I have
class holiday(models.Model):
h_date=models.DateField()
h_description=models.CharField(max_length=200)
when I query
h_list=holiday.objects.all().values('h_date')
I get output as
<QuerySet[{'h_date':datetime.date(2017,5,1)},{'h_date':datetime.date(2017,4,2)}]>
But I need output as
2017-05-01
2017-04-02
How can I get that output with just querying the database?
values() method returns a QuerySet object. (BTW, you could use dates, which directly evaluates to a list of datetime.date objects). You could use list comprehension to apply format though strftime method to each item:
[d.strftime('%Y-%m-%d') for d in holiday.objects.all().dates('h_date', 'day')]
Related
we all know that if we need to retrieve data from the database the data will back as a queryset but the question is How can I retrieve the data from database which is the name of it is queryset but remove that name from it.
maybe I can't be clarified enough in explanation so you can look at the next example to understand what I mean:
AnyObjects.objects.all().values()
this line will back the data like so:
<QuerySet [{'key': 'value'}]
now you can see the first name that is on the left side of retrieving data which is: "QuerySet" so, I need to remove that name to make the data as follows:
[{'key': 'value'}]
if you wonder about why so, the abbreviation of answer is I want to use Dataframe by pandas so, to put the data in Dataframe method I should use that layout.
any help please!!
You don't have to change it from a Queryset to anything else; pandas.DataFrame can take any Iterable as data. So
df = pandas.DataFrame(djangoapp.models.Model.objects.all().values())
Gives you the DataFrame you expect. (though you may want to double check df.dtypes. If there are Nones in your data, the column may end up to be of object type.)
You can use list(…) to convert it to a list of dictionaries:
list(AnyObjects.objects.values())
You will need to serialize it with the json package to obtain a JSON blob, since strings with single quotes are not valid JSON, in order to make it a JSON blob, you can work with:
import json
json.dumps(list(AnyObjects.object.values()))
Using Django and Python 3.7. I'm tryhing to write a query to give me the average of the difference between two dates. I have two fields in my model, both "DateTimeField"s, and I try to calculate the average difference like so
everything_avg = Article.objects.aggregate(
avg_score=Avg(F('removed_date') - F('created_on'), output_field=models.DateTimeField())
).filter(removed_date__isnull=False)
return everything_avg
but I end up getting this error when running the above
AttributeError: 'dict' object has no attribute 'filter'
What's the right way to get my average?
As the documentation says:
aggregate() is a terminal clause for a QuerySet that, when invoked, returns a dictionary of name-value pairs. *
aggregate method returns a dictionary, thus you need to make your filtering before it. Thus if you alter your code as following you would get your result:
everything_avg = Article.objects.filter(removed_date__isnull=False)\
.aggregate(
avg_score=Avg(
F('removed_date') - F('created_on'),
output_field=models.DateTimeField()
)
)
return everything_avg
When I do this,
>>> b = Blog.objects.all()
>>> b
I get this:
>>>[<Blog: Blog Title>,<Blog: Blog Tile>]
When I query what type b is,
>>> type(b)
I get this:
>>> <class 'django.db.models.query.QuerySet'>
What does this mean? Is it a data type like dict, list, etc?
An example of how I can build data structure like a QuerySet will be appreciated.
I would want to know how Django builds that QuerySet (the gory details).
A django queryset is like its name says, basically a collection of (sql) queries, in your example above print(b.query) will show you the sql query generated from your django filter calls.
Since querysets are lazy, the database query isn't done immediately, but only when needed - when the queryset is evaluated. This happens for example if you call its __str__ method when you print it, if you would call list() on it, or, what happens mostly, you iterate over it (for post in b..). This lazyness should save you from doing unnecessary queries and also allows you to chain querysets and filters for example (you can filter a queryset as often as you want to).
Yes, it's just another type, built like every other type.
A QuerySet represents a collection of objects from your database. It can have zero, one or many filters. Filters narrow down the query results based on the given parameters. In SQL terms, a QuerySet equates to a SELECT statement, and a filter is a limiting clause such as WHERE or LIMIT.
https://docs.djangoproject.com/en/1.8/topics/db/queries/
A QuerySet is a list of objects of a given model, QuerySet allow you to read data from database
I have a model which looks like this:
class MyModel(models.Model)
value = models.DecimalField()
date = models.DatetimeField()
I'm doing this request:
MyModel.objects.aggregate(Min("value"))
and I'm getting the expected result:
{"mymodel__min": the_actual_minimum_value}
However, I can't figure out a way to get at the same time the minimum value AND the associated date (the date at which the minimum value occured).
Does the Django ORM allow this, or do I have to use raw SQL ?
What you want to do is annotate the query, so that you get back your usual results but also have some data added to the result. So:
MyModel.objects.annotate(Min("value"))
Will return the normal result with mymodel__min as an additional value
In reply to your comment, I think this is what you are looking for? This will return the dates with their corresponding Min values.
MyModel.objects.values('date').annotate(Min("value"))
Edit: In further reply to your comment in that you want the lowest valued entry but also want the additional date field within your result, you could do something like so:
MyModel.objects.values('date').annotate(min_value=Min('value')).order_by('min_value')[0]
This will get the resulting dict you are asking for by ordering the results and then simply taking the first index which will always be the lowest value.
See more
Is there a way to filter and get only specific columns?
For example, get all entries with the column first_name.
QuerySet.values() or QuerySet.values_list(), e.g.:
Entry.objects.values('first_name')
If you want a list of only the values, use:
Entry.objects.values_list('first_name', flat=True)
To only get a column's values from the table but still return an object of that model, use only:
record = Entry.objects.only('first_name')
This will defer all other columns from the model but you can still access them all normally.
record.first_name # already retrieved
record.last_name # retrieved on call