I've been struggling for this issue for a few hours - I know there's probably a simple solution that I'm overlooking.
I have a one to many relationship with my models.
I have need to return all rows of one object with the rows for the related object.
In a sense I have this:
object
object
object_relationship.property
object_relationship.property
object
object_relationship.property
object
Now - I can run through all of these fine, but I run into an issue when I want to send these back to the html template.
I can send the object back - but how do I send the object_relationship back in the order that I have it above?
Does this make sense?
You might not need to worry too much about this, acutally... look at these models:
class Venue(base.NamedEntity, HasPerformances, HasUrl, HasLocation):
city = db.ReferenceProperty(City, collection_name='venues')
url = db.StringProperty(required=True, validator=validators.validate_url)
location = db.GeoPtProperty()
class Performance(base.Entity):
show = db.ReferenceProperty(Show, collection_name='performances', required=True)
utc_date_time = db.DateTimeProperty(required=True)
venue = db.ReferenceProperty(Venue, collection_name='performances', required=True)
In a case like this, nothing stops you from using venue.performances from either code or templates and treating it as a list. The API will automatically fire queries as needed to fetch the actual objects. The same thing goes for performance.venue.
The only problem here is performance - you've got a variant of the n+1 problem to deal with. There are workarounds, though, like this article by Nick Johnson. I'd suggest reading the API code too... it makes for interesting reading how the property get is captured and dereferenced.
My first suggestion is to denormalize the data if you are going to do many reports like that. For example, maybe you could include object.name on the object_relationship entity.
That said, you could send a list of dicts to your template, so maybe something like:
data = []
for entity in your_query:
children = [{'name': child.name} for child in entity.object_relation]
data.append({'name': object.name,
'children': children,
...
})
Then pass the data list to your template, and process it.
Please note, this will perform very badly. It will execute another query for every one of the items in your first query. Use Appstats to profile your app.
Related
I have a Wagtail site and I'm making a listing page for a few different Page types/Content types. I filter by a snippet field first:
def highlights_list(category='Highlight'):
tag = ContentCategory.objects.get(name=category)
highlights = Page.objects.filter(
Q(event__categories=tag)|
Q(newspage__categories=tag)|
Q(contentpage__categories=tag)).live()
return highlights.order_by('-last_published_at')
In Wagtail all content types inherit from the base Page class which makes creating a queryset with all the content types I want really easy. But I can't work out how to sort nicely.
Sorting by last_published_at is fine for NewsPage and ContentPage but not for Event where I'd like to sort by the DateTimeField for the event.
I thought about making a #property on all the models called sort_date which uses the datetime field specific to each model that I'd like to sort on, but that just doesn't work.
Any suggestions are very welcome!
I can think of two solutions for this, but only one seems to be easier to do. I changed your function above to this:
def highlights_list(category='Highlight'):
tag = ContentCategory.objects.get(name=category)
highlights = Page.objects.filter(
Q(event__categories=tag)|
Q(newspage__categories=tag)|
Q(contentpage__categories=tag)
).live().specific()
return sorted(
highlights,
key=lambda p: getattr(p, 'date') or getattr(p, 'last_published_at'),
reverse=True
)
What this does is it takes all the pages from highlights and sorts them based on the value in date or last_published_at as if they're the same field. Because only the Event page has a date field, last_published_at is used as the fallback, a field all live pages have in common. If you leave out specific() in your queryset like in your question, it throws an AttributeError saying 'Page' object has no attribute 'date'.
As a bonus, the sorting part doesn't use any database queries since it is done in Python. Note that this returns a list based on the original QuerySet.
The second solution involves database transactions and Wagtail hooks and has more code, but that is something I don't want to share as I haven't thought of how to write that out.
I am trying to make use of a column's value as a radio button's choice using below code
Forms.py
#retreiving data from database and assigning it to diction list
diction = polls_datum.objects.values_list('poll_choices', flat=True)
#initializing list and dictionary
OPTIONS1 = {}
OPTIONS = []
#creating the dictionary with 0 to no of options given in list
for i in range(len(diction)):
OPTIONS1[i] = diction[i]
#creating tuples from the dictionary above
#OPTIONS = zip(OPTIONS1.keys(), OPTIONS1.values())
for i in OPTIONS1:
k = (i,OPTIONS1[i])
OPTIONS.append(k)
class polls_form(forms.ModelForm):
#retreiving data from database and assigning it to diction list
options = forms.ChoiceField(choices=OPTIONS, widget = forms.RadioSelect())
class Meta:
model = polls_model
fields = ['options']
Using a form I am saving the data or choices in a field (poll_choices), when trying to display it on the index page, it is not reflecting until a server restart.
Can someone help on this please
of course "it is not reflecting until a server restart" - that's obvious when you remember that django server processes are long-running processes (it's not like PHP where each script is executed afresh on each request), and that top-level code (code that's at the module's top-level, not in a function) is only executed once per process when the module is first imported. As a general rule: don't do ANY db query at a module's top-level or at the top-level of a class statement - at best you'll get stale data, at worse it will crash your server process (if you're doing query before everything has been properly setup by django, or if you're doing query based on a schema update before the migration has been applied).
The possible solutions are either to wait until the form's initialisation to setup your field's choices, or to pass a callable as the formfield's choices options, cf https://docs.djangoproject.com/en/2.1/ref/forms/fields/#django.forms.ChoiceField.choices
Also, the way you're building your choices list is uselessly complicated - you could do it as a one-liner:
OPTIONS = list(enumerate(polls_datum.objects.values_list('poll_choices', flat=True))
but it's also very brittle - you're relying on the current db content and ordering for the choice value when you should use the polls_datum's pk instead (which is garanteed to be stable).
And finally: since you're working with what seems to be a related model, you may want to use a ModelChoiceField instead.
For future reference:
What version of Django are you using?
Have you read up on the documentation of ModelForms? https://docs.djangoproject.com/en/2.1/topics/forms/modelforms/
I'm not sure what you're trying to do with diction to dictionary to tuple. I think you could skip a step there and your future self will thank you for that.
Try to follow some tutorials and understand why certain steps are being taken. I can see from your code that you're rather new to coding or Python and there's room for improvement. Not trying to talk you down, but I'm trying to push you into the direction of becoming a better developer ;-)
REAL ANSWER:
That being said, I think the solution is to write the loading of the data somewhere in your form model, rather than 'loose' in forms.py. See bruno's answer for more information on this.
If you want to reload the data on each request that loads the form, you should create a function that gets called every time the form is loaded (for example in the form's __init__ function).
Actually I have two things that don't seem to work. I'll list couple of models and their dependencies (shortened). The StudentGroup has students (which may be active/inactive), and messages, which are listed as chat.
In views.py, when I call delete_group(), I want to make all students inactive and delete all of the messages relevant to the group.
class StudentsGroup(models.Model):
students = models.ManyToManyField(User,limit_choices_to={'is_staff': False}, related_name="user_groups",blank=True)
finished = models.ManyToManyField(User,limit_choices_to={'is_staff': False}, related_name="finished_user_groups",blank=True)
class Message(models.Model):
group=models.ForeignKey(StudentsGroup)
def delete_group(request,group):
Message.objects.filter(group=group).delete()
groupl=StudentsGroup.objects.get(id=group)
for s in group1.students.all():
groupl.finished.add(s)
group1.save()
Nothing changes. I've tried similar things in console and it seemed to be ok. Tried bunch of similar code.
Tried to add makemigrations to the server restarting but still no result.
Kinda noob in django and webdev overall, any help would be appreciated.
In your line
Message.objects.filter(group=group).delete()
it looks like you haven't yet converted group from the raw ID value to an actual StudentGroup instance. If group is supplied to the function as a StudentGroup instance, then this should work. If, as I suspect, group is just a raw ID value, then your filter queries should be:
Message.objects.filter(group_id=group).delete()
It's nicer however to work with objects rather than IDs. Try instead doing this:
group = StudentGroup.objects.get(id=group)
Message.objects.filter(group=group).delete()
Then you can do other things like group = get_object_or_404(StudentGroup, id=group).
As for the second part, moving the Students to the group's finished list, your code looks reasonable, though you don't need the .save() part. I suspect the errors may be due to the object not being correctly instantiated yet, as with above.
However you should be able to do all this in a single line in Django 1.9+, see https://docs.djangoproject.com/en/1.10/ref/models/relations/#django.db.models.fields.related.RelatedManager.add
group1.finished.add(*list(group1.students.all()))
Sorry for noobster question again.
But I'm trying to do some very easy stuff here, and I don't know how. Documentation gives me hints which do not work, or apply.
I recieve a POST request and grab a variable out of it. It says "name".
I have to search all over my entities Object (for example) and find out if there's one that has the same name. Is there's none, I must create a new Entity with this name. Easy it may look, but I keep Failing.
Would really appreciate any help.
My code currently is this one:
objects_qry = Object.query(Object.name == data["name"])
if (not objects_qry ):
obj = Object()
obj .name = data["name"]
obj .put()
class Object(ndb.Model):
name = ndb.StringProperty()
Using a query to perform this operation is really inefficient.
In addition your code is possibly unreliable, if name doesn't exist and you have two requests at the same time for name you could end up with two records. And you can't tell because your query only returns the first entity with the name property equal to some value.
Because you expect only one entity for name a query is expensive and inefficient.
So you have two choices you can use get_or_insert or just do a get, and if you have now value create a new entity.
Any way here is a couple of code samples using the name as part of the key.
name = data['name']
entity = Object.get_or_insert(name)
or
entity = Object.get_by_id(name)
if not entity:
entity = Object(id=name)
entity.put()
Calling .query just creates a query object, it doesn't execute it, so trying to evaluate is as a boolean is wrong. Query object have methods, fetch and get that, respectively, return a list of matching entities, or just one entity.
So your code could be re-written:
objects_qry = Object.query(Object.name == data["name"])
existing_object = objects_qry.get()
if not existing_object:
obj = Object()
obj.name = data["name"]
obj.put()
That said, Tim's point in the comments about using the ID instead of a property makes sense if you really care about names being unique - the code above wouldn't stop two simultaneous requests from creating entities with the same name.
I'm using google app engine with django 1.0.2 (and the django-helper) and wonder how people go about doing recursive delete.
Suppose you have a model that's something like this:
class Top(BaseModel):
pass
class Bottom(BaseModel):
daddy = db.ReferenceProperty(Top)
Now, when I delete an object of type 'Top', I want all the associated 'Bottom' objects to be deleted as well.
As things are now, when I delete a 'Top' object, the 'Bottom' objects stay and then I get data that doesn't belong anywhere. When accessing the datastore in a view, I end up with:
Caught an exception while rendering: ReferenceProperty failed to be resolved.
I could of course find all objects and delete them, but since my real model is at least 5 levels deep, I'm hoping there's a way to make sure this can be done automatically.
I've found this article about how it works with Java and that seems to be pretty much what I want as well.
Anyone know how I could get that behavior in django as well?
You need to implement this manually, by looking up affected records and deleting them at the same time as you delete the parent record. You can simplify this, if you wish, by overriding the .delete() method on your parent class to automatically delete all related records.
For performance reasons, you almost certainly want to use key-only queries (allowing you to get the keys of entities to be deleted without having to fetch and decode the actual entities), and batch deletes. For example:
db.delete(Bottom.all(keys_only=True).filter("daddy =", top).fetch(1000))
Actually that behavior is GAE-specific. Django's ORM simulates "ON DELETE CASCADE" on .delete().
I know that this is not an answer to your question, but maybe it can help you from looking in the wrong places.
Reconsider the data structure. If the relationship will never change on the record lifetime, you could use "ancestors" feature of GAE:
class Top(db.Model): pass
class Middle(db.Model): pass
class Bottom(db.Model): pass
top = Top()
middles = [Middle(parent=top) for i in range(0,10)]
bottoms = [Bottom(parent=middle) for i in range(0,10) for middle in middles]
Then querying for ancestor=top will find all the records from all levels. So it will be easy to delete them.
descendants = list(db.Query().ancestor(top))
# should return [top] + middles + bottoms
If your hierarchy is only a small number of levels deep, then you might be able to do something with a field that looks like a file path:
daddy.ancestry = "greatgranddaddy/granddaddy/daddy/"
me.ancestry = daddy.ancestry + me.uniquename + "/"
sort of thing. You do need unique names, at least unique among siblings.
The path in object IDs sort of does this already, but IIRC that's bound up with entity groups, which you're advised not to use to express relationships in the data domain.
Then you can construct a query to return all of granddaddy's descendants using the initial substring trick, like this:
query = Person.all()
query.filter("ancestry >", gdaddy.ancestry + "\U0001")
query.filter("ancestry <", gdaddy.ancestry + "\UFFFF")
Obviously this is no use if you can't fit the ancestry into a 500 byte StringProperty.