Django如何显示 <bound method … > &gt;在django-admin中将对象作为字符串对象？

Question

How does Django translate this <bound method Child.parent_identity of <Child: >> object in a string object, and displays it as such in my django-admin "inline" Child class idparent field ? What does Django do ?

I have the following application structure:

##========================models.py
...
from django_extensions.db.fields import UUIDField

class Parent(models.Model):
    id           = UUIDField(primary_key=True)

class Child(models.Model):
    parent      = models.ForeignKey(Parent)
    idparent    = models.CharField(max_length=100)
    def parent_identity(self):
        return self.parent_id

#========================admin.py
class ChildForm(forms.ModelForm):
    class Meta:
        model   = Child
        exclude = []
    def __init__(self, *args, **kwargs):
        super(ChildForm, self).__init__(*args, **kwargs)
        #print self.instance.parent_identity
        self.initial['idparent']   = self.instance.parent_identity        

class ChildInline(admin.TabularInline):
    model = Child
    extra = 1
    form  = ChildForm

class ParentAdmin(admin.ModelAdmin):
    exclude = []
    inlines = [ChildInline]
    #list_display, etc
admin.site.register(Parent,ParentAdmin)

My inline idparent field displays the Parent id field CORRECTLY in the admin inline interface. Being a newbie, it's magic for me, because self.instance.parent_identity is initially not a string object.

print self.instance.parent_identity
#it prints  : <bound method Child.parent_identity of <Child: >>

But how to explictly print the string content as follows

>>print self.instance.parent_identity
#would print  : fffeee29-7ac6-42eb-8a8d-eb212d2365ff

That is, how to get it so as to deal with it in the ChildForm class ?

UPDATE

I do not mind specifically about "UUID in the form when the instance hasn't been created yet" and i do not want to provide an initial value myself.

I want my still empty (extra) Child fields (one field in my example code: idparent) to contain by default something which is Parent variable.

Is it possible ?

Answer 1

Django templates automatically call any object that is callable; eg the callable() function returns True when you pass the object in. From the Variables section in the template documentation :

If the resulting value is callable, it is called with no arguments. The result of the call becomes the template value.

Bound methods are callable, so instead of using self.instance.parent_identity , the template uses the output of self.instance.parent_identity() .

In your own code, you generally already know that something is a method and you call it explicitly:

def __init__(self, *args, **kwargs):
    super(ChildForm, self).__init__(*args, **kwargs)
    self.initial['idparent']   = self.instance.parent_identity()

You can treat the parent_identity method as an attribute; have Python call it automatically without you having to call it explicitly. If you never have to pass in an argument, then that might make sense. You do this by decorating the method with the @property decorator :

class Child(models.Model):
    parent      = models.ForeignKey(Parent)
    idparent    = models.CharField(max_length=100)

    @property
    def parent_identity(self):
        return self.parent_id

at which point self.instance.parent_identity will give you the return value of that method.

Take into account that the UUIDField only is given a value on pre-save ; it'll be None until the object is saved in a database.

If you really wanted to UUID in the form when the instance hasn't been created yet, you'll have to provide an initial value yourself:

import uuid 

class ParentAdmin(admin.ModelAdmin):
    exclude = []
    inlines = [ChildInline]

    def __init__(self, *args, **kwargs):
        super(ParentAdmin, self).__init__(*args, **kwargs)
        self.fields['id'].initial = uuid.uuid4

Answer 2

You are calling a function, which means you need to use it as such:

self.initial['idparent']   = self.instance.parent_identity()

Alternately you could wrap it with the @property decorator and continue using it as you are, notice that you need to use self.parent.id if you want to access the parent's id :

class Child(models.Model):
    parent      = models.ForeignKey(Parent)
    idparent    = models.CharField(max_length=100)

    @property
    def parent_identity(self):
        return self.parent.id