On Fri, 8 Jun 2007, Daniel McBrearty wrote:
as my app is developing I'm seeing more and more that having
flexibility in the db layer is one of the things that saves a lot of
stress and hassle.
something that happens a lot is that you get the basic objects and
relationships between them correct, but don't know all the attributes
that they will need until later. so (thanks Alan Humphrey for this)
there is a way (that is likely not new to many people here) ... :
TABLE my_object {
id INTEGER PRIMARY KEY,
.
. // whatever
.
};
TABLE my_object_attributes {
id INTEGER PRIMARY KEY,
name TEXT
}
TABLE my_object_attributes {
my_object_id REFERENCES my_object(id),
attribute_id REFERENCES my_object_attributes(id)
text_value TEXT,
number_value NUMBER
}
so now you have a way to add a new attribute to the object, and have
any instance then have a text/number value for that.
You can also write some API functions in the db layer to
create/assign/delete attributes, taking table name as an arg. So at
the cost of two tables for the object, you move a lot of design
decisions downstream.
Comments? horrendous hackery? laziness? a landmine? ok in some cases?
Been there, done that, bought the T-Shirt.
Pros: Flexibility, no need to design fully up-front, less work..
Cons: No constraints, no types or extra cpde to check types/number/text
fields, which normally the db would do for you, no default values per
attribute "type", no nullable check.. etc..
If you take this all the way, you're designing a db within the db, without
all the benefits of its built-in checking/optimising.
That's not to say it can't be useful, it can, but you should make sure you
know/define the "rules" for the type/subset of data you are storing this
way, and don't overstep them.
Jess
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