On Apr 6, 2008, at 11:53 AM, Daniel DeCovnick wrote:
I've got a situation where I'm about 99% sure I should use a one-to-
one relationship. I've got about 100 attributes that can be
logically divided into 14 different categories. A single entity
(Note) would then have one of each of these categories (if modeled
as separate entities). There's no reuse possible of instances of
the categories.
Problem: The Create Relationship dialog in Entity Modeler won't let
me create a one-to-one relationship.
Question: Does this mean you can't have one-to-one relationships?
And if you can, how do you set one up?
Thanks,
Daniel
I think you are right. Entity Modeler does not seem to give you a way
to create this in the UI. One can create a relationship that is to-
one in one direction and then to-many in the other direction and then
one can edit the resulting relationships to make it to-one and also
to-one in the reverse direction. There seem to be a bunch of warnings
in the Problems tab as one does this, but it should work.
FYI, I find that the way a one-to-one relationship makes sense is
that the join needs to be based on the primary keys of the entities.
The old EOModeler would complain if one set up a one-to-one in any
other way. If you are not wanting to join the two primary keys, you
may not really want a one-to-one relationship. I often find that I
have given myself some flexibility if I create the relationship as to-
one in one direction and to-many in the other and then, in the
application logic, I can decide how to make the to-many point to only
one object. One can re-write the accessors in the entity classes so
that one can forget that this really is not a true one-to-one. A real
one-to-one implies quite a lot about the structure of the schema. If
you have real one-to-one relationship, you might actually be wanting
to use a table with vertical entity inheritance. It is at least worth
thinking through the issues. If you are not so sure of the purity of
your schema, are you really sure you want a true one-to-one? There
are lots of situations where I want a one-to-one but then find
reality to be more messy than that.
One should respect one's primary keys and rely on them appropriately
but sparingly.
cheers - ray
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