models for dynamic attributes

attribute`s models

# models
from django import models
from django.utils.translation import ugettext_lazy as _


class RowAttribute(models.Model):
    """Product/look row attribute model."""
    product = models.ForeignKey(
        'ModelWithAttribute', verbose_name=_('Product'),
        on_delete=models.CASCADE, null=True,
        blank=True, related_name='row_attribute'
    )
    attribute = models.ForeignKey(
        'Attribute', on_delete=models.CASCADE,
        verbose_name=_('Attribute'),
        related_name='row_attribute'
    )
    value = models.ForeignKey(
        'AttributeValue', on_delete=models.CASCADE,
        verbose_name=_('Value'),
        related_name='row_attribute',
        null=True, blank=True
    )

    class Meta:
        verbose_name = _('Row attribute')
        verbose_name_plural = _('Row attributes')

    def __str__(self) -> str:
        return self.attribute.name


class Attribute(models.Model):
    """Attribute model"""
    name = models.CharField(verbose_name=_('Name'), max_length=255)
    
    # some more relatied stuff
    
    class Meta:
        verbose_name = _('Attribute')
        verbose_name_plural = _('Attributes')

    def __str__(self) -> str:
        return self.name


class AttributeValue(models.Model):
    """Attribute variation model"""
    value = models.CharField(verbose_name=_('Attribute value'), max_length=255)
    attr = models.ForeignKey(
        Attribute, verbose_name=_('Attribute'),
        on_delete=models.CASCADE,
        related_name='value'
    )

    class Meta:
        verbose_name = _('Attribute variation')
        verbose_name_plural = _('Attribute variations')

    def __str__(self) -> str:
        return f'{self.attr.name} - {self.value}'

# admin 

class RowAttributeForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        super(RowAttributeForm, self).__init__(*args, **kwargs)
        # fix value when instance already exists 
        if self.initial:
            self.fields['attribute'].disabled = True
            self.fields['value'].queryset = AttributeValue.objects.filter(
                attribute=self.initial.attribute
            )


class RowAttributeInline(admin.StackedInline):
    form = RowAttributeForm
    model = RowAttribute
    exotra = 0
    
    def get_fields(self, request, obj=None):
        fields = super().get_fields(request, obj)
        
        if not obj:
            fields.remove('value')
        
        return fields