I'm trying to design a CoreData model that will allow my object to
have a mutable set of associated attributes. My goal is to create a
generic database record that will allow users to add their own
attributes to. But I'm having a hard time with the design. In
particular I'm having a hard time figuring out how to design things
so that it's possible to query multiple attributes with a single
NSPredicate.
My design consists of an Entry managed object that has a relationship
to multiple EntryAttribute objects. Each EntryAttribute has a string
"key" (the name of the attribute) and a "value". Values are typed, so
there is actually a hierchy of EntryAttributes entities. An abstract
one that defines the "key" attribute. And concrete subclass entities
for each value type. For example there is a EntryStringAttribute for
subclass who's value is of type string and an EntryInteger16Attribute
subclass who's value is of type integer16. A screenshot of this
design is here:
With this design the user can create an Entry (generic record) and
then define two attributes for that entry; a string "name" attribute
and a integer "age" attribute. The problem is I can't find a good way
to query across both attributes at the same time. For instance I'm
not sure how to formulate a query (name == "joe" and age > 25) that
will allow me to find the matching records.
So what's the best coredata design to support these features?
Querying across attributes doesn't seem to work in this case, I think
because of the fact that there is a hierarchy of EntryAttributes and
the "value" of those different subclasses will have different types.
It might be that I'm just not forming the query correctly, so please
let me know if I'm wrong and querying across attributes should be
possible in this case.
Thanks for any help.
Jesse
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