Course referrers
****************

Courses are referred to by certificate courses.

We can 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'].addCourseRef(
    ...   fac['DEPT'].courses['CRS1'])

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

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

    >>> fac['DEPT'].certificates['CERT'].addCourseRef(
    ...   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']