Hi Poe,
You have just implemented relational many to many relation.
But this design is perfectly fine but just think if you need one to many
relation only then in your case you will be writing 2 entities while u could
just write one by saving catehoryGroup id in category it self.
And when u want to read category and catehorygroup, u need to read three
entities while u could read just two.
So for one to many relations I will use unowned relation and save some write
and read :).
GAE doesn't say u can't think relational :).
Thanks
Ravi
Sent from my iPhone
On 7 Jul 2010, at 15:18, poe <stefan.poe...@googlemail.com> wrote:
> I've got a question regarding this.
>
> Isn't it kind of "nicer" regarding the design to not store relational
> Attributes in a core table/class. In the given example I would
> implement this like this:
>
>
> @PersistenceCapable
> public class Categories
> {
> @PrimaryKey
> @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
> private Long Id;
>
> No owned or unowner object here
> ...
> }
>
> @PersistenceCapable
> public class CategoryGroups
> {
> @PrimaryKey
> @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
> private Long Id;
> ...
> }
>
> @PersistenceCapable
> public class CategoryGroupRelation {
>
>
> @PrimaryKey
> @Persistent(mappedBy="category")
> private Long categoryId;
>
> @Persistent(mappedBy="categorygroups"
> private Long categoryGroupId;
>
> }
>
> Isn't it better to design the tables this way? I don't like relational
> attributes in a core database class. What do you think?
>
> Greets
> Poe
>
>
> On 7 Jul., 01:25, AC <acol...@gmail.com> wrote:
>> Thanks. That answers my question.
>>
>> On Jul 6, 3:18 pm, Ravi Sharma <ping2r...@gmail.com> wrote:
>>
>>
>>
>>> You need to understand the concept of Entity group here, and need to
>>> understand the owned relation and unowned relation.
>>> Think of owned and unowned relation like this
>>> You(your house) and your TV has owned relationship, your house has TV, no ne
>>> can see it from outside, and if your friend want the same tv he need to buy
>>> the same TV and own it. And at one point if you ask who all watching my TV
>>> then it will be just you not your friend as he has his own TV(although same
>>> kind of TV).
>>
>>> But You and your favourite movie theatre has unowned relationship, you dont
>>> own it, anyone who knows the address of theatre can go and watch movie. and
>>> at one moment 1000s of people might be watching the same movie/theatre
>>
>>> Back to your question.....
>>
>>> Categories(3)/CategoryGroups(4) is an example for owned relation.
>>> CategoryGroup(4) will be available to Category(3) only and if you create
>>> another category Cateory(10) with same category group it will just create
>>> another version of categoryGroup CategoryGroup(11) in that category. But you
>>> may see that CategoryGroup 11 and CategoryGroup 4 both are Entertainment.
>>
>>> In your case many categories can have same categoryGroup.So instead of using
>>> owned relationship, you should use unowned relationship.
>>
>>> Instead of this
>>> @Persistent
>>> private CategoryGroups Group;
>>
>>> define this
>>> @Persistent
>>> private Key categoryGroupId;
>>
>>> and save key of CategoryGroup rather then full object. (Dont buy a TV for
>>> every other rmovie you want to watch, just buy a movie theatre ticket go
>>> there and use that :) )
>>
>>> I hope u get the idea, you can learn more from here....
>>
>>> http://code.google.com/appengine/docs/java/datastore/relationships.html
>>
>>> spare me if you dont like my example... :)
>>
>>> Ravi.
>>
>>> On Tue, Jul 6, 2010 at 5:47 PM, AC <acol...@gmail.com> wrote:
>>>> I just started fooling around w/ GAE this weekend.
>>
>>>> I have a Categories class and a CategoryGroup class. The idea is that
>>>> every category must be grouped. For example the CategoryGroup
>>>> "Entertainment" can be assigned to many Categories, such as "movies",
>>>> "music" and "television".
>>
>>>> Here are my classes.
>>
>>>> @PersistenceCapable
>>>> public class Categories
>>>> {
>>>> @PrimaryKey
>>>> @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
>>>> private Key Id;
>>
>>>> @Persistent
>>>> private CategoryGroups Group;
>>
>>>> @Persistent
>>>> private boolean IsDeleted;
>>
>>>> @Persistent
>>>> private String Name;
>>
>>>> @Persistent
>>>> private Date CreateDate;
>>
>>>> @Persistent
>>>> @Column(allowsNull="true")
>>>> private Date ModifiedDate;
>>>> }
>>
>>>> @PersistenceCapable
>>>> public class CategoryGroups
>>>> {
>>>> @PrimaryKey
>>>> @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
>>>> private Key Id;
>>
>>>> @Persistent
>>>> boolean IsDeleted;
>>
>>>> @Persistent
>>>> String Name;
>>
>>>> @Persistent
>>>> Date CreateDate;
>>
>>>> @Persistent
>>>> @Column(allowsNull="true")
>>>> Date ModifiedDate;
>>>> }
>>
>>>> First I add a record to CategoryGroups. When I query all, the reult
>>>> is :
>>
>>>> CategoryGroups(2)
>>
>>>> Next, I add a record to Categories. When I query all CategoryGroups,
>>>> the result is :
>>
>>>> Categories(3)/CategoryGroups(4)
>>>> CategoryGroups(2)
>>
>>>> I do not doubt that this works as it was designed to, however, coming
>>>> from a RDBMS background, this is very confusing to me. The screen I
>>>> created to manage Categories now has "Entertainment" listed twice in
>>>> the CategoryGroups select box. Is there a better approach to achieve
>>>> my goal?
>>
>>>> Any advice is much appreciated.
>>
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