New user question: Where to put non-form validation code?

[Dean Karres]

Hi,

I am learning Django.  I am using CBVs.  My default "index.py" view is basically a dashboard for the app I am playing with.  As my models and views become more complicated I want to be able to ask, for any model instance, is this instance "valid" or "complete".  Valid means that all Required elements are present and have reasonable values.  "Complete" means that an instance is "valid" but also some specific bits of additional info are also ok.

For example I have a Student model that has name and address info.  There is a ManyToMany relation to the Class(es) in which that Student is enrolled.  A "student" instance is valid if the name and address fields are filled.  A student is "complete" if the student is valid and has signed up for one or more classes.

So, my question is: where should the valid and complete methods live?  Should they be in the Student model or CBV?  Someplace else?  Does it matter?

Cheers

[Mike Dewhirst]

I like to put that sort of stuff into model methods then add model.clean() to call them and raise whatever error may be appropriate if they fail.

https://docs.djangoproject.com/en/2.1/topics/forms/modelforms/#interaction-with-model-validation

and

https://docs.djangoproject.com/en/2.1/ref/models/instances/#validating-objects

and

https://docs.djangoproject.com/en/2.1/ref/models/instances/#django.db.models.Model.clean

You can raise your own errors which inherit from ValidationError to fine tune the process. For example, I differentiate between BusinessRuleViolation and InvalidToken and a couple of others where that delivers better control.

Cheers

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[Dean Karres]

Thank you. There are way more parts to this than I would have imagined.

[laya]

Hi

If I got your meaning Truly. It should be mentioned that as I know there is not any difference for using validation in model or View . Usually in model, we use regex for validation and in View  it needs to use some methods for Email Validation and etc.

 

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[Mike Dewhirst]

Well yes it could be called multifaceted. 

Usually but not always the interface with the user is the form. You can have non-database fields as well as model fields so either way there has to be a full suite of validation functionality available in both types of forms. Luckily, model forms automatically call model.clean() for you.

Unit tests don't though. You have deliberately call obj.clean() if you want to test things. obj.save() doesn't call obj.clean().

Actually, I also do a bit in view code too especially if there are non-database or hidden fields in the form. I know you are not supposed to validate data in a view for proper separation of concerns but it does keep my forms neater.

The bottom line for me is that if I can imagine non-form access ... eg through an API ... then all validation possible has to be via model.clean() and the API has to remember to call clean() before saving every time there is a POST

Hence I always put business rules validation in model.clean() and leave 'local' validation for the form.

You are right. There are quite a few moving parts

Connected by Motorola

Dean Karres <dean....@gmail.com> wrote:

Thank you. There are way more parts to this than I would have imagined.

On Sat, Jul 13, 2019, 8:01 PM Mike Dewhirst <mi...@dewhirst.com.au> wrote:

On 14/07/2019 10:37 am, Dean Karres wrote:

Hi,

I am learning Django.  I am using CBVs.  My default "index.py" view is basically a dashboard for the app I am playing with.  As my models and views become more complicated I want to be able to ask, for any model instance, is this instance "valid" or "complete".  Valid means that all Required elements are present and have reasonable values.  "Complete" means that an instance is "valid" but also some specific bits of additional info are also ok.

For example I have a Student model that has name and address info.  There is a ManyToMany relation to the Class(es) in which that Student is enrolled.  A "student" instance is valid if the name and address fields are filled.  A student is "complete" if the student is valid and has signed up for one or more classes.

So, my question is: where should the valid and complete methods live?  Should they be in the Student model or CBV?  Someplace else?  Does it matter?

I like to put that sort of stuff into model methods then add model.clean() to call them and raise whatever error may be appropriate if they fail.

https://docs.djangoproject.com/en/2.1/topics/forms/modelforms/#interaction-with-model-validation

and

https://docs.djangoproject.com/en/2.1/ref/models/instances/#validating-objects

and

https://docs.djangoproject.com/en/2.1/ref/models/instances/#django.db.models.Model.clean

You can raise your own errors which inherit from ValidationError to fine tune the process. For example, I differentiate between BusinessRuleViolation and InvalidToken and a couple of others where that delivers better control.

Cheers

[Jim Illback]

I had a slight variation on this thread - where to put some M2M field validation/deletion logic. 

I have a purely model-based form where a checkbox’s value determines whether another field (it’s a M2M field) in the form should be NULL or keep its values to be saved in the database. So, following the appropriate separation of concerns as advocated below, I added logic in both the models' clean and save override methods. Neither approach set the other field to NULL when it should have been. The reason is the form still contained M2M values even though the model said to delete them (delete the set, actually). 

After a lot of trial and error, it turns out that the model’s save seems to be run BEFORE the form’s save. To me, that seems backwards. Shouldn’t the model’s processes (which are directly relate to the database, the ultimate data source) occur last and well before the form’s (which are merely an interaction with the user)? What was happening was my model’s delete actually did the deletions (those IDs were gone), but then the form’s processing came along afterwards and re-inserted them again (with brand new IDs). 

Can someone help me understand why Django has chosen this seemingly “inversion” of processing - first models’ processes, then forms’? And, perhaps more importantly, where should this either/or logic should be placed so it will take effect?

Thanks very much,

Jim Illback

On Jul 13, 2019, at 11:48 PM, Mike Dewhirst <mi...@dewhirst.com.au> wrote:

Well yes it could be called multifaceted.  

Usually but not always the interface with the user is the form. You can have non-database fields as well as model fields so either way there has to be a full suite of validation functionality available in both types of forms. Luckily, model forms automatically call model.clean() for you.

Unit tests don't though. You have deliberately call obj.clean() if you want to test things. obj.save() doesn't call obj.clean().

Actually, I also do a bit in view code too especially if there are non-database or hidden fields in the form. I know you are not supposed to validate data in a view for proper separation of concerns but it does keep my forms neater.

The bottom line for me is that if I can imagine non-form access ... eg through an API ... then all validation possible has to be via model.clean() and the API has to remember to call clean() before saving every time there is a POST

Hence I always put business rules validation in model.clean() and leave 'local' validation for the form. 

You are right. There are quite a few moving parts <ic_list_happy.png>

Connected by Motorola

To view this discussion on the web visit https://groups.google.com/d/msgid/django-users/ajujeafh6d4qa4vvpfv2bxf9.1563085360028%40email.android.com.

[Mike Dewhirst]

On 25/07/2019 1:03 pm, Jim Illback wrote:
> I had a slight variation on this thread - where to put some M2M field
> validation/deletion logic.
>
> I have a purely model-based form where a checkbox’s value determines
> whether another field (it’s a M2M field) in the form should be NULL or
> keep its values to be saved in the database. So, following the
> appropriate separation of concerns as advocated below, I added logic
> in both the models' clean and save override methods. Neither approach
> set the other field to NULL when it should have been. The reason is
> the form still contained M2M values even though the model said to
> delete them (delete the set, actually).

There's a bit of trapeze happening here. The form is always in mid-air.
No matter what happens in the form the model save is only called when
all the validation (ie form first for the obvious errors then model for
business rules) has succeeded.

So, boolean (or any other) field values don't make it into the database
until model.save() succeeds.

And, the framework logic has to cope with database rejection of the
values as well. So the entire transaction needs to be able to roll back
if something goes awry at the last moment. Keep your eye on
transactions. I think PostgreSQL is probably the best of the open source
RDBMSs for that. I know Django seems to have no problems with it.

My typical approach in the model to see things before and after the save
is ...

def save(self, *args, **kwargs:
    var_to_be_passed = self._pre_save()
    super(ThisModel, self).save(*args, **kwargs)
    self._post_save(var_to_be_passed)

I use self._post_save() usually to do additional database things like
creating child models.

If your user-initiated change relies on a value in the database then you
have to save it first so it gets there.

Also, remember that the model's clean() method is only ever called by
Django when a ModelForm is used. Otherwise you have to call it yourself.
That's where I put all my business rule validation. I typically have
separate mofel methods and just call thin in clean() and raise
validation errors if necessary. That is totally appropriate because a
Django form knows how to deal with them.

As I said previously, in unti tests you have to call clean() explicitly.

>
> After a lot of trial and error, it turns out that the model’s save
> seems to be run BEFORE the form’s save. To me, that seems backwards.
> Shouldn’t the model’s processes (which are directly relate to the
> database, the ultimate data source) occur last and well before the
> form’s (which are merely an interaction with the user)? What was
> happening was my model’s delete actually did the deletions (those IDs
> were gone), but then the form’s processing came along afterwards and
> re-inserted them again (with brand new IDs).
>
> Can someone help me understand why Django has chosen this seemingly
> “inversion” of processing - first models’ processes, then forms’? And,
> perhaps more importantly, where should this either/or logic should be
> placed so it will take effect?
>
> Thanks very much,
> Jim Illback
>
>> On Jul 13, 2019, at 11:48 PM, Mike Dewhirst <mi...@dewhirst.com.au

>> <mailto:mi...@dewhirst.com.au>> wrote:
>>
>> Well yes it could be called multifaceted.
>>
>> Usually but not always the interface with the user is the form. You
>> can have non-database fields as well as model fields so either way
>> there has to be a full suite of validation functionality available in
>> both types of forms. Luckily, model forms automatically call
>> model.clean() for you.
>>
>> Unit tests don't though. You have deliberately call obj.clean() if
>> you want to test things. obj.save() doesn't call obj.clean().
>>
>> Actually, I also do a bit in view code too especially if there are
>> non-database or hidden fields in the form. I know you are not
>> supposed to validate data in a view for proper separation of concerns
>> but it does keep my forms neater.
>>
>> The bottom line for me is that if I can imagine non-form access ...
>> eg through an API ... then all validation possible has to be via
>> model.clean() and the API has to remember to call clean() before
>> saving every time there is a POST
>>

>> Hence I*always*put business rules validation in model.clean() and

>> leave 'local' validation for the form.
>>
>> You are right. There are quite a few moving parts<ic_list_happy.png>
>>

>> /Connected by Motorola/

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>>
>>
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>
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[Mike Dewhirst]

On 25/07/2019 1:03 pm, Jim Illback wrote:

> I had a slight variation on this thread - where to put some M2M field
> validation/deletion logic.
>
> I have a purely model-based form where a checkbox’s value determines
> whether another field (it’s a M2M field) in the form should be NULL or
> keep its values to be saved in the database. So, following the
> appropriate separation of concerns as advocated below, I added logic
> in both the models' clean and save override methods. Neither approach
> set the other field to NULL when it should have been. The reason is
> the form still contained M2M values even though the model said to
> delete them (delete the set, actually).
>
> After a lot of trial and error, it turns out that the model’s save
> seems to be run BEFORE the form’s save. To me, that seems backwards.
> Shouldn’t the model’s processes (which are directly relate to the
> database, the ultimate data source) occur last and well before the
> form’s (which are merely an interaction with the user)? What was
> happening was my model’s delete actually did the deletions (those IDs
> were gone), but then the form’s processing came along afterwards and
> re-inserted them again (with brand new IDs).

Don't know what's happening here but you can be sure Django is doing it
the right way.

Maybe you need to look at your on_delete constants. models.CASCADE
versus models.SET_NULL or models.PROTECT

>
> Can someone help me understand why Django has chosen this seemingly
> “inversion” of processing - first models’ processes, then forms’? And,
> perhaps more importantly, where should this either/or logic should be
> placed so it will take effect?
>
> Thanks very much,
> Jim Illback
>
>> On Jul 13, 2019, at 11:48 PM, Mike Dewhirst <mi...@dewhirst.com.au

>> <mailto:mi...@dewhirst.com.au>> wrote:
>>
>> Well yes it could be called multifaceted.
>>
>> Usually but not always the interface with the user is the form. You
>> can have non-database fields as well as model fields so either way
>> there has to be a full suite of validation functionality available in
>> both types of forms. Luckily, model forms automatically call
>> model.clean() for you.
>>
>> Unit tests don't though. You have deliberately call obj.clean() if
>> you want to test things. obj.save() doesn't call obj.clean().
>>
>> Actually, I also do a bit in view code too especially if there are
>> non-database or hidden fields in the form. I know you are not
>> supposed to validate data in a view for proper separation of concerns
>> but it does keep my forms neater.
>>
>> The bottom line for me is that if I can imagine non-form access ...
>> eg through an API ... then all validation possible has to be via
>> model.clean() and the API has to remember to call clean() before
>> saving every time there is a POST
>>

>> Hence I*always*put business rules validation in model.clean() and

>> leave 'local' validation for the form.
>>
>> You are right. There are quite a few moving parts<ic_list_happy.png>
>>

>> /Connected by Motorola/

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>>> To post to this group, send email

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>>
>>
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>
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[Matt Zand]

Hi everyone, 

Does anyone know how to post blog at below page:

https://www.djangoproject.com/community/blogs/ 

I am really frustrated as I submitted last week but nothing is there. Who is in charge of blog posts on Django site?

Thx,

Matt

 

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Cheers,

Matt Zand

Cell: 202-420-9192
Work: 240-200-6131

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