Optionally, you could use a reflexive relationship for this. I've
used them in the past for this type of situation and it seemed to
work out well. This would allow you to have an arbitrarily deep tree
structure using a single database table and single EO.
I can't think of any particular disadvantages to this type of
relationship, and the structure is very simple. You create a one-to-
many relationship that references a different row in the same table.
The only caveat I'm aware of is to watch out for recursive associations.
I've built similar database structures for bills of material
(although they are more complex than this, and involve a many-to-many
reflexive relationship). Basically you just have to make sure that
any "root" node is never used anywhere within that particular branch
of the tree. This can usually be accomplished though the UI, but it's
best to recursively validate the branch of the tree on save.
------------------------------------------------------------------------
---------
Location
------------------------------------------------------------------------
---------
id
name
type (Country, Region, State, Metro Area, County, or City)
parentID
------------------------------------------------------------------------
---------
parent (to-one)
children (to-many)
------------------------------------------------------------------------
---------
United States (Country)
--> South East (Region)
----> Alabama (State)
----> Arkansas (State)
----> Georgia (State)
------> Atlanta (Metro Area)
--------> Fulton (County)
----------> Atlanta (City)
----> ....
--> North West (Region)
----> Washington (State)
----> Oregon (State)
----> Montana (State)
Watch out for infinite recursion problems as illustrated below:
United States (Country)
--> South East (Region)
----> Alabama (State)
------> United States (Country) <------- Causes infinite recursion
--------> South East (Region)
----------> Alabama (State)
------------> United States (Country) <------ As is obvious here
since you never find leaf nodes
On Apr 22, 2008, at 8:33 AM, Mr. Frank Cobia wrote:
Thanks. I will look into this and see if I can make it work.
Frank
On Apr 21, 2008, at 3:45 PM, Chuck Hill wrote:
On Apr 20, 2008, at 6:15 PM, Mr. Frank Cobia wrote:
I have a database design question. I am posting it to the
WebObjects list because I am building a WebObjects app and want a
solution that works well for EOF.
I have a table of data that has to have a location associated
with it. Unfortunately the level of the location is not constant.
The data can be associated with a Country, Region, State, Metro
Area, County or City. Each of those levels is its own table with
relationships pointing to the related locations. i.e. a State
knows which Region it belongs to and which Metro Areas belong to it.
I have been unable to come up with a design that seems elegant. I
have thought of having 6 separate relationships to each of the
location levels, but it seems to duplicate data and the data
could get out of sync if a state is moved to a different region
or a City is moved to a different county.
Has anyone had a situation like this and come up with a good
solution?
This seems like a good place for inheritance with an abstract EO
of Location with concrete subclasses of Country, Region, State,
Metro Area, County or City. For some operations, having them in
separate tables might cause more fetches than you want. Using
single table inheritance would address that.
Chuck
--
Practical WebObjects - for developers who want to increase their
overall knowledge of WebObjects or who are trying to solve
specific problems.
http://www.global-village.net/products/practical_webobjects
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