I am developing a dictionary application and I have an Expression model defined as follows:
class Expression(models.Model):
expression = models.CharField()
In one of my views, I am using Expression.objects.get_or_create(expression=expression).
Importantly, I want the comparison between "expression" and all existing Expression objects in my database to be case-insensitive.
In other words, even though this is not valid syntax, I would like the logic of my search to be: Expression.objects.get_or_create(expression.lower()=expression.lower())
How can I achieve that?
You can do case-insensitive filter as below:
Expression.objects.get_or_create(expression__iexact=expression.lower())
Look at documentation for other field lookups.
Note: get_or_create method can create duplicate items in database because of race condition. Be sure that you pass unique field or fields that are unique together to this method for avoiding possible duplicates.
Related
I have a queryset like this:
predicts = Prediction.objects.select_related('match').filter(match_id=pk)
I need to annotate this with a new field is_correct. I need to compare two string fields and the result should be annotated in this new field. the fields that I want to compare are:
predict from Prediction table
result from Match table (that has been joined through select_related)
I need to know what expression should I put inside my annotate function; below I have my current code which throughs a TypeError exception:
predicts = predicts.annotate(is_correct=(F('predict') == F('result')))
all help will be greatly appreciated.
UPDATE:
I found an alternative solution that does the job for me (filtering the Prediction based on Match result using filter and exclude), but I still like to know how to address this specific case where the new annotated field is the result of the comparison between two other fields of the queryset. For those who may need it, in Django 2.2 and later the Nullif database function does a comparison between two fields.
You can use the extra function, a hook for injecting specific clauses into the SQL.
First of all, we must know the names of the apps and the models, or the name of the tables in the database.
Assuming that in your case, the two tables are called "app_prediction" and "app_match".
The sentence would be as follows:
Prediction.objects.select_related('match').extra(
select={'is_correct': "app_prediction.predict = app_match.result"}
)
This will add a field called is_correct in your result,
in the database, the fields and tables must be called in the same way.
It would be best to see the models.
I have a model with some HStoreField attributes and I can't seem to use Django's ORM HStoreField to query those values using LIKE.
When doing Model.objects.filter(hstoreattr__values__contains=['text']), the queryset only contains rows in which hstoreattr has any value that matches text exactly.
What I'm looking for is a way to search by, say, te instead of text and those same rows be returned as well. I'm aware this is possible in a raw PostgreSQL query but I'm looking for a solution that uses Django ORM.
If you want to check value of particular key in every object if it contains 'te', you can do:
Model.objects.filter(hstoreattr__your_key__icontains='te')
If you want to check if any key in your hstore field contains 'te', you will need to create your own lookup in django, because by default django won't do such thing. Refer to custom lookups in django docs for more info.
As far as I can remember, you cannot filter in values. If you want to filter in values, you have to pass a column and value you are referencing to. When you want it to be case insensitive use __icontains.
Although you cannot filter by all values, you can filter by all keys. Just like you showed in your code.
If you want to search for 'text' in all objects in key named let's say 'fo' - just do smth like this:
Model.objects.filter(hstoreattr__icontains={'fo': 'text'})
I am using Django, with mongoengine. I have a model Classes with an inscriptions list, And I want to get the docs that have an id in that list.
classes = Classes.objects.filter(inscriptions__contains=request.data['inscription'])
Here's a general explanation of querying ArrayField membership:
Per the Django ArrayField docs, the __contains operator checks if a provided array is a subset of the values in the ArrayField.
So, to filter on whether an ArrayField contains the value "foo", you pass in a length 1 array containing the value you're looking for, like this:
# matches rows where myarrayfield is something like ['foo','bar']
Customer.objects.filter(myarrayfield__contains=['foo'])
The Django ORM produces the #> postgres operator, as you can see by printing the query:
print Customer.objects.filter(myarrayfield__contains=['foo']).only('pk').query
>>> SELECT "website_customer"."id" FROM "website_customer" WHERE "website_customer"."myarrayfield_" #> ['foo']::varchar(100)[]
If you provide something other than an array, you'll get a cryptic error like DataError: malformed array literal: "foo" DETAIL: Array value must start with "{" or dimension information.
Perhaps I'm missing something...but it seems that you should be using .filter():
classes = Classes.objects.filter(inscriptions__contains=request.data['inscription'])
This answer is in reference to your comment for rnevius answer
In Django ORM whenever you make a Database call using ORM, it will generally return either a QuerySet or an object of the model if using get() / number if you are using count() ect., depending on the functions that you are using which return other than a queryset.
The result from a Queryset function can be used to implement further more refinement, like if you like to perform a order() or collecting only distinct() etc. Queryset are lazy which means it only hits the database when they are actually used not when they are assigned. You can find more information about them here.
Where as the functions that doesn't return queryset cannot implement such things.
Take time and go through the Queryset Documentation more in depth explanation with examples are provided. It is useful to understand the behavior to make your application more efficient.
I want to attach a field value (id) to a QS like below, but Django throws a 'str' object has no attribute 'lookup' error.
Book.objects.all().annotate(some_id='somerelation__id')
It seems I can get my id value using Sum()
Book.objects.all().annotate(something=Sum('somerelation__id'))
I'm wondering is there not a way to simply annotate raw field values to a QS? Using sum() in this case doesn't feel right.
There are at least three methods of accessing related objects in a queryset.
using Django's double underscore join syntax:
If you just want to use the field of a related object as a condition in your SQL query you can refer to the field field on the related object related_object with related_object__field. All possible lookup types are listed in the Django documentation under Field lookups.
Book.objects.filter(related_object__field=True)
using annotate with F():
You can populate an annotated field in a queryset by refering to the field with the F() object. F() represents the field of a model or an annotated field.
Book.objects.annotate(added_field=F("related_object__field"))
accessing object attributes:
Once the queryset is evaluated, you can access related objects through attributes on that object.
book = Book.objects.get(pk=1)
author = book.author.name # just one author, or…
authors = book.author_set.values("name") # several authors
This triggers an additional query unless you're making use of select_related().
My advice is to go with solution #2 as you're already halfway down that road and I think it'll give you exactly what you're asking for. The problem you're facing right now is that you did not specify a lookup type but instead you're passing a string (somerelation_id) Django doesn't know what to do with.
Also, the Django documentation on annotate() is pretty straight forward. You should look into that (again).
You have <somerelation>_id "by default". For example comment.user_id. It works because User has many Comments. But if Book has many Authors, what author_id supposed to be in this case?
I have Django code as follows
qs = Result.objects.only('time')
qs = qs.filter(organisation_id=1)
qs = qs.annotate(Count('id'))
And it gets translated into the following SQL:
SELECT "myapp_result"."id", "myapp_result"."time", COUNT("myapp_result"."id") AS "id__count" FROM "myapp_result" WHERE "myapp_result"."organisation_id" = 1 GROUP BY "myapp_result"."id", "myapp_result"."organisation_id", "myapp_result"."subject_id", "myapp_result"."device_id", "myapp_result"."time", "myapp_result"."tester_id", "myapp_result"."data"
As you can see, the GROUP BY clause starts with the field I intended (id) but then it goes on to list all the other fields as well. Is there any way I can persuade Django not to specify all the individual fields like this?
As you can see, even with .only('time') that doesn't stop Django from listing all the other fields anyway, but only in this GROUP BY clause.
The reason I want to do this is to avoid the issue described here where PostgreSQL doesn't support annotation when there's a JSON field involved. I don't want to drop native JSON support (so I'm not actually using django-jsonfield). The query works just fine if I manually issue it without the reference to "myapp_result"."data" (the only JSON field on the model). So if I could just persuade Django not to refer to it, I'd be fine!
only only defers the loading of certain fields, i.e. it allows for lazy loading of big or unused fields. It should generally not be used unless you know exactly what you're doing and why you need it, as it is nothing more than a performance booster than often decreases performance with improper use.
What you're looking for is values() (or values_list()), which actually excludes certain fields instead of just lazy loading. This will return a dictionary (or list) instead of a model instance, but this is the only way to tell Django to not take other fields into account:
qs = (Result.objects.filter_by(organisation_id=1)
.values('time').annotate(Count('id')))