How to Query for Many to Many Fields in Django - django

Question

I have these classes -
class DocumentType(models.Model):
type_id = models.AutoField(primary_key=True)
name = models.CharField('type name', max_length=200)
class MetaData(models.Model):
metadata_id = models.AutoField(primary_key = True)
name = models.CharField('metadata name', max_length=200, unique=True)
description = models.TextField('description')
class DocumentTypeMetaData(models.Model):
documentType_id = models.ManyToManyField(DocumentType,)
metadata_id = models.ManyToManyField(MetaData,)
required = models.BooleanField(default=False)
For example, a DocumentType value of 'Photo' would have Required Metadata of 'Decade' and 'Orientation'.
In the DocumentTypeMetaData class I would like to have a def __str__(self) function that returns something like the following in the admin page -
Photo: (Decade, Photo Type) required
The format is not critical, I just want to know which metadata is required. Currently, all that is displayed is
DocumentTypeMetaData object
on the admin page.
I am struggling with how to write the queries for this function.
Thanks!
Mark

Answer 1

These relationships are not right. The many-to-many relationship is between DocumentType and MetaData; DocumentTypeMetaData is the through table. So:
class DocumentType(models.Model):
type_id = models.AutoField(primary_key=True)
name = models.CharField('type name', max_length=200)
metadata = models.ManyToManyField('MetaData', through='DocumentTypeMetaData')
class MetaData(models.Model):
metadata_id = models.AutoField(primary_key = True)
name = models.CharField('metadata name', max_length=200, unique=True)
description = models.TextField('description')
class DocumentTypeMetaData(models.Model):
document_type = models.ForeignKey(DocumentType)
metadata = models.ForeignKey(MetaData)
required = models.BooleanField(default=False)
def __str__(self):
return '{} {} {}'.format(self.document_type, self.metadata, self.required)