Django使用“通过”关系表从模型中获取所有相关对象

Question

在我们的应用程序中，我们有几个关系和几个模型，我试图实现一种通用的方法来获取一个对象的所有相关对象，甚至是反向对象。

如果我从我的模型Pessoa 打印._meta.get_fields()，我会得到这些关系字段（我省略了“正常”字段）：

<ManyToManyRel: cadastroimoveis.pessoa>
<ManyToOneRel: cadastroimoveis.pessoa_pessoa>
<ManyToOneRel: cadastroimoveis.pessoa_pessoa>
<ManyToOneRel: cadastroimoveis.pessoa_itr>
<ManyToManyRel: cadastroimoveis.doc>
<ManyToOneRel: cadastroimoveis.doc_pessoa>
cadastroimoveis.Pessoa.relacoes
cadastroimoveis.Pessoa.itrs

此特定模型只有 M2M 关系，并且都包含指定 Here 的“直通”模型。

如您所见，它重复了它们，一个用于模型，一个用于“通过”中间表（我猜也是一个模型）。而在递归关系的情况下，它会重复两次。

我的问题是，有没有办法让这些不重复？

一种知道哪些重复字段最终“指向”相同关系的方法（即使它向两个表发送垃圾邮件）？因为如果直通表有字段，我想以不同的方式显示它们。

根据Model _meta API 文档，您可以使用它来获取所有相关对象：

[
    f for f in MyModel._meta.get_fields()
    if (f.one_to_many or f.one_to_one)
    and f.auto_created and not f.concrete
]

但是“通过”表不被认为是自动创建的并且是具体的。

示例：

<ManyToManyRel: cadastroimoveis.ccir>
<ManyToOneRel: cadastroimoveis.ccir_pessoa>

这两个字段'指向'相同的关系，一个是中间表，另一个是模型，有没有（自动）方法可以知道这两个是相关的？我找不到他们共享的任何属性。

这是因为当直通表有字段时，我需要对其进行编辑，而不是模型本身的 M2M 字段

Models.py : http://pastebin.com/szDfhHQ3 我尽我所能地清理了

Answer 1

例如，我们有这组模型。我是从this django example 收到的。

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

class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(
        Person,
        through='Membership',
        through_fields=('group', 'person'),
    )

class Membership(models.Model):
    group = models.ForeignKey(Group, on_delete=models.CASCADE)
    person = models.ForeignKey(Person, on_delete=models.CASCADE)
    inviter = models.ForeignKey(
        Person,
        on_delete=models.CASCADE,
        related_name="membership_invites",
    )
    invite_reason = models.CharField(max_length=64)

该解决方案看起来有点难看，但可以根据您的需要进行优化。

def get_through_field(f):                                                                                              
    opts = f.through._meta                                                                                             
    if f.through_fields:
        return opts.get_field(f.through_fields[1])                                                                     
    for field in opts.fields:                                                                                          
        rel = getattr(field, 'remote_field', None)                                                                     
        if rel and rel.model == f.model:                                                                               
            return field

model = models.Person

rels = dict(
    (f.field, f) for f in model._meta.get_fields()
    if f.is_relation
)

excludes = set()
for f in model._meta.get_fields():
    if f.many_to_many:
        through = get_through_field(f)
        excludes.add(rels[through])

for f in model._meta.get_fields():
    if f not in excludes:
        print f.name, f

输出：

group <ManyToManyRel: m.group>
membership_invites <ManyToOneRel: m.membership>
id m.Person.id
name m.Person.name

如您所见，没有membership 字段。

Answer 2

对于 Django 1.10，以下代码的灵感来自 BaseModelForm 代码（Django 原创）。

如果你有以下关系：

class Group(Model):
    field = ....

class Person(Model):
    groups = ManyToManyField(Group, through='Membership')

class Membership(Model):
    person = ForeignKey(Person)
    group = ForeignKey(Group)
    position = TextField(...)

然后可以这样查询相关的字段和属性：

opts = Person._meta
for f in chain(opts.many_to_many, opts.virtual_fields):
    if f.rel.through:
        # this would return "group"
        attr_field = f.m2m_reverse_field_name()

        # this is the Membership class (a class instance)
        m2m_model = f.rel.through

        # this would return "person"
        join_field = field.m2m_field_name()

        # to get all "Membership" objects for "person" personXY
        qs_filter = {join_field: personXY}
        qs = m2m_model.objects.filter(**qs_filter)

        # get the PKs of all groups where personXY is a member of
        lookup_by_pk = '{}__pk'.format(attr_field)
        current_pks = qs.values_list(lookup_by_pk, flat=True) if qs.exists() else []

Answer 3

其他答案肯定帮助我弄清楚了这一点，特别是在我的情况下，我所有的关系都是 M2M 并且有一个直通表，而且一切都是在 AJAX/Javascript 中完成的，所以我的答案非常 JSON-y。

目前它只能通过 m2m 模型的表获取所有内容，因为您必须在其中创建对象才能创建关系，但它可以轻松扩展为获取所有其他关系

def get_relationships(model):
    fields = list(model._meta.get_fields())

    m2m_fields = {}
    #Getting m2m relationships first
    for i, field in enumerate(fields):
        print(field)
        if field.is_relation:
            if field.many_to_many:
                fields.pop(i)
                try:
                    #If its a forward field, we want the relationship instead
                    if not hasattr(field,'field'):
                        field = field.remote_field
                except AttributeError:
                    pass
                if hasattr(field,'through'):
                    through = field.through
                    #In case of recursive relationships, there will be duplicates so we don't need to do it again
                    if m2m_fields.get(through._meta.model.__name__):
                        continue
                    m2m_fields[through._meta.model.__name__] = {}
                    m2m = m2m_fields[through._meta.model.__name__]
                    #Finding the models which participate in the through table and the direction
                    m2m['owner'] = {'model' : field.model.__name__}
                    m2m['related'] = {'model' : field.related_model.__name__}
                    recursive = False
                    #Checking recursivity, will use this later
                    #Finding field names for the foreignkeys of the through table
                    for through_field in through._meta.get_fields():
                        if not (through_field.related_model is None):
                            if m2m['owner']['model'] == through_field.related_model.__name__ and not m2m['owner'].get('field'):
                                m2m['owner']['field'] = through_field.name
                            elif m2m['related']['model'] == through_field.related_model.__name__ and not m2m['related'].get('field'):
                                m2m['related']['field'] = through_field.name
                        elif not through_field.primary_key:
                            if not m2m.get('rel_fields'):
                                m2m['rel_fields'] = []
                            m2m['rel_fields'].append(through_field.name)
    #Now removing the through tables from the fields list, because they appear as a regular ManyToOne relationship otherwise
    for through_table in m2m_fields.keys():
        name = through_table
        for i, field in enumerate(fields):
            if field.many_to_one:
                if field.__name__ and field.related_model:
                    if field.related_model.__name__ == name:
                        fields.pop(i)
    #Todo : OneToOne and ManyToOne relationships

    return m2m_fields


for key,value in get_relationships(Pessoa).items():
    print(key, " = ", value)

这是一个非常丑陋的代码，但我不太擅长 Python，只是想学习一些东西，但我保证它对我的问题很有吸引力