可在字段集中使用的有序 ManyToManyField

Question

我一直在研究一个有序的 ManyToManyField 小部件，并且它的前端方面工作得很好：

< img src="https://i.sstatic.net/APMce.png" alt="替代文本">

不幸的是，我在让后端工作时遇到了很多麻烦。连接后端的明显方法是使用 through 表将模型与关系两侧的 ForeignKey 联系起来，并覆盖 save 方法。这会很好用，但由于内容的特殊性，绝对要求将此小部件放置在字段集中（使用 ModelAdmin fieldsets 属性），即 显然不可能。

我没主意了。有什么建议吗？

谢谢！

Answer 1

关于如何设置模型，您是对的，带有“订单”列的贯穿表是表示它的理想方式。你也是对的，Django 不会让你在字段集中引用该关系。解决此问题的技巧是记住，您在 ModelAdmin 的“fieldsets”或“fields”中指定的字段名称实际上并不引用 Model 的字段>，但是到 ModelForm 的字段，我们可以根据自己的喜好自由地覆盖它。对于many2many字段，这会变得很棘手，但请耐心等待：

假设您试图代表比赛和参加其中竞争的竞争对手，在比赛和竞争对手之间使用有序的many2many，其中顺序代表竞争对手在该比赛中的排名。您的 models.py 将如下所示：

from django.db import models

class Contest(models.Model):
    name = models.CharField(max_length=50)
    # More fields here, if you like.
    contestants = models.ManyToManyField('Contestant', through='ContestResults')

class Contestant(models.Model):
    name = models.CharField(max_length=50)

class ContestResults(models.Model):
    contest = models.ForeignKey(Contest)
    contestant = models.ForeignKey(Contestant)
    rank = models.IntegerField()

希望这与您正在处理的内容类似。现在，对于管理员来说。我已经编写了一个示例 admin.py ，其中包含大量注释来解释发生的情况，但这里有一个摘要可以帮助您：

由于我没有您所订购的 m2m 小部件的代码编写时，我使用了一个占位符虚拟小部件，它只是继承自 TextInput。输入包含参赛者 ID 的逗号分隔列表（不带空格），它们在字符串中出现的顺序决定了 ContestResults 模型中“rank”列的值。

发生的情况是，我们用自己的重写 Contest 的默认 ModelForm，然后在其中定义一个“结果”字段（我们不能将该字段称为“参赛者”，因为会有一个名称）与模型中的 m2m 字段冲突）。然后，我们重写__init__()，该函数在后台显示表单时调用，因此我们可以获取可能已为竞赛定义的任何 ContestResults，并使用它们来填充小部件。我们还重写 save()，以便我们可以依次从小部件获取数据并创建所需的 ContestResults。

请注意，为了简单起见，此示例省略了诸如验证小部件中的数据之类的内容，因此如果您尝试在文本输入中输入任何意外的内容，事情就会中断。此外，创建 ContestResults 的代码非常简单，并且可以大大改进。

我还应该补充一点，我实际上已经运行了这段代码并验证了它的工作原理。

from django import forms
from django.contrib import admin
from models import Contest, Contestant, ContestResults

# Generates a function that sequentially calls the two functions that were
# passed to it
def func_concat(old_func, new_func):
    def function():
        old_func()
        new_func()
    return function

# A dummy widget to be replaced with your own.
class OrderedManyToManyWidget(forms.widgets.TextInput):
    pass

# A simple CharField that shows a comma-separated list of contestant IDs.
class ResultsField(forms.CharField):
    widget = OrderedManyToManyWidget()

class ContestAdminForm(forms.models.ModelForm):
    # Any fields declared here can be referred to in the "fieldsets" or
    # "fields" of the ModelAdmin. It is crucial that our custom field does not
    # use the same name as the m2m field field in the model ("contestants" in
    # our example).
    results = ResultsField()

    # Be sure to specify your model here.
    class Meta:
        model = Contest

    # Override init so we can populate the form field with the existing data.
    def __init__(self, *args, **kwargs):
        instance = kwargs.get('instance', None)
        # See if we are editing an existing Contest. If not, there is nothing
        # to be done.
        if instance and instance.pk:
            # Get a list of all the IDs of the contestants already specified
            # for this contest.
            contestants = ContestResults.objects.filter(contest=instance).order_by('rank').values_list('contestant_id', flat=True)
            # Make them into a comma-separated string, and put them in our
            # custom field.
            self.base_fields['results'].initial = ','.join(map(str, contestants))
            # Depending on how you've written your widget, you can pass things
            # like a list of available contestants to it here, if necessary.
        super(ContestAdminForm, self).__init__(*args, **kwargs)

    def save(self, *args, **kwargs):
        # This "commit" business complicates things somewhat. When true, it 
        # means that the model instance will actually be saved and all is
        # good. When false, save() returns an unsaved instance of the model.
        # When save() calls are made by the Django admin, commit is pretty
        # much invariably false, though I'm not sure why. This is a problem
        # because when creating a new Contest instance, it needs to have been
        # saved in the DB and have a PK, before we can create ContestResults.
        # Fortunately, all models have a built-in method called save_m2m()
        # which will always be executed after save(), and we can append our
        # ContestResults-creating code to the existing same_m2m() method.
        commit = kwargs.get('commit', True)
        # Save the Contest and get an instance of the saved model
        instance = super(ContestAdminForm, self).save(*args, **kwargs)
        # This is known as a lexical closure, which means that if we store
        # this function and execute it later on, it will execute in the same
        # context (i.e. it will have access to the current instance and self).
        def save_m2m():
            # This is really naive code and should be improved upon,
            # especially in terms of validation, but the basic gist is to make
            # the needed ContestResults. For now, we'll just delete any
            # existing ContestResults for this Contest and create them anew.
            ContestResults.objects.filter(contest=instance).delete()
            # Make a list of (rank, contestant ID) tuples from the comma-
            # -separated list of contestant IDs we get from the results field.
            formdata = enumerate(map(int, self.cleaned_data['results'].split(',')), 1)
            for rank, contestant in formdata:
                ContestResults.objects.create(contest=instance, contestant_id=contestant, rank=rank)
        if commit:
            # If we're committing (fat chance), simply run the closure.
            save_m2m()
        else:
            # Using a function concatenator, ensure our save_m2m closure is
            # called after the existing save_m2m function (which will be
            # called later on if commit is False).
            self.save_m2m = func_concat(self.save_m2m, save_m2m)
        # Return the instance like a good save() method.
        return instance

class ContestAdmin(admin.ModelAdmin):
    # The precious fieldsets.
    fieldsets = (
        ('Basic Info', {
            'fields': ('name', 'results',)
        }),)
    # Here's where we override our form
    form = ContestAdminForm

admin.site.register(Contest, ContestAdmin)

如果您想知道，我自己在我一直在从事的一个项目中遇到了这个问题，所以大部分代码都来自该项目。我希望你觉得它有用。