Question

I think this may be more SQL than Django but Django is what I’m working in. What I am trying to do is to come up with a object model which can have many properties but is constrained to only 1 property type per object.

Say we have 3 property types:

• is_cool
• is_happy
• is_mean

Suppose I have an object (MyObject) which can have * (0-All) of these properties applied to it but only one of each.

So I think this is diagramed as follows (please correct me if I’m wrong):

In Django I am stuggling with this constraint. I want it at the db level i.e using unique_together.

Here is what I have..

PROP_VALUE_CHOICES = (("URL", "url"),
                      ("Boolean", "bool"),
                      ("String", "char"),
                      ("Person", "person"))

class PropertyType(models.Model):
    name = models.CharField(max_length=32)
    value_type = models.CharField(max_length=32, choices=PROP_VALUE_CHOICES)

class Property(models.Model):
    type = models.ForeignKey(PropertyType)
    value = models.CharField(max_length=32)

class MyObjectA(models.Model):
    properties = models.ManyToManyField(Property, related_name="MyObjectA")

class MyObjectB(models.Model):
    properties = models.ManyToManyField(Property, related_name="MyObjectB")

So the questions:

1. Is the above picture the correct way to document what I’m trying to accomplish.
2. My model is not complete – what am I missing and where do I apply the unique together constraint on the Object name and property type.

BTW – This is similar to this post but they used a through which I’m not sure I need??

Thanks!!

Answer 1

In case anyone really is looking for this answer…

Using Abstract Base Class I created the following structure which should work. Granted it no longer represents the picture completely but does solve the problem.

PROP_VALUE_CHOICES = (("URL", "url"),
                      ("Boolean", "bool"),
                      ("String", "char"),
                      ("Person", "person"))

class PropertyType(models.Model):
    name = models.CharField(max_length=32)
    value_type = models.CharField(max_length=32, choices=PROP_VALUE_CHOICES)

class Property(models.Model):
    type = models.ForeignKey(PropertyType, unique=True, related_name="%(app_label)s_%(class)s_related")
    value = models.CharField(max_length=32)

    class Meta:
        abstract = True

class ObjectAProperties(Property): pass

class ObjectA(models.Model):
    properties = models.ManyToManyField(Property, through="ObjectAProperties")

class ObjectBProperties(Property): pass

class ObjectB(models.Model):
    properties = models.ManyToManyField(Property, through="ObjectBProperties")

Posted in case I need this again in the future!