Re: First models.py needs tuning

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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