schema dumps. Detects db.String(50) → db.String(100), db.Numeric precision changes, and nullable=False additions that break existing inserts.

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Alembic Migration Review

Validate flask db migrate auto-generated scripts against actual schema differences. Catch when Alembic misses index changes, constraint renames, or enum modifications before running flask db upgrade.

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Index & Constraint Detection

Diffs db.Index(), db.UniqueConstraint(), and db.CheckConstraint() declarations. Catches missing indexes that could slow down SQLAlchemy query performance.

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Relationship Validation

Validates db.ForeignKey() and db.relationship() constraints. Detects missing or changed foreign keys that could break SQLAlchemy backref and lazy-load behavior.

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Enum & Check Constraint

Detects changes to SQLAlchemy Enum types and CheckConstraint expressions. Validates that enum values match your Python Enum class definitions.

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Privacy First

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

How it works with Flask

Dump your schemas

Run pg_dump --schema-only for PostgreSQL, or use sqlite3 .schema for SQLite, or export from MySQL Workbench on both environments.

Paste into SchemaLens

Copy the SQL into the two editor panes. Dialect auto-detects PostgreSQL, MySQL, or SQLite based on your Flask SQLALCHEMY_DATABASE_URI.

Review the diff

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

Copy the migration

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

Flask migration examples

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

Add a new column

ALTER TABLE users
  ADD COLUMN email_verified BOOLEAN DEFAULT FALSE;

Change VARCHAR length

ALTER TABLE products
  ALTER COLUMN description TYPE VARCHAR(2000);

Add a foreign key

ALTER TABLE orders
  ADD CONSTRAINT fk_orders_user_id
  FOREIGN KEY (user_id) REFERENCES users(id)
  ON DELETE CASCADE;

Add a unique constraint

ALTER TABLE users
  ADD CONSTRAINT uq_users_email
  UNIQUE (email);

Related guides

Python