Re: Backwards in the relationship chain?

First, is this a pre-existing data base whose format you are stuck with?

While your models probably can be made to work, it is more common to let Django take care of creating a single primary key in a model (called "id" (lower case) by default).

Also, while ForeignKey fields are implemented by storing the primary key value of the desired row in the other table, that, too, is intended to be hidden, so when you make a reference through a ForeignKey field, the ORM actually hands you an instance of the foreign Model, not the id.  That is:

    instanceOfCourseCatalog.TeacherID

is an instance of Teacher, not the TeacherID of a teacher.  To get the id integer, for those very rare cases in which it is valuable, you woudl use:

    instanceOfCourseCatalog.TeacherID.TeacherID

If you go the normal way and let Django create the primary key fields by the default name "id", and if, to better describe what is happening, removed the "ID from the end of your ForeignKey field names and, also a convention, made them lowercase, the lines above lines would become, respectively:

   instanceOfCourseCatalog.teacher

and

   instanceOfCourseCatalog.teacher.id

In this case Django would have added a "manager" to the Teacher model called "coursecatalog_set" (or just possibly "course_catalog_set", not certain and don't have time to search the documents now), but whose name your can specify with the "related_name" argument to the ForeignKey field.  This manager has all the usual queryset methods, so you can append ".all()' to get an array of all the CourseCatalog objects with that Teacher.  See https://docs.djangoproject.com/en/1.5/topics/db/queries/#backwards-related-objects

Bill

On Wed, Jul 24, 2013 at 8:37 AM, Chris Ryan <chris.ryan.12345@gmail.com> wrote:

I have spent hours trying to find the answer to this. What makes it even more frustrating is that I'm new to django development and not a real programmer by trade (I am a UNIX admin) so I am struggling to label what I am trying to do so that I can search for an answer.

Here's what I have:

In my models.py file I have the following class definitions:

class Family(models.Model):

...

FamilyID = models.AutoField(primary_key=True)

Name = models.CharField(verbose_name="Family Name")

------------------------------------------------------------------------------------------------

class FamilyMember(AbstractUser):

....

ID = models.AutoField(primary_key=True)

FamilyID = models.ForeignKey(Family, blank=True, null=True)

...

------------------------------------------------------------------------------------------------

class Teacher(models.Model):

...

TeacherID = models.AutoField(primary_key=True)

ID = models.OneToOneField(FamilyMember)

...

------------------------------------------------------------------------------------------------

class CourseCatalog(models.Model):

...

TeacherID = models.ForeignKey(Teacher, blank=True, null=True)

...

------------------------------------------------------------------------------------------------

So, there is a table called Family that I belong to. It holds my family name and a couple of other fields.   

There is then a one-to-many relationship from that to the FamilyMember table. In this table I have a record for each member of my family.   

The FamilyMember table is also being used as the authentication table for my site. I have not replaced the default security model but only abstracted it and added additional fields.

I am a teacher so there is also a record in the Teacher table.  This table contains the FamilyMember.ID field.

That's the setup for me. My main information is in the FamilyMember table and my FamilyMember.ID field is stored in the Family and the Teacher tables.

On to the list of classes...

There is a table called CourseCatalog that contains information about a class that I will teach. One of the columns is called TeacherID and this stores the TeacherID from the Teacher table.   I set it up this way so that an admin can add a class and then assign it only to a teacher. The drop down list should not include FamilyMembers that are not teachers.

Here are my questions and problems:

-   I want to log into the website as myself and modify only my classes. It would also be OK to modify the classes of everyone in my family. Either solution would be fine.  However, I can't figure out how to do either of these things. 

-   It seems to me that what I'm doing is trying to go backwards in the relationship chain. I have my CourseCatalog record that includes the TeacherID. I need to look that ID up in the Teacher table to get the FamilyMember.ID. I then need to use that to look up the record in the FamilyMember table. Once I have that, I can get the username and compare it to the username of the logged in user (me).   Finally, I can filter on records where these two match.  

-   I'm wondering if I have architected my database wrong. Is there an easier way to do this? Maybe I should get rid of the Teacher table and just use a boolean for teacher in the FamilyMember table. The concern that I have with this is when I start working on the Student table. I need to store more information about a student (allergy, emergency contacts, etc) and don't think it is appropriate to add those fields to the FamilyMember table because they are only used for students.

-  I'm asking for you experts to help guide me through this. Any advice is greatly appreciated.

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