I am defining the following models, where Category has a non-integer primary key code of type Text.
Category
code Text
Primary code
deriving Show Typeable
CategoryTreeNode
name Text
code CategoryId Maybe
lft Int MigrationOnly default=0
rgt Int MigrationOnly default=0
deriving Show Typeable
For models defined without Primary, I could use toSqlKey to convert arbitrary Int64s into Key Category. Is there an equivalent function for models with custom primary keys?
I found the answer by looking through persistent's TH module:
https://github.com/yesodweb/persistent/blob/9396c278fc181fdac4a97c53637700417f41a478/persistent-template/Database/Persist/TH.hs#L857-L858
This line suggest that a CategoryKey is generated via TH and that is indeed the case:
let x :: Key Category
x = CategoryKey "foobar"
Related
I have following two query set as properties of django model
1 - Comment.objects.filter(ForeignKeyField=self)
2 - Comment.objects.filter(ForeginKeyField=self.id)
I am not why both these lines are giving same result , namely CommentObjectList ? Why I am able to filter the Comment Resultset with self and self.id both ??
The FK field actually holds the primary key for the related record. So
Comment.objects.filter(ForeginKeyField=self.id)
should give you the result you want (assuming self here is an instance of the foreign key model)
Comment.objects.filter(ForeginKeyField=self)
would try and stuff the entire instance in and match that, so you'd be matching an int or UUID (depending on what kind of key you have) with an instance of the FK object, and of course this will not match.
Side note: field names should use underscores in Django, by convention, so foreign_key_field, not ForeignKeyField.
I have these two models:
class A(models.Model):
name=models.CharField(max_length=10)
class D(models.Model):
code=models.IntegerField()
the code field can have a number that exists in model A but it cant be related due to other factors. But what I want know is to list items from A whose value is the same with code
items=D.objects.values('code__name')
would work but since they are not related nor can be related, how can I handle that?
You can use Subquery() expressions in Django 1.11 or newer.
from django.db.models import OuterRef, Subquery
code_subquery = A.objects.filter(id=OuterRef('code'))
qs = D.objects.annotate(code_name=Subquery(code_subquery.values('name')))
The output of qs is a queryset of objects D with an added field code_name.
Footnotes:
It is compiled to a very similar SQL (like the Bear Brown's solution with "extra" method, but without disadvantages of his solution, see there):
SELECT app_d.id, app_d.code,
(SELECT U0.name FROM app_a U0 WHERE U0.id = (app_d.code)) AS code_name
FROM app_d
If a dictionary output is required it can be converted by .values() finally. It can work like a left join i.e. if the pseudo related field allows null (code = models.IntegerField(none=True)) then the objects D are not restricted and the output code_name value could be None. A feature of Subquery is that it returns only one field expression must be eventually repeated for another fields. (That is similar to extra(select={...: "SELECT ..."}), but thanks to object syntax it can be more readable customized than an explicit SQL.)
you can use django extra, replace YOUAPP on your real app name
D.objects.extra(select={'a_name': 'select name from YOUAPP_a where id=code'}).values('a_name')
# Replace YOUAPP^^^^^
I'm working on simple ratings and comments apps to add to my project and am looking for advice on creating the models.
Normally, I'd create these database schemas like this:
comment_
id - primary key
type - varchar (buyer_item, buyer_vendor, vendor_buyer)
source_id - int (primary key of the table based on the type)
target_id - int (primary key of the table based on the type))
timestamp - timestamp
subject - varchar
comment - text
rating_
id - primary key
type - varchar (buyer_item, buyer_vendor, vendor_buyer)
source_id - int (primary key of the table based on the type)
target_id - int (primary key of the table based on the type)
timestamp - timestamp
rating - int (the score given, ie: 1-5 stars)
This would let me have simple methods that would allow me to apply comments or ratings to any type of thing by setting the proper type and setting the id's of who submitted it (source_id) what it applies to (target_id), like:
add_comment('user_product', user.pk, product.pk, now, subject, comment)
add_comment('user_vendor', user.pk, vendor.pk, now, subject, comment)
I know in the models you define the relationships to other tables as part of the model. How would I define the relationship in these types of tables where the TYPE field determines what table the SOURCE_ID and TARGET_ID link to.
Or should I omit the relationships from the model and set the joins up when I get the QuerySets?
Or just trash the who common table idea and make a whole bunch of different tables to be used for each relationship (eg: user_ratings, product_ratings, transaction_ratings, etc)?
What's the best practice here? My DBA senses say use common tables, but Django newbie me isn't sure what the natives do.
Thanks!
I think what you are looking for is a Generic Relation, and you can find this type of thing in the contenttypes framework: https://docs.djangoproject.com/en/1.0/ref/contrib/contenttypes/#generic-relations
I wanted to know is there anything equivalent to:
select columnname from tablename
Like Django tutorial says:
Entry.objects.filter(condition)
fetches all the objects with the given condition. It is like:
select * from Entry where condition
But I want to make a list of only one column [which in my case is a foreign key]. Found that:
Entry.objects.values_list('column_name', flat=True).filter(condition)
does the same. But in my case the column is a foreign key, and this query loses the property of a foreign key. It's just storing the values. I am not able to make the look-up calls.
Of course, values and values_list will retrieve the raw values from the database. Django can't work its "magic" on a model which means you don't get to traverse relationships because you're stuck with the id the foreign key is pointing towards, rather than the ForeignKey field.
If you need to filters those values, you could do the following (assuming column_name is a ForeignKey pointing to MyModel):
ids = Entry.objects.values_list('column_name', flat=True).filter(...)
my_models = MyModel.objects.filter(pk__in=set(ids))
Here's a documentation for values_list()
To restrict a query set to a specific column(s) you use .values(columname)
You should also probably add distinct to the end, so your query will end being:
Entry.objects.filter(myfilter).values(columname).distinct()
See: https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.values
for more information
Depending on your answer in the comment, I'll come back and edit.
Edit:
I'm not certain if the approach is right one though. You can get all of your objects in a python list by getting a normal queryset via filter and then doing:
myobjectlist = map(lambda x: x.mycolumnname, myqueryset)
The only problem with that approach is if your queryset is large your memory use is going to be equally large.
Anyway, I'm still not certain on some of the specifics of the problem.
You have a model A with a foreign key to another model B, and you want to select the Bs which are referred to by some A. Is that right? If so, the query you want is just:
B.objects.filter(a__isnull = False)
If you have conditions on the corresponding A, then the query can be:
B.objects.filter(a__field1 = value1, a__field2 = value2, ...)
See Django's backwards relation documentation for an explanation of why this works, and the ForeignKey.related_name option if you want to change the name of the backwards relation.
in models i have start and end date.
How to get all element where start and end date are diffrents.
>>> Entry.objects.exclude(start = end)
>>> NameError: name 'end' is not defined
I have no idea please help.
https://docs.djangoproject.com/en/dev/topics/db/queries/#filters-can-reference-fields-on-the-model
In the examples given so far, we have constructed filters that compare the value of a model field with a constant. But what if you want to compare the value of a model field with another field on the same model?
Django provides the F() object to allow such comparisons. Instances of F() act as a reference to a model field within a query. These references can then be used in query filters to compare the values of two different fields on the same model instance.
For your case, the following should work.
from django.db.models import F
Entry.objects.exclude(start=F('end'))