.. _removecertificatecourses:

Persistence of Certificate Courses
==================================

If a certificate course requires a certain course and this is course
is deleted, also the referring certificate course is deleted.

We setup a data structure that reflects typical usage. It looks like
this::

    Department-Instance
    |
    +---> courses
    |        |
    |        +--------------------> Course-Instance
    |                                        ^
    +---> certificates                       |
             |                               |
             +-----> Certificate-Instance    |
                        |                    |
                        +------> Certificate-Course

The Certifcate Course here refers to a Course instance.

In Python we build such a structure like this (from top to bottom):

    >>> from zope.component import createObject
    >>> mydept = createObject('waeup.Department')

In real world use this data will be stored in a ZODB. We setup our own
litte ZODB backend (which is easy!):

    >>> from ZODB import FileStorage, DB
    >>> dbpath = 'tinyData.fs'
    >>> class TinyZODB(object):
    ...   def __init__(self, path=dbpath):
    ...     self.storage = FileStorage.FileStorage(path)
    ...     self.db = DB(self.storage)
    ...     self.connection = self.db.open()
    ...     self.dbroot = self.connection.root()
    ...   def close(self):
    ...     self.connection.close()
    ...     self.db.close()
    ...     self.storage.close()

Now we can use this ZODB as backend database and store our data
structure:

    >>> import transaction
    >>> db = TinyZODB()
    >>> dbroot = db.dbroot
    >>> dbroot['mydept'] = mydept
    >>> mycourse = createObject('waeup.Course')
    >>> mycourse.code = 'MYCOURSE'
    >>> mydept.courses.addCourse(mycourse)
    >>> mycert = createObject('waeup.Certificate')
    >>> mycert.code = 'MYCERT'
    >>> mydept.certificates.addCertificate(mycert)
    >>> mycert.addCertCourse(mycourse)

    >>> transaction.commit()

The data is now stored in the ZODB. We can close the DB, reopen it
later and the data will still be there:

    >>> db.close()
    >>> newdb = TinyZODB()
    >>> newdbroot = newdb.dbroot
    >>> list(newdbroot)
    ['mydept']

The certificate course we stored in the certificate is indeed a
referrer of the course, not a copy of it:

    >>> course = newdbroot['mydept'].courses['MYCOURSE']
    >>> certcourse = newdbroot['mydept'].certificates['MYCERT']['MYCOURSE_100']
    >>> certcourse.course is course
    True

So, we can be sure that modifications to the course are immediately
reflected in the certcourse.

We can also be sure, that when removing courses or referrers of them,
other data will be handled in a way we expect. That is,

* when we remove a course, we want all referrers to it be removed as
  well

* when we remove a referrer of a course, nothing else should be
  touched but only this very referrer.


As correct deletion of referrer needs support of catalogs (we lookup
catalogs to find referrers), we need a fully blown ZODB here,
including an instance of :class:`waeup.kofa.app.University`, as this is our
`ISite` object where the catalogs are stored.

We create a complete university tree, starting with a
:class:`University` instance and going down to courses, certificats
and course certificates:

    >>> from waeup.kofa.app import University
    >>> root = getRootFolder()
    >>> root['app'] = University()

We have to set the site, to enable proper working of catalogs and
`IObjectRemoved` handlers:

    >>> from zope.app.component.hooks import setSite
    >>> setSite(root['app'])

    >>> from zope.component import createObject
    >>> faculty = createObject('waeup.Faculty')
    >>> faculty.code = 'FAC'
    >>> root['app']['faculties'].addFaculty(faculty)
    >>> fac = root['app']['faculties']['FAC']

    >>> dept = createObject('waeup.Department')
    >>> dept.code = 'DEPT'
    >>> root['app']['faculties']['FAC'].addDepartment(dept)

    >>> course = createObject('waeup.Course')
    >>> course.code = 'CRS1'
    >>> fac['DEPT'].courses.addCourse(course)

Now let's add another department, which will also hold a course:

    >>> dept2 = createObject('waeup.Department')
    >>> dept2.code = 'DEPT2'
    >>> root['app']['faculties']['FAC'].addDepartment(dept2)

    >>> course = createObject('waeup.Course')
    >>> course.code = 'CRS2'
    >>> fac['DEPT2'].courses.addCourse(course)

Finally, we add a certificate with referrers (certificate courses) to
the created courses:

    >>> cert = createObject('waeup.Certificate')
    >>> cert.code = 'CERT'
    >>> fac['DEPT'].certificates.addCertificate(cert)

    >>> fac['DEPT'].certificates['CERT'].addCertCourse(
    ...   fac['DEPT'].courses['CRS1'])

    >>> fac['DEPT'].certificates['CERT'].addCertCourse(
    ...   fac['DEPT2'].courses['CRS2'])

We add the latter course again, but this time for the second level:

    >>> fac['DEPT'].certificates['CERT'].addCertCourse(
    ...   fac['DEPT2'].courses['CRS2'], level=200)

So, we have three referrers in our certificate of two different
courses stored somewhere in the application:

    >>> list(fac['DEPT'].certificates['CERT'])
    [u'CRS1_100', u'CRS2_100', u'CRS2_200']

Both, the courses themselves and the referrers of them, are the same
(not only equal):

    >>> a = root['app']['faculties']['FAC']['DEPT']
    >>> b1 = a.courses['CRS1']
    >>> b2 = a.certificates['CERT']['CRS1_100'].course
    >>> b1 is b2
    True

If we remove the course, also the select item in the certificate course
will be gone:

    >>> list(a.certificates['CERT'])
    [u'CRS1_100', u'CRS2_100', u'CRS2_200']

    >>> list(a.courses)
    [u'CRS1']

    >>> del root['app']['faculties']['FAC']['DEPT'].courses['CRS1']
    >>> list(a.certificates['CERT'])
    [u'CRS2_100', u'CRS2_200']

    >>> list(a.courses)
    []

If we remove a referrer of a course, this will not touch the course
nor other referrers:

    >>> del fac['DEPT'].certificates['CERT']['CRS2_200']
    >>> list(a.certificates['CERT'])
    [u'CRS2_100']

    >>> list(fac['DEPT2'].courses)
    [u'CRS2']

If a single course is registered for several levels in a single
certificate, this will not block things if the course is removed
(regression test):

We add the same course as a reference for level 200 and level 300:

    >>> fac['DEPT'].certificates['CERT'].addCertCourse(
    ...   fac['DEPT2'].courses['CRS2'], level=200)
    >>> fac['DEPT'].certificates['CERT'].addCertCourse(
    ...   fac['DEPT2'].courses['CRS2'], level=300)

Deleting the course (thus also the refererrers) will work:

    >>> del root['app']['faculties']['FAC']['DEPT2'].courses['CRS2']

Now all references are gone:

    >>> list(a.certificates['CERT'])
    []

and also the course itself vanished:

    >>> list(fac['DEPT2'].courses)
    []