A set of operations such as addition, removal, or updating database entries is called a database transaction. A database session consists of one or more transactions. The act of committing ends a transaction by saving the transactions permanently to the database. In contrast, rollback rejects the pending transactions and changes are not permanently saved in the database.

In Flask-SQLAlchemy, a database is changed in the context of a session, which can be accessed as the session attribute of the database instance. An entry is added to a session with the add() method. The changes in a session are permanently written to a database when .commit() is executed.

For example, we create new readers and would like to add them to our database:

from app import db, Reader
new_reader1 = Reader(name = "Nova", surname = "Yeni", email = "[email protected]")
new_reader2 = Reader(name = "Nova", surname = "Yuni", email = "[email protected]")
new_reader3 = Reader( name = "Tom", surname = "Grey", email = "[email protected]")

Note that we didn’t specify the primary key id value. Primary keys don’t have to be specified explicitly, and the values are automatically generated after the transaction is committed.

Adding each new entry to the database has the same pattern:

db.session.add(new_reader1)
try:
    db.session.commit()
except:
    db.session.rollback()

Notice that we surrounded db.session.commit() with a try-except block. Why did we do that? If you look more carefully, new_reader1 and new_reader2 have the same e-mail, and when we declared the Reader model, we made the e-mail column unique (see the app.py file). As a consequence, we want to undo the most recent addition to the transaction by using db.session.rollback() and continue with other additions without interruption.