CODEWITHSUNDEEP
djangodjango-modelsdjango-managers
FAM_Maurice
FAM_Maurice

Reputation: 423

When to use a custom model manager?

Right now i'm reading about custom managers that you can use to add additional logic when doing a CRUD action like Create. You make a custom manager class and then initialize the objects attribute of the table class with an instance of the custom manager class.

I've also read that you can also use the pre_save and post_save signals to add additional save logic to the save action. My question is: When should i use a custom manager class over signals and is the use of a custom manager slower then the default manager of the Model class?

Thank you

Upvotes: 0

Views: 276

Answers (1)

nigel222
nigel222

Reputation: 8202

OK, here's one I wrote earlier. I've cut out a lot of irrelevant stufff, hoping all the essentials remain. You need to know there's a Foreign Key chain PRSBlock -> PRS2 -> Jobline -> Description. Using this, all queries for PRSBlock.objects.filter(... will return PRSBlock objects with extra fields membrane_t, substrate_t and material from the Description found by following this chain. They're almost always needed in this context, unlike most of the other things along that chain. select_related would be serious overkill. 

class PRSBlock_Manager(models.Manager):   

    def get_queryset(self):
        return super().get_queryset().annotate(
            membrane_t = F('PRS__jobline__description__mt'), 
            substrate_t= F('PRS__jobline__description__ft'),
            material   = F('PRS__jobline__description__material')
        )

class PRSblock( models.Model):

    # override the default manager
    objects=PRSBlock_Manager()  # annotate always with description parametrs relevant to block LUTs

    PRS = models.ForeignKey( PRS2, models.CASCADE, related_name='prs_blocks')
    # and lots of other fields that aren't relevant

Upvotes: 1

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