How to check the type of a many-to-many-field in django?

Question

How can you check the type of a many-to-many-field in django?

I wanted to do it this way:

import django  
field.__class__ == django.db.models.fields.related.ManyRelatedManager

This doesn't work, because the class ManyRelatedManager can't be found. But if i do field.__class__ the output is django.db.models.fields.related.ManyRelatedManager

Why does it refer to a class that doesn't seem to exist and how can i bring it to work?

Many thanks for your help.

Soapy

Answer 1

If you already have the field instance you can simply do:

if isinstance(field, ManyToManyField):
    pass // stuff

If you only have the related manager instance, you can reverse lookup the field instance:

>>> print fm
<class 'django.db.models.fields.related.ManyRelatedManager'>
>>> print fm.instance._meta.get_field_by_name('fieldnamehere')
(<django.db.models.fields.related.ForeignKey: fieldnamehere>, None, True, False)

This has only been tested on Django 1.5

Moonwort

This would be the perfect way, but i don't know how to get the field-instance of a many-to-many-field, that is located at the target-model.. if you could help me with this, i'll accecpt your answer :) – Soapy

I can list all the fields with object._meta.get_all_field_names() but the corresponding fieldname isn't accessible via object._meta.get_field('fieldname'). I can use the function object._meta.get_field_by_name('fieldname') which puts out (<RelatedObject: projectname:modelname1 related to modelname2>, None, False, True). Actually i don't know how to get the field-instance out of that.. – Soapy

got it :).. it is: object._meta.get_field_by_name("fieldname")[0].field – Soapy

Answer 2

You should be able to check it as a string.

field.__class__.__name__ == 'ManyRelatedManager'

Effusion

Thanks for the quick response! But isn't that a little bit undynamic? Actually, i don't think that the class "ManyRelatedManager" will be ever renamed, but isn't it a little bit sloppy? But however, i think I'll use this answer, becuase it is more performant than lazerscience's one. Thanks! – Soapy

Answer 3

(Using Django 1.11)

This question has become confusing, because the reported behavior is not the modern behavior for a related field. Example here, where JobTemplate is the model class, and credentials is a many-to-many related field:

>>> JobTemplate._meta.get_field('credentials').__class__
 django.db.models.fields.related.ManyToManyField

Is it different if we inspect the _meta of an object?

>>> JobTemplate.objects.first()._meta.get_field('credentials').__class__
django.db.models.fields.related.ManyToManyField

no.

So here's when I insert what I think is the most likely scenario of someone coming here. You have this:

>>> JobTemplate.objects.first().credentials
<django.db.models.fields.related_descriptors.ManyRelatedManager at 0x6f9b390>

Note, this is what the OP had.

I will stipulate that the related model is Credential. I can check if this is a related credential manager!

>>> isinstance(JobTemplate.objects.first().credentials, Credential.objects.__class__)
True

ManyToMany fields can be very hard to process, because the attribute kills itself and replaces itself with the manager sub-class. You could also cross-reference this information with the field obtained from get_field('credentials') to be extra sure. The above isinstance check could also erroneously pick up other managers that you have set on the model. However, this is still a valuable test to see if the attribute you have "quacks" how a ManyToMany field of that particular related model should.

Thomasthomasa

Answer 4

I'm not quite sure what you are trying to achieve, but i guess you should better look into your model's _meta attribute for determining the field class! check out _meta.fields and _meta.many_to_many!

You could do something like:

field_class = [f for f in yourmodel._meta.many_to_many if f.name=='yourfield'][0].__class__

Medlock

Answer 5

One of the scenario where I had to check for field type of Django model was when I was Trying to update or create a Django record and some data I am passing also require me to remove the m2m relationship data since django it does not support direct assignment for m2m relationship.

def extract_many_to_many_relationship(self, data:dict):
    m2m_fields = {}
    
    for field_name, value in list(data.items()):
        if isinstance(Person._meta.get_field(field_name), ManyToManyField):
            m2m_fields[field_name] = value
            data.pop(field_name)
    return m2m_fields

def set_many_to_many_relationship(self, m2m_fields:dict, instance):
    for field_name, value in m2m_fields.items():
        getattr(instance, field_name).set(value)


def create(self, data: dict):
    m2m_fields = extract_many_to_many_relationship(data)
    with transaction.atomic():
        instance = Person.objects.create(**data)
        set_many_to_many_relationship(m2m_fields, instance)
    return instance, None

With this snippet of code, i was able to check m2m relationship field remove and later set it back to the instance of the created field. This solution also works for the above case.

Note: Every field in Django model is an instance of another class. For example, Person._meta.get_field(field_name) returns the field in the model, and the field is an instance of ManyToMany relationship.

Pentha