Template helpers (that is, everything in ``kallithea.lib.helpers``)

should only be referenced from templates. If you need to call a

helper from the Python code, consider moving the function somewhere

else (e.g. to the model).

Notes on the SQLAlchemy session

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Each HTTP request runs inside an independent SQLAlchemy session (as well

as in an independent database transaction). Database model objects

(almost) always belong to a particular SQLAlchemy session, which means

that SQLAlchemy will ensure that they're kept in sync with the database

(but also means that they cannot be shared across requests).

Objects can be added to the session using ``Session().add``, but this is

rarely needed:

* When creating a database object by calling the constructor directly,

  it must explicitly be added to the session.

* When creating an object using a factory function (like

  ``create_repo``), the returned object has already (by convention)

  been added to the session, and should not be added again.

* When getting an object from the session (via ``Session().query`` or

  any of the utility functions that look up objects in the database),

  it's already part of the session, and should not be added again.

  SQLAlchemy monitors attribute modifications automatically for all

  objects it knows about and syncs them to the database.

SQLAlchemy also flushes changes to the database automatically; manually

calling ``Session().flush`` is usually only necessary when the Python

code needs the database to assign an "auto-increment" primary key ID to

a freshly created model object (before flushing, the ID attribute will

be ``None``).