[Seaside] How do you handle constraints on persisted collections?
mike at sharedlogic.ca
Tue May 19 21:59:18 UTC 2009
On Mon, May 18, 2009 at 6:50 PM, Pat Maddox <pat.maddox at gmail.com> wrote:
> I'm trying to make the leap from RDBMS/ORM thinking to pure objects,
> and am running into some problems. The first is how to handle
> constraints on collections, when the objects inside of that collection
> are mutable. Quick example, ensuring that only one user exists with a
> certain username. It's trivial to check that a username is available
> before adding the user to a UserRepository. If a user can update his
> own username though, it'll bypass any check on the UserRepository and
> invalidate that constraint. A couple options...
First, expressing the constraint itself, to which you alluded. In
loose Relational Model parlance you're talking about a table
constraint, specifically, a key constraint. From the OO perspective,
I think of these as constraints on the class. However, I implement
them as instance methods to allow reference of the instance itself
and, thus, use of a single method for both insert and update checks.
| count |
count := self repository count: [ :each | each name = aString and
each ~= self ].
(count = 0) ifFalse: [ self error: 'Constraint violation: ',
aString, ' is not unique.' ]
Second, where to send the constraint check message. For updates, I
just send it as a guard message in every accessor that mutates the
variable in question. For inserts, it depends on how intimately you
tie instantiation and insertion. #checkUniqueName above works
correctly whether the instance is already in the repository or not.
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