which table structure would be better?

I cant decide which way is better to create a key/value Tag model in Django,

class Tag(models.Model):
    tag = models.CharField(max_length=35, unique=True)
    description = models.CharField(max_length=250, null=True)
    is_key = models.BooleanField(default=False)
    parent = models.ForeignKey("self", blank=True, related_name="key")

or

class TagKey(models.Model):
    key = models.CharField(max_length=35, unique=True)
    description = models.CharField(max_length=250, null=True)    

class TagValue(models.Model):
    value = models.CharField(max_length=35, unique=True)
    description = models.CharField(max_length=250, null=True)
    port = models.PositiveIntegerField(default=0)
    key = models.ForeignKey(TagKey)

All I intent to do is create a key:value based tag model which I can use to tag my applications.

Explanation: What I am doing in Tag class is that I am giving a self relation to itself when I will be adding a value tag, but if I am adding a key type Tag then I wont be populating the parent field.

P.S I have to use my own tag model so please don't suggest third party Django tag app