Re: Organization hierarchy

I understand the *preoccupations* (forgive my limited english vocabulary) regarding the standard parent_id case, which indeed would cover 9 out of 10 use cases.
What we came up with in some of the projects is de-normalizing the database, and it's pretty much the solution you're proposing.

So, for example, the Organization model would still have a parent (or parent_id) attribute, that I would call current_parent, for clarity.
The JSON serialization explicitly would contain both an array of parents and children, with start and end dates, extracted from the external Relation model.
The current_parent would usually be the last element of the parents list, and it must have a Null end_date.

This would allow to represent time-dependend father-child compositions.


An example (pseudo-python) for an organization with parentships changing dynamically over time:

Organization
 { id: ID,
   current_parent_id: PID3,
   parents: 
   [
     { id: PID1, start_date: '2006/07', end_date: '2008/09/01' },
     { id: PID2, start_date: '2008/09/02', end_date: '2012/04' },
     { id: PID3, start_date: '2012/04', end_date: NULL },
   ],
   childresn: []
 }
    



As for N-N aggregations, it's a very rare use-case, in the institutional context we're focusing on, I can only think of 
a rather stretched example.

Since in Italy there is a minimum number of MPs required to form a group (in both chambers of the parliament), 
we have a so called mixed group, with members from various small (usually regional) electoral parties.
Now, from time to time, an MP exits from a big group and enters into the mixed group, usually a few months before
passing into another different big group alltogether, just in order to disguise the actual flip.

If I want to know the composition of the mixed group at any given time, in terms of electoral parties, a single party could easily be into two groups.
The electoral party as an organization exists independently of the parliament groups.

Of course I could just count the memberships and obtain the same result, but I was just trying to make an example.
In other contexts these situation could arise more frequently.

I would agree in considering aggregation issue a minor one.
Given the focus and context of the popolo project, it could be left out of the specs.


Guglielmo



Il giorno 04/set/2013, alle ore 00:46, James McKinney <james@opennorth.ca> ha scritto:

> Hi Guglielmo,
> 
> For your second question about aggregations (N-N relations between organizations), can you give an example from your work where this is the case?
> 
> For the first question: indeed, there is an issue in the tracker: https://github.com/opennorth/popolo-spec/issues/27 Very few existing standards handle changes over time, so we will likely have to come up with our own solution like the one you suggest.
> 
> The relation you propose would work. It's actually very similar to a Membership in Popolo. (Perhaps an eventual solution would have a Relation superclass with your new class and Membership as subclasses.)
> 
> The challenge when dealing with historical use cases is to make sure that the common use cases are still easy to implement. Here, a common use case is to represent the *current* organizational hierarchy/tree/graph. There already exist many treelibraries in various languages for storing tree structures in databases. Most of these have no method of tracking changes over time, and use a single field like "parent_id" to track the tree structure. An ideal solution to the challenge would allow people to continue to use such libraries.
> 
> Perhaps a solution would be to maintain "parent_id" and "parent" as-is, and to add a new "parents" property, whose value is an array of Relation objects? Implementations can then choose whether to implement either "parent_id" or "parents" or both.
> 
> Depending on how the aggregations issue is resolved, it may make sense to encourage the use of "parents" only.
> 
> Would anyone be against eliminating parent_id, if that were part of a solution?
> 
> James
> 
> On 2013-09-03, at 9:38 AM, Guglielmo Celata wrote:
> 
>> James,
>> the Popolo protocol currently allows hierarchical relations between organizations to be mapped through the parent_id attribute.
>> 
>> One possible shortcoming is that this is a permanent relation (it has no start nor end dates), and sometimes, especially in political groups, relations do depend on time.
>> 
>> Another lesser shortcoming is it maps compositions (a group, or a big company and its departments), but leaves out aggregations (members can join more than one group).
>> 
>> In a relational world, I would map it with an external entity:
>> 
>>     ------------
>>   1|            |N
>>  -----        -----
>> | Org |------| Rel |
>>  ----- 1    N -----
>> 
>> Where Rel is the relation and it would have these fields:
>> id
>> from_id
>> to_id
>> start_date
>> end_date
>> 
>> from_id and to_id are references to the Org, organizaiton entities.
>> 
>> Don't know how it would translate into the protocol and if the complexity it introduces are worth the issues it tries to solve. 
>> 
>> 
>> Any ideas?
>> 
>> Guglielmo Celata
>> Developer
>> Associazione Openpolis
>> 
>