This is a follow-up question on Delete field from standard Django model. In short: a field can be dynamically deleted from a model that is already created, in this case the field User.email . So field email would be deleted from User without changing the code for User. See code below for example.
I can dynamically delete a a field from a model(1), but that happens when the server starts and is undone when it exists. Since syncdb doesn't require the server to be running, and generally seems to ignore the deletion code (somehow), this approach doesn't prevent the field from appearing in the database(2).
Is there a way to do delete the field from the model (without changing the file it's in, as it's a Django model), in a way that also makes it not appear in the database?
Thanks in advance!
Mark
EDIT: I problem is not that I am deleting the text "m = models.IntegerField()" from the model file and want the field removed from the database. The problem is that I am using the code below to remove a field from a model that has already been declared in another file. I do not think that creating a migration with South for every time I run syncdb is a solution(3).
Additional information:
1) Currently, the code is in models.py, but I suppose Where to put Django startup code? works.
2) I can delete it on post_syncdb signal with custom query, but I hope for something more elegant... Or elegant at all, more accurately.
3) If it even works at all, because obviously syncdb is still seeing the 'removed' field), so I think South would to as it's still somehow there.
This is the code (models.py):
class A(models.Model):
m = models.IntegerField()
for i, f in enumerate(A._meta.fields):
if f.name == 'm':
del A._meta.fields[i]
break
class B(A):
n = models.IntegerField()
for i, f in enumerate(B._meta.fields):
if f.name == 'm':
del B._meta.fields[i]
break
EDIT: I checked (with print) and the deletion code is executed on syncdb. It is executed before tables are created
django does a lot of meta class magic and i would guess that the meta class is responsible for defining the database table to back your model. Subsequently just deleting the field is not enough to alter the generated table.
as several people have pointed out, south is the way to deal with these problems.
Related
In my Django app, I want to insert a record with a composite primary key. Apparently this should be possible by making use of "unique_together". I'm quite sure this code was working in the past, but for some reason it does not seem to be working now. This code used to run on a Linux VM, and now I'm hosting it in Google App Engine. However I don't see how this can be the cause for this error.
class TermsAndConditionsDocument(models.Model):
organization = models.ForeignKey(Organization, on_delete=models.CASCADE, verbose_name=_("Organization"))
language = models.CharField(_('Language'),choices=LANGUAGE_CHOICES, max_length=5, help_text=_("The language of the content."))
content = models.TextField()
class Meta:
unique_together = ('organization', 'language')
The error:
IntegrityError at /transactions/settings/terms_and_conditions
null value in column "id" violates not-null constraint
DETAIL: Failing row contains (null, nl-BE, <p>B</p>, 10).
According to what I've read, using "unique_together" should cause Django to not need or include an ID as primary key. I checked the database, and the ID field DOES exist. I do not understand where the database constraint and the ID field are still coming from?
Apparently, as pointed out in the comments, a primary key "id" field is always added, even if you don't need it. It's supposed to get out of your way, so you don't even notice its existence. In my case, it required me to give it a value when I created a new record, which is not how things are supposed to work.
A while back I migrated this database from one Postgres database to another Postgres database. I used an SQL dump and load method for this. Some sequences seem to have been lost during that migration.
Because there are no sequences, some fields now lacked autoincrement capabilities, explaining the IntegrityError on insertion.
In order to fix this, I did the following:
1) Export the current data:
manage.py dumpdata > data.json
2) Drop your database and create a new empty one.
3) Run database migrations:
manage.py migrate
4) Load the data again, excluding some default data already recreated by Django.
manage.py loaddata --exclude auth.permission --exclude contenttypes data.json
This procedure seems to have recreated the sequences while also keeping the data.
The unique_together only creates a DB constraint (https://docs.djangoproject.com/en/2.2/ref/models/options/#unique-together).
You could create a custom primary key with the option primary_key https://docs.djangoproject.com/en/2.2/ref/models/fields/#django.db.models.Field.primary_key but you could only do that for one field.
But I suggest to just keep the auto increment id field, this works better with Django.
For the error are you saving a model? or doing a raw import?
To make a long story short, I am very grateful for hints on how I can accomplish the following. I have an app A that I don't want to change. I have an app B that needs to select data from A or to request data to be added/changed if necessary. Think of B as an app to suggest data that should end in A only after review/approval. By itself, B is pretty useless. Furthermore, a significant amount of what B's users will enter needs to be rejected. That's why I want A to be protected so to say.
# in app A
class Some_A_Model(models.Model): #e.g., think artist
some_value = models.TextField()
# in app B
class MyCustomField(models.ForeignKey):
...
class Some_B_Model(models.Model): # e.g., think personal preference
best_A = MyCustomField('Some_A_Model')
worst_A = MyCustomField('Some_A_Model')
how_many_times_I_like_the one_better_than_the_other = models.FloatField()
class Mediator(models.Model):
# already exists: generic foreign key
content_type = models.ForeignKey(
ContentType,
on_delete=models.CASCADE
)
object_id = models.PositiveIntegerField()
content_object = GenericForeignKey(
'content_type',
'object_id'
)
#does not yet exist or needs to be changed:
add_or_change = PickledObjectField()
Django should create a form for Some_B_Model where I can select instances of Some_A_Model for best_A and worst_A, respectively; if, however, my intended best_A is not yet in A's database, I want to be able to request this item to be added. And if I find worst_A is present but has a typo, I want to be able to request this item to be corrected. An editor should be required to review/edit the data entered in B and either reject or release all the associated changes to A's database as an atomic transaction. I don't want any garbage in A and refrain from adding some status field to track what is considered valid, requiring filtering all the time. If it's in A, it must be good.
I figured I need to define a MyCustomField, which could be a customized ForeignKey. In addition, I need some intermediate model ('mediator' maybe?) that MyCustomField would actually be pointing to and that can hold a (generic) ForeignKey to the item I selected, and a pickled instance of the item I would like to see added to A's database (e.g., a pickled, unsaved instance of Some_A_model), or both to request a change. Note that I consider using PickledObjectField from 'django-picklefield', but this is not a must.
As there is only some documentation on custom model fields but not on the further steps regarding form fields and widgets, it seems I have to dig through django's source to find out how to tie my intended functionality into its magic. That's where I am hoping for some comments and hints. Does my plan sound reasonable to you? Is this a known pattern, and if so, what is it called? Maybe someone has already done this or there is a plugin I could look into? What alternatives would you consider?
Many thanks in advance!
Best regards
I have a model that have a URLField field and I need to make a migration that turns this field into a foreign key, where the string is a unique field of the other object, and, if the object does not exist create it.
for example, turn this:
class Event_UserVisit(Event_Base):
dest_url = models.URLField(max_length=1000)
into this:
class Event_UserVisit(Event_Base):
dest_url = models.ForeignKey(Page)
I've never done a manual migration like this and didn't find any tutorial or instructions to do something like this.
obviously doing a naive migration return errors like this:
django.db.utils.ProgrammingError: column "source_url_id" cannot be cast automatically to type integer
what's the best approach to do it?
note: I need to do this on a production db with lots of data, so I can't have long down time and can't lose any data.
thanks! :)
I don't think you can do this at the ORM level in one go (unless someone corrects me) You would need create a new FK maybe dest_url2 run migrations, then write a script to migrate the data. Next,delete dest_url again run migrations. Then rename dest_url2 to dest_url Django will detect the name change here.
However, I don't understand why you are linking an FK ID on page to a field called dest_url. A PK in Page should not be a 1000 max URL! It has no order and would make indexing hard and slow down your app. It would make more sense do have...
class Event_UserVisit(Event_Base):
page = models.ForeignKey(Page, related_name='eventvisits')
The I assume Page looks something like this...
class Page(models.Model):
dest_url = models.URLField(max_length=1000)
I am doing a migration that adds a referral code to my custom user model. The field I've added looks like this:
refer_code = models.CharField(max_length=10, default=UUID_10)
In my datamigration I've added the following:
for u in orm['app.User'].objects.all():
u.refer_code = UUID_10()
print u.name + ': ' + u.refer_code
u.save()
However, when I perform the migrations, all of my users have the same code. Different codes for each user are spat out from the print command, so it's not a problem with my function.
Weirdly, if I roll the datamigration back (it has no backwards method) as well as the previous migration that added the field, and re-apply them, not only does every model have the same data in, it exactly has the same data in as before.
There are other parts of the datamigration (adding objects to the database with get_or_create) that are working fine.
What on earth is going on?
I think your function was called rather than new field was created. So try to do two migrations. First add new field with default value. And second fill that field.
I created this field in my model:
numero_str = models.AutoField(primary_key=True, unique=True, default = 0)
The default value seems to invalidate the auto increment of AutoField but if I take it out I receive an error saying that the field can't be null. What I don't understand is: if I declareted it as a AutoField, isn't it supposed to generate a sequencial integer automatically? Or should I declare something when saving an item?
To be more especific, my app is basically a form that is send by e-mail and saved in a database. The error occur when sending (in case I take out the default value). It says:
IntegrityError at /solicitacaodetreinamento/
str_solicitacao.numero_str may not be NULL
I found a solution: I deleted the DB file and all the migration of South (include the Initial). Then I recreated the database and made the migrations. This time, using the default primary key, i.e., "id". In my case it was simpler because I had no real data at all (it is not in production), otherwise I would have to export the data, recreate the database and then import the data. Thank you all.