Re: Retrieve field values from ManyToMany field in model

Melvyn,

Thanks for your response. My original question was how to get the string names for the metadata and document out of the many to many table in the __str__ model method. I was able to accomplish this by the following

class DocumentMetaData(models.Model):

    document = models.ManyToManyField(Document)

    metadata = models.ManyToManyField(MetaData)

    metadatavalue = models.ManyToManyField(MetaDataValue)

    

    def __str__(self):

        title = self.document.all()[0].title

        metadata = self.metadata.all()[0].name

        return "%s - %s" % (title, metadata)

However, you raise a good question about my model design. This is the complete model design (with some of the model methods removed for clarity)

class MetaData(models.Model):

    metadata_id = models.AutoField(primary_key = True)

    name = models.CharField('metadata name', max_length=200)

    description = models.TextField('description')

    required = models.BooleanField(default=False)

          

class MetaDataValue(models.Model):

    metadata = models.ForeignKey(MetaData, on_delete=models.CASCADE,)

    value = models.CharField('value', max_length=200)

                

class Document(models.Model):

    document_id = models.AutoField(primary_key=True)

    document_type = models.ForeignKey(DocumentType, on_delete=models.CASCADE,)

    title = models.CharField('title', max_length=200)

    description = models.TextField('description')

    created_date = models.DateTimeField('date created', editable=False)

    storage_file_name = models.FileField('File name', upload_to=unique_file_path)

    original_file_name = models.CharField(editable=False, max_length=200)

    

class DocumentMetaData(models.Model):

    document = models.ManyToManyField(Document)

    metadata = models.ManyToManyField(MetaData)

    metadatavalue = models.ManyToManyField(MetaDataValue)

The problem I am having is that when I add data to the DocumentMetaData screen in the admin portal, the model does not maintain the relationship between the MetaData and the MetaDataName. For example, I can create the metadata entry People with values Mark, Sally, Sam, and another metadata entry of Events with values birthday, wedding, funeral. When I enter data into the DocumentMetaData, I am able to make an entry for events with the value Sally, which is wrong. So, my model is not really working as expected. 

What I want is to add metadata to documents, where metadata has one to many values. Many documents can have the metadata People with various values from the set of metadata People. Same with Events. But the Metadata Events can only have one or more of these values - birthday, wedding, funeral. 

Thanks for any help you can provide!

Mark

On Mon, Jun 26, 2017 at 12:19 AM, Melvyn Sopacua <m.r.sopacua@gmail.com> wrote:

On Sunday 25 June 2017 15:08:25 Mark Phillips wrote:

> I have this class

>

> class DocumentMetaData(models.Model):

> document = models.ManyToManyField(Document)

> metadata = models.ManyToManyField(MetaData)

> metadatavalue = models.ManyToManyField(MetaDataValue)

 

That design is wrong. This is the design you want:

 

class DocumentMetaData(models.Model):

name = models.CharField()

value = models.TextField()

 

class Document(models.Model):

title = models.CharField()

metadata = models.ManyToManyField(DocumentMetaData)

 

This design allows many metadata with name and value to be linked to a each document. I'm not entirely sure how to describe what your data design accomplishes and I'm guessing you can't either or it doesn't accomplish what you think it should.

--

Melvyn Sopacua

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