Re: Mapping one to many

Ahh but groups dont know about there members. It it was implemented
the way that it has been suggested to me (a table for each users,
groups and mapping user to groups) you would end up with a many to
many relationship. The way I wanted to do it was just map user to
groups without mapping groups to users. In other words map one user
to many groups, the groups know nothing about the users.

On Jun 13, 2007, at 1:29 PM, Steve Lefevre wrote:

> I'm a little confused...
>
> Is it true that one member can belong to many groups, and that one
> group can have many members?
>
> On 6/13/07, Nabil < Nabil(at)kramer-smilko(dot)com> wrote:
> Kind of sort of no.... A group doest really need to know it members
> but users (members) need to know what groups they belong to. In
> other words your mapping one user to many groups. But it is also
> true that the two different users can map to the same group.
>
> On Jun 13, 2007, at 12:26 PM, Steve Lefevre wrote:
>
>> Nabil --
>>
>> Are you sure you need a one-many-mapping? If I'm reading between
>> the lines correctly, it sounds like you may need a many-to-many
>> relationship.
>>
>> Here's a question. Can one class have many students, and one
>> student have many classes? If so, you need a many-to-many
>> relationship.
>>
>> It would go something like this:
>>
>> students
>> -----------
>> id
>> first_name
>> last_name
>>
>> classes
>> -----------
>> id
>> class_name
>>
>> enrollment
>> -------------
>> student_id
>> class_id
>>
>>
>>
>> On 6/13/07, Nabil <Nabil(at)kramer-smilko(dot)com> wrote:
>> First of thanks Michael for clearing things up, I do appreciate the
>> help. But I must say... wow... umm.... ok... this is not the first
>> time, and not the last, that I'v had to hammer data that doesn't fit
>> into a row/column format into a row/column format. I't kinda funny
>> that there isn't a tree equivalent to a RMDB. I'm sure alot of people
>> have to map tree like data into a RMDB and end up creating extra
>> tables to do the one to many mapping. I know this isn't the best
>> place to post this rant but hasn't any one come up with a better
>> solution. Idk may be the is some wonderful magic in RMDBs that I seem
>> to be missing, if so will some one please enlighten me. This problem
>> isn't limited to just tree like data but to other data organization
>> formats too. I guess what I'm trying to say is that the storage
>> medium should fit the data, not hammer the data to fit storage, but
>> that seems to be what were doing with RMDBs.
>>
>> On Jun 13, 2007, at 11:49 AM, Michael Glaesemann wrote:
>>
>> >
>> > On Jun 13, 2007, at 10:05 , Nabil wrote:
>> >
>> >> Ok this is a very simple problem but for some reason I'm suffering
>> >> from brain freeze. I have two tables Users and Groups. A user can
>> >> be a member of many different groups.
>> >
>> >> What I was thinking of doing is creating a column called groups in
>> >> users of type int[] that contains the ids of the groups the user
>> >> is a member of. I want to make sure the group exists. The problem
>> >> is I cant have Users.groups reference Groups.id.
>> >
>> > Only use arrays for data types that are naturally arrays, i.e.,
>> > you're treating the array as a value rather than accessing
>> > individual elements of the array. As you've seen, relational
>> > databases are not at their best when operating on array elements:
>> > relational databases operate on tables, columns, and rows.
>> >
>> >> Is there some kind of check I can do?
>> >
>> > Not easily.
>> >
>> >> If so what would happen if I delete a group that has members
>> in it?
>> >
>> > Good question :)
>> >
>> >> One other way I though about was having a user_group_mapping table
>> >> so that would have something like user_id that references Users.id
>> >> and group_id that references Groups.id and when I want to figure
>> >> out what groups a user is a member of I would do "SELECT group_id
>> >> FROM user_group_mapping WHERE user_id=(the id I need)" but that
>> >> seems kind of messy.
>> >
>> > That's exactly how you *should* do it. It's a lot less messy than
>> > what you'll go through trying to do it using arrays. :)
>> >
>> > Michael Glaesemann
>> > grzm seespotcode net
>> >
>> >
>> >
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>> --
>> "Computers are useless. They can only give you answers"
>> -- Pablo Picasso
>
>
>
>
> --
> "Computers are useless. They can only give you answers"
> -- Pablo Picasso