Skip to main content

Django Custom Chainable QuerySets

How to reuse custom defined 'models.Manager' QuerySets

Django recommends to define custom query sets on your sublcassed models.Manager, there is one problem however that these are not chainable, that is you can only call your custom queryset once - assuming I have defined an '.active()' queryset the following will not work:

>> PaymentTerm.objects.active().active()
Traceback (most recent call last):
  File "", line 1, in 
AttributeError: 'PaymentTermManager' object has no attribute 'active'
>>>

Current solutions on the web involve creating a subclassed 'models.Manager' and a custom 'QuerySet' which do work but are to code verbose for my linking. I believe the below approach is simpler, cleaner and more succinct.

class OfferManager(models.Manager):
 
 ...
 
 STATUS_CHOICES = (
  (STATUS_DISABLED, "Disabled"),
  (STATUS_ENABLED, "Enabled"),
  (STATUS_NEGOTIATED, "Negotiated"),
  (STATUS_ARCHIVED, "Archived"),
 )
 QUERYSET_PUBLIC_KWARGS = {'status__gte': STATUS_ENABLED}
 QUERYSET_ACTIVE_KWARGS = {'status': STATUS_ENABLED}
  def get_query_set(self):
   """ Assing a modified QuerySet to include our chainable QuerySets """
   class OfferManagerQuerySet(QuerySet): # clone QuerySet
                pass
  # explictly copy our QuerySet methods
  OfferManagerQuerySet.public = self.public
  OfferManagerQuerySet.active = self.active

  ...
 
  return QuerySetDynamicallyCreated(self.model, using=self._db)
 def public(self):
  """ Returns all entries accessible through front end site"""
  return self.all().filter(**OfferManager.QUERYSET_PUBLIC_KWARGS)
 public.chainable = True
 def active(self):
  """ returns offers that are open to negotiation """
  return self.public().filter(**OfferManager.QUERYSET_ACTIVE_KWARGS)

 ...

Django patch for model.Manager chainable QuerySets

Please take a look at Django Ticket #20625 with a github Django branch for new proposed method for defining chainable filters based on this idea which works through a 'chainable == True' attribute as shown below:

class OfferManager(models.Manager):

    ...
    
    def public(self):
        """ Returns all entries accessible through front end site"""
        return self.all().filter(...)
    public.chainable = True     # instructs to dynamically tranplat this method onto
                                # returned QuerySet as .public(...) 
                                # effectively providing chainable custom QuerySets

    def active(self):
        """ Returns offers that are open to negotiation """
        return self.public().filter(**OfferManager.QUERYSET_ACTIVE_KWARGS)
                                    # an example of how to reffer to OfferManager
                                    # constants as 'self' context changes
    active.chainable = True
    ...

As always feel free to comment or follow me on twitter @danielsokolowski

Comments

Popular posts from this blog

Duplicate value found: duplicates value on record with id: <unknown>.

System.DmlException: Insert failed. First exception on row 0; first error: DUPLICATE_VALUE, duplicate value found: <unknown> duplicates value on record with id: <unknown>. The above error is triggered in the database layer and caused by a trigger or workflow outside of your main code of block that is bubbling this exception. This is rather difficult to track down especially if you are unfamiliar with the code, I am sharing my procedure in the hopes this saves you time - if you find this helpful drop me a line or follow me on twitter @danielsokolows . This error is caused by unique field constraint on the object, so the first step is to examine the object and locate the API names of all unique fieds. You can do this through SF direclty 'Setup < Customize &lt <object being inserted> &lt Fields' or by downloading the `src/objects` metadata information and searching for <unique> ; I preffer the latter and actually download ALL matadata i

Softeher 'Error occurred. (Error code: 2)' sollution

Protocol error occurred. Error was returned from the destination server. The Softether server by default to run on port 443 , if you server also hosts normal https then 443 is already taken and so Softether can't bind to it. When you run `vpncmd` it attempts to connect, find an active port, but of course fails with 'Protocol error occurred. Error was returned from the destination server.' because it's not actually connecting to the vpn server. By default Softether also listens on 992 , 1194 , and 5555 so the sollution is to modify specify `localhost:5555` when executing the `vpncmnd`. If this has helped you feel free to comment or follow me on twitter @danielsokolows .

How to child proof a fireplace

DIY - Do it yourself fireplace child guard Our wonderful 8.5 month old Sofia has become a crawling race car with an untamed thirst for exploration. And so with the cold nights approaching we needed to child proof the fireplace. This however proved to be more difficult than would reasonably expect, I've checked the local Toys "R" Us, Walmart, and even a Canadian Tire with no success for a ready to use product. Internet search was more fruitful and returned a few online stores one could order from, however in all honestly they didn't look too sturdy to me. So I build my own relatively quickly and inexpensively. Materials needed is a privacy plastic lattice - the smallest hole pattern - a few screws and anchors; tools needed are a drill, and a handsaw if you don't have the lattice cut at the store - that’s it. The construction consits of screwing the lattice into the wall and the final product is easiest explained through following pictures.