Rich
I've run out of time this week. Maybe someone else can help.
Mike
On 21/04/2017 11:45 AM, Rich Shepard wrote:
On Fri, 21 Apr 2017, Mike Dewhirst wrote:
In the more usual scenario you specify your models using Python and
let the Django ORM framework do the SQL. If that is your case ...
Mike,
I wrote the schema for postgres but had not created the database. So I
renamed the file models.py and converted the tables to classes.
1. No need for a PK. Django automatically inserts a PK. You can refer to
this in your code as model_id or model.id where "model" is the
lower-case
class name of the model. This is best-practice.
I understand that Django creates the PK when a class has only a single
variable. I have a table that contains one FK and two other variables
form a
combined primary key. Perhaps this is a case for the many-to-one model
type
in Django?
3. In the case of 1 above I would expect three models being Company,
Person and Project each of which would have a PK managed by Django.
If so,
the model you are designing (possibly Project) presumably has a field
called 'proj_nbr' and foreign keys to Company and Person. Those FKs are
represented in your model as company = models.ForeignKey('Company') and
person = models.ForeignKey('Person')
4. Within each model is a class Meta: to carry various options for
the model as a whole. The option you are looking for is:
unique_together = (('company', 'person', 'proj_nbr'),)
Here's one class which requires three columns to make each row unique:
class Projects (models.Model):
company=models.ForeignKeyField('Companies', on_delete=models.CASCADE)
person=models.ForeignKeyField('Contacts', on_delete=models.CASCADE)
proj_nbr=models.CharField(max_length=8, Field_null=True)
proj_name=models.TextField(Field_null=True)
description=models.TextField()
PRIMARY KEY unique_together=('company', 'person', 'proj_nbr')
If I delete 'PRIMARY KEY' and leave the rest of the line, does this
provide the relational integrity?
This I'll need to ponder more to figure out where to put the clean
method
so it validates entries before they're saved.
I put the clean() method after the save() method which follows the
__str__() method which follows the Meta class. It doesn't really
matter so long as you are consistent.
Okay. I'll try writing these methods after I get the multi-variable PKs
correct.
Another place I sometimes use is the save() method. Django also
provides pre_save and post_save hooks so you never need to use
database triggers.
This goes beyond what I've learned. I'll get there step-by-step.
Yes, you are helping me smooth off the rough spots.
Thanks,
Rich
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