Question

In django, I m trying to do something like this:

# if form is valid ...
article = form.save(commit=False)
article.author = req.user

product_name = form.cleaned_data[ product_name ]
try:
    article.product = Component.objects.get(name=product_name)
except:
    article.product = Component(name=product_name)

article.save()
# do some more form processing ...

But then it tells me:

null value in column "product_id" violates not-null constraint

But I don t understand why this is a problem. When article.save() is called, it should be able the create the product then (and generate an id).

I can get around this problem by using this code in the except block:

product = Component(name=product_name)
product.save()
article.product = product

But the reason this concerns me is because if article.save() fails, it will already have created a new component/product. I want them to succeed or fail together.

Is there a nice way to get around this?

Answer 1

You could get around this by using :

target_product, created_flag = Component.objects.get_or_create(name=product_name)
article.product = target_product

as I m pretty sure get_or_create() will set the id of an object, if it has to create one.

Alternatively, if you don t mind empty FK relations on the Article table, you could add null=True to the definition.

Answer 2

The way the Django ManyToManyField works is that it creates an extra table. So say you have two models, ModelA and ModelB. If you did...

ModelA.model_b = models.ManyToManyField(ModelB)

What Django actually does behind the scenes is it creates a table... app_modela_modelb with three columns: id, model_a_id, model_b_id.

Hold that thought in your mind. Regarding the saving of ModelB, Django does not assign it an ID until it s saved. You could technically manually assign it an ID and avoid this problem. It seems you re letting django handle that which is perfectly acceptable.

Django has a problem then doing the M2M. Why? If ModelB doesn t have an id yet, what goes in the model_b_id column on the M2M table? The error for null product_id is more than likely a null constraint error on the M2M field, not the ModelB record id.

If you would like them to "succeed together" or "fail together" perhaps it s time to look into transactions. You, for example, wrap the whole thing in a transaction, and do a rollback in the case of a partial failure. I haven t done a whole lot of work personally in this area so hopefully someone else will be of assistance on that topic.

Answer 3

There s little value in including a code snippet on transactions, as you should read the Django documentation to gain a good understanding.

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