How it works...

Now, let's say we modify the model of our product table to add a new field called company, as shown here:

class Product(db.Model): 
    # ... 
    # Same product model as last recipe 
    # ... 
    company = db.Column(db.String(100)) 

The result of migrate will be something like the following snippet:

    $ flask db migrate
    INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
    INFO  [alembic.runtime.migration] Will assume non-transactional 
    DDL.
    INFO  [alembic.autogenerate.compare] Detected added column 
    'product.company'
     Generating 
<path/to/application>/flask_catalog/migrations/versions/2c08f71f9253_.py ... done 

In the preceding code, we can see that Alembic compares the new model with the database table and detects a newly added column for company in the product table (created by the Product model).

Similarly, the output of upgrade will be something like the following snippet:

    $ flask db upgrade
    INFO  [alembic.runtime.migration] Context impl SQLiteImpl.
    INFO  [alembic.runtime.migration] Will assume non-transactional 
    DDL.
    INFO  [alembic.runtime.migration] Running upgrade None -> 
    2c08f71f9253, empty message

Here, Alembic performs the upgrade of the database for the migration detected earlier. We can see a hex code in the preceding output. This represents the revision of the migration performed. This is for internal use by Alembic to track the changes to database tables.