Now that you understand the power of SQLAlchemy, you can also understand what the SQLAlchemy session object is and why web apps should never be made without them. As stated before, the session can be simply described as an object that tracks the changes in our models and commits them to the database when we tell it to. However, there is a bit more to it than this.

First, the session is the handler for transactions. Transactions are sets of changes that are flushed to the database on commit. Transactions provide a lot of hidden functionality. For example, transactions automatically determine which objects will be saved first when objects have relations. You might have noted this when we were saving tags in the previous section. When we added tags to the posts, the session automatically knew to save the tags first despite the fact that we did not add it to be committed. If we are working with raw SQL queries and a database connection, we would have to...