My problem is that I don't really know how to put my design in terms of a relational db.
I have a class 'Feat', which should contain several other classes, namely lists of 'Stat's, 'Skill's and 'Attribute's. The exact number of those depends on the instance of 'Feat', so I can't just define a static number of fields.
In normal Python, I'd simply use lists:
class Feat():
name
desc
att_effects = []
skill_effects = []
stat_effects = []
I tried to replicate this in Django using the Many-to-Many relationship, but that didn't quite work out.
class Attribute_Name(models.Model):
name = models.CharField(max_length=100)
desc = models.TextField()
def __unicode__(self):
return str(self.name)
...
class Attribute(models.Model):
name = models.ForeignKey(Attribute_Name)
value = models.IntegerField()
def __unicode__(self):
return "%s: %s" % (self.name, self.value)
...
class Feat(models.Model):
name = models.CharField(max_length=100)
desc = models.TextField()
att_effects = models.ManyToManyField(Attribute)
skill_effects = models.ManyToManyField(Skill)
stat_effects = models.ManyToManyField(Stat)
With these models, I could create new 'Attribute's when creating a new 'Feat' in the admin interface. However, I would always see all existing 'Attribute's, not just those pertaining to my new 'Feat'. Is there a way to restrict the view to only those created along with the new 'Feat'? Something like a back-link to the 'Feat' which created the 'Attribute'?
Basically, when I create or edit a 'Feat', I should only be able to use 'Attribute's created from the page of this instance or add a new 'Attribute' instance. I don't want 'Attribute's belonging to another 'Feat' (or another unrelated class) to show up.
Note that I don't want to add a field to the 'Attribute' class, since it should also be used along with other classes (eg 'Character')
I'd also like to do this within the admin interface, since it saves me quite a bit of work.
On a related note: Is there a way to leave a Many-to-Many field empty? Or should I just create a magic 'empty' value and set it as the default?
"there a way to restrict the view to only those created along with the new 'Feat'? Something like a back-link to the 'Feat' which created the 'Attribute'?"
Yes: use the queryset feature of ModelAdmin as described in this question
Related
I have the following model in django
class params(models.Model):
name = models.CharField(max_length=30, default = 'no_name')
cs_n = models.IntegerField(default=16)
alt_n = models.IntegerField(default=2)
opt_out = models.BooleanField(default=1)
at_n = models.IntegerField(default=4)
I want to create a new model with as many fields as at_n. For example, if the user enter "4" in at_n, I want this to create automatically:
class params(models.Model):
at_1 = models.IntegerField(default=2)
at_2 = models.IntegerField(default=2)
at_3 = models.IntegerField(default=2)
at_4 = models.IntegerField(default=2)
Thanks
This probably isn't a good data model to follow as Django models are intended to closely mirror database tables. For example, you wouldn't want to dynamically update the DDL of a table in a database because doing so places you at risk of messing up data that already exists in said table.
Instead, I think it would be a better approach for you to re-evaluate your data model.
For example, if there was a main object you were trying to tie these attributes to, then make a model for that main object and then make a separate model for main object attributes/values.
From there, you could use view logic to actually validate that the appropriate number of attributes assigned to a particular main object.
I'm thinking something kind of like this:
class MainModel(models.Model):
....
{Your main model attributes}
at_n = models.IntegerField(default=4)
....
class MainModelAttributes(model.Model):
main_model = models.ForeignKey(MainModel)
attr_value = models.IntegerField()
Then in your views.py file, you could use logic to make sure that the number of attributes on the MainModelAttributes model match the number stored in MainModel.at_n.
I have a simple question but multiple google searches left me without a nice solution. Currently I am doing the following:
allowed_categories = self.allowed_view.all().difference(self.not_allowed_view.all())
users = []
for cat in allowed_categories:
for member in cat.members.all():
users.append(member)
return users
I have a ManyToManyField to Objects that also have a ManyToManyField for instances of Users. In the code above, I am trying to get all the users from all those categories and get a list of all Users.
Later I would like the same in a method allowed_to_view(self, user_instance) but that's for later.
How would I achieve this using Django ORM without using nested for-loops?
[edit]
My models are as follows:
class RestrictedView(models.Model):
allowed_view = models.ManyToManyField(Category)
not_allowed_view = models.ManyToManyField(Category)
class Category(models.Model):
name = models.CharField(max_length=30)
members = models.ManyToManyField(User)
So, I've made the following one-liner with only one query towards the database. It took me some time...
users = User.objects.filter(pk__in=self.allowed_view.all().values("users").difference(self.not_allowed_view.all().values("users")))
This gives me a nice queryset with only the users that are in the allowed_view and explicitly not in the not_allowed_view.
Without seeing you database structure / models.py file its hard to say, but you can do a search on member objects like so:
members_queryset = Member.objects.filter(
category = <allowed categories>,
...
)
users += list(members.all())
class PurchaseOrder(models.Model):
purchase_order_id = models.AutoField(primary_key=True)
purchase_order_number = models.CharField(unique=True)
vendor = models.ForeignKey(Vendor)
i am creating Purchase Order(po) table. when po created i have to update purchase_order_number as "PO0"+purchase_order_id ex PO0123 (123 is Primary key). so i am using def save in models to accomplish this
def save(self):
if self.purchase_order_id is not None:
self.purchase_order_number = "PO"+str(self.purchase_order_id)
return super(PurchaseOrder, self).save()
It is working fine with single creation but when i try to create bulk of data using locust(Testing tool) its giving an error duplicate entry for PurchseOrdernumber Can we modify field value in models itself some thing like this
purchase_order_number = models.CharField(unique=True,default=("PO"+self.purchase_order_id )
To be honest, I don't think it should work when you create multiple instances. Because as I can see from the code:
if self.purchase_order_id is not None:
self.purchase_order_number = "PO"+str(self.purchase_order_id)
Here purchase_order_id will be None when you are creating new instance. Also, until you call super(PurchaseOrder, self).save(), it will not generate purchase_order_id, meaning purchase_order_number will be empty.
So, what I would recommend is to not store this information in DB. Its basically the same as purchase_order_id with PO in front of it. Instead you can use a property method to get the same value. Like this:
class PurchaseOrder(models.Model):
purchase_order_id = models.AutoField(primary_key=True)
# need to remove `purchase_order_number = models.CharField(unique=True)`
...
#property
def purchase_order_number(self):
return "PO{}".format(self.purchase_order_id)
So, you can also see the purchase_order_number like this:
p = PurchaseOrder.objects.first()
p.purchase_order_number
Downside of this solution is that, you can't make any query on the property field. But I don't think it would be necessary anyway, because you can do the same query for the purchase_order_id, ie PurchaseOrder.objects.filter(purchase_order_id=1).
I'm trying to figure out how to design my model. I've been going over the documentation, and it ultimately seems like I should be using the "through" attribute, but I just can't figure out how to get it to work how I want.
If someone could take a look and point out what I'm missing, that would be really helpful. I have pasted my model below.
This is what I am trying to do:
1) Have a list of server types
2) Each server type will need to have different parts available to that specific server type
3) The asset has a FK to the servermodel, which has a M2M to the parts specific to that server type.
My question is, how can each "Asset" store meta data for each "Part" specific to that "Asset"? For example, each "Asset" should have it's own last_used data for the part that's assigned to it.
Thanks! :)
class Part(models.Model):
part_description = models.CharField(max_length=30,unique=1)
last_used = models.CharField(max_length=30)
def __unicode__(self):
return self.part_description
class ServerModel(models.Model):
server_model = models.CharField(max_length=30,unique=1)
parts = models.ManyToManyField(Part)
def __unicode__(self):
return self.server_model
class Asset(models.Model):
server_model = models.ForeignKey(ServerModel)
serial_number = models.CharField(max_length=10,unique=1)
def __unicode__(self):
return self.server_model.server_model
EDIT:
Thank you for the help!
I may have not explained myself clearly, though. It's probably my confusing model names.
Example:
ServerModel stores the type of server being used, say "Dell Server 2000".
The "Dell Server 2000" should be assigned specific parts:
"RAM"
"HARD DISK"
"CDROM"
Then, I should be able to create 10x Assets with a FK to the ServerModel. Now, each of these assets should be able to mark when the "RAM" part was last used for this specific asset.
I'm not sure I exactly understand what you want to do, but basically you can solve that with a "through" model, as you expected:
import datetime
class Part(models.Model):
name = models.CharField(max_length=30,unique=1)
class ServerModel(models.Model):
server_model = models.CharField(max_length=30,unique=1)
parts = models.ManyToManyField(Part,through='Asset')
class Asset(models.Model):
server_model = models.ForeignKey(ServerModel)
part = models.ForeignKey(Part)
serial_number = models.CharField(max_length=10,unique=1)
used = models.DateTimeField(default=datetime.datetime.now())
First thing to notice is the relation of the parts to the servermodel using the "through"-model: that way for each Part instance assigned to the "parts"-property of a ServerModel instance a new Asset instance is created (Phew - hope that doesn't sound too complicated). At the time of creation the "used"-property of the Asset instance is set to the current date and time (thats what default=datetime.datetime.now() does).
If you do that, you can then just query the database for the last asset containing your part. That queryset can then be sorted by the "used" property of the Asset model, which is the date when the Asset instance has been created.
ServerModel.objects.filter(parts__name='ThePartYouAreLookingFor').order_by('asset__used')
I'm not absolutely sure if the queryset is correct, so if someone finds huge nonsense in it, feel free to edit ;)
edit:
The models above do not exactly that. But you do not even need a through model for what you want:
class ServerModel(models.Model):
server_model = models.CharField(max_length=30,unique=1)
parts = models.ManyToManyField(Part)
class Asset(models.Model):
server_model = models.ForeignKey(ServerModel)
parts = models.ForeignKey(Part)
serial_number = models.CharField(max_length=10,unique=1)
used = models.DateTimeField(default=datetime.datetime.now())
Basically you can just add assets and then query all assets that have a RAM in parts.
Asset.objects.filter(parts__contains='RAM').order_by('used')
Get the date of the first (or last) result of that queryset and you have the date of the last usage of your 'RAM'-part.
this is a model of the view table.
class QryDescChar(models.Model):
iid_id = models.IntegerField()
cid_id = models.IntegerField()
cs = models.CharField(max_length=10)
cid = models.IntegerField()
charname = models.CharField(max_length=50)
class Meta:
db_table = u'qry_desc_char'
this is the SQL i use to create the table
CREATE VIEW qry_desc_char as
SELECT
tbl_desc.iid_id,
tbl_desc.cid_id,
tbl_desc.cs,
tbl_char.cid,
tbl_char.charname
FROM tbl_desC,tbl_char
WHERE tbl_desc.cid_id = tbl_char.cid;
i dont know if i need a function in models or views or both. i want to get a list of objects from that database to display it. This might be easy but im new at Django and python so i having some problems
Django 1.1 brought in a new feature that you might find useful. You should be able to do something like:
class QryDescChar(models.Model):
iid_id = models.IntegerField()
cid_id = models.IntegerField()
cs = models.CharField(max_length=10)
cid = models.IntegerField()
charname = models.CharField(max_length=50)
class Meta:
db_table = u'qry_desc_char'
managed = False
The documentation for the managed Meta class option is here. A relevant quote:
If False, no database table creation
or deletion operations will be
performed for this model. This is
useful if the model represents an
existing table or a database view that
has been created by some other means.
This is the only difference when
managed is False. All other aspects of
model handling are exactly the same as
normal.
Once that is done, you should be able to use your model normally. To get a list of objects you'd do something like:
qry_desc_char_list = QryDescChar.objects.all()
To actually get the list into your template you might want to look at generic views, specifically the object_list view.
If your RDBMS lets you create writable views and the view you create has the exact structure than the table Django would create I guess that should work directly.
(This is an old question, but is an area that still trips people up and is still highly relevant to anyone using Django with a pre-existing, normalized schema.)
In your SELECT statement you will need to add a numeric "id" because Django expects one, even on an unmanaged model. You can use the row_number() window function to accomplish this if there isn't a guaranteed unique integer value on the row somewhere (and with views this is often the case).
In this case I'm using an ORDER BY clause with the window function, but you can do anything that's valid, and while you're at it you may as well use a clause that's useful to you in some way. Just make sure you do not try to use Django ORM dot references to relations because they look for the "id" column by default, and yours are fake.
Additionally I would consider renaming my output columns to something more meaningful if you're going to use it within an object. With those changes in place the query would look more like (of course, substitute your own terms for the "AS" clauses):
CREATE VIEW qry_desc_char as
SELECT
row_number() OVER (ORDER BY tbl_char.cid) AS id,
tbl_desc.iid_id AS iid_id,
tbl_desc.cid_id AS cid_id,
tbl_desc.cs AS a_better_name,
tbl_char.cid AS something_descriptive,
tbl_char.charname AS name
FROM tbl_desc,tbl_char
WHERE tbl_desc.cid_id = tbl_char.cid;
Once that is done, in Django your model could look like this:
class QryDescChar(models.Model):
iid_id = models.ForeignKey('WhateverIidIs', related_name='+',
db_column='iid_id', on_delete=models.DO_NOTHING)
cid_id = models.ForeignKey('WhateverCidIs', related_name='+',
db_column='cid_id', on_delete=models.DO_NOTHING)
a_better_name = models.CharField(max_length=10)
something_descriptive = models.IntegerField()
name = models.CharField(max_length=50)
class Meta:
managed = False
db_table = 'qry_desc_char'
You don't need the "_id" part on the end of the id column names, because you can declare the column name on the Django model with something more descriptive using the "db_column" argument as I did above (but here I only it to prevent Django from adding another "_id" to the end of cid_id and iid_id -- which added zero semantic value to your code). Also, note the "on_delete" argument. Django does its own thing when it comes to cascading deletes, and on an interesting data model you don't want this -- and when it comes to views you'll just get an error and an aborted transaction. Prior to Django 1.5 you have to patch it to make DO_NOTHING actually mean "do nothing" -- otherwise it will still try to (needlessly) query and collect all related objects before going through its delete cycle, and the query will fail, halting the entire operation.
Incidentally, I wrote an in-depth explanation of how to do this just the other day.
You are trying to fetch records from a view. This is not correct as a view does not map to a model, a table maps to a model.
You should use Django ORM to fetch QryDescChar objects. Please note that Django ORM will fetch them directly from the table. You can consult Django docs for extra() and select_related() methods which will allow you to fetch related data (data you want to get from the other table) in different ways.