Rails - Jan 2006 - To Chris Hall - Re: DB Modelling the Rails way - Opinions??

Chris,
Finally got time to fully play with your suggestions. Had to re-code a 
bit of stuff but the concept works - just like a bought one :~). Many 
thanks for the solution,
Kind Regards,
Eric.

For those curious - here''s what it was about;

On Monday 02 January 2006 02:10, Eric Sloane tried to type something like:
 > Hi,
 > I''m trying to figure the most efficient way to model the
following. I
 > can think of at least two ways to relate the tables but from a
 > client/server perspective! I''m wondering how to best (and
 > elegantly)relate them from an AR perspective.
 >
 > A project has many people,
 > A person can work on many projects at any time,
 > A project has many roles,
 > A role is performed by a person,
 > A person may perform multiple roles,
 > An organisation has many people,
 > An organisation is a stakeholder (God, I hate that word - makes me feel
 > like Dracula surrounded!)in one or more projects,
 > A stakeholder has many roles within a project.
 >
 > So one way I have
 >
 > Projects HABTM Roles
 > Roles HABTM People
 > Organisation Has_Many People
 > An Organisation Belongs_to a Stakeholder
 > A Stakeholder HABTM Projects
 > A Stakeholder Has_Many Roles
 >
 > Or
 >
 > Projects HABTM People,
 > A Project has_many roles,
 > People HABTM Roles,
 > An Organisation has_many People,
 > An Organisation is a Stakeholder in a Project,
 > A Stakeholder has_many Roles in a Project.
 >
 > I guess the outcome I''m after is a way to view this data from
various
 > perspectives. For example, I have a project view that presents static
 > project data at the head of the screen with a set of tabs containing
 > partials with forms for editing stuff like e.g. People
 > Acting_For(Stakeholder), Acting_As (Role). Other perspectives would be
 > like seeing which organisations are doing what within any number of
 > projects - that sorta thing.
 >
 > Whatya think?
 > Eric.
 >

TO which Chris replied;


I might suggest the following (and I am making the asusmption that the 
relationships between projects, people and roles is unlimited

(untested)

people_projects_roles (join table between people and projects and roles)
----------
person_id
role_id
project_id

class Project < ActiveRecord::Base
   has_and_belongs_to_many :people, :join_table =>
"people_projects_roles"
   has_and_belongs_to_many :roles, :join_table =>
"people_projects_roles"
end

class Person < ActiveRecord::Base
   has_and_belongs_to_many :projects, :join_table =>
"people_projects_roles"
   has_and_belongs_to_many :roles, :join_table =>
"people_projects_roles"
end

class Role < ActiveRecord::Base
   has_and_belongs_to_many :people, :join_table =>
"people_projects_roles"
   has_and_belongs_to_many :projects, :join_table =>
"people_projects_roles"
end

now, this allows you to do things such as

project = Project.find(1) # "my project"

# all people who are associated with "my project" (any role)
project.people

# all roles associated with "my project"
project.roles

person = Person.find(1) # "John Smith"

# all projects assocated with "John Smith"
person.projects

# all roles assocated with "John Smith"
person.roles

role = Role.find(1) # "programmer"

# all projects with a "programmer" role
role.projects

# all people with a "progammer" role
role.people


now say you want to add John as a "manager" (id = 2) role to Project
10

john = Person.find_by_name("John")
manager = Role.find_by_name("Manager")
project.find(10)

with this information, you could do it several different 
ways...depending on the situation

project.people.push_with_attributes(john, :role_id => manager.id)
project.roles.push_with_attributes(manager, :person_id => john.id)
john.projects.push_with_attributes(project, :role_id => manager.id)
john.roles.push_with_attributes(manager, :project_id => project.id)
role.projects.push_with_attributes(project, :person_id => john.id)
role.people.push_with_attributes(john, :project_id => project.id)

each of these accomplish the same thing, they add John as a Manager to 
Project 10

now, as far as organisation/projects/roles go, that sounds strange (not 
being critical)...can an organisation have the same roles as a person? 
i would assume that organisations have different roles than people so 
you will want to setup a separate "org roles" table to manage those.

can an organisation be involved in many projects and can a project have 
many organisations (stakeholders)?  if so, then i would setup another 
join table between organisations/projects/org roles and follow the same 
idea as above

now, one thing i would be concerned about is corss referencing (not sure 
the proper term).  you have people associated with projects, 
organisations assocated with projects and people associated with 
organisations...this can get messy when you want to start limiting who 
can do based upon their other associations (ie, given a 
project/organisation association, can only people associated with the 
same organisation be assocated with that project?)

hope this helps.

Not to get all semantic or anything, but what do you mean when you say a 
"project has many roles"?

I''ve always considered a role to be an attribute of a relationship, not
a thing.

-- 
Posted via http://www.ruby-forum.com/.

Kevin Olbrich wrote:
> Not to get all semantic or anything, but what do you mean when you say a 
> "project has many roles"?
> 
> I''ve always considered a role to be an attribute of a
relationship, not
> a thing.
> 
Yeah - And, your point is? It''s expressed this way for flexibility

Eric Sloane wrote:
> Kevin Olbrich wrote:
>> Not to get all semantic or anything, but what do you mean when you say
a
>> "project has many roles"?
>> 
>> I''ve always considered a role to be an attribute of a
relationship, not
>> a thing.
>> 
> Yeah - And, your point is? It''s expressed this way for flexibility
I didn''t have a point, I had a question.  I have no doubt there is 
reason for that setup, I just didn''t see the logic.

-- 
Posted via http://www.ruby-forum.com/.

Kevin,
I had a complex relationship to build between Projects, Roles and 
People. I wanted maximum flexibility in addressing each, any or all of 
those and I wanted to have uncluttered schema entities. So forgetting 
attributes and entities for the moment the solution proposed by Chris 
gave me that flexibility in terms of associations. Hey, I''m finding my 
way here, so I''ve put previous experience with C/S design on hold 
temporarily. Actually, I started down this path with a full blown BUD 
schema painstakingly drawn in DBDesigner - it no longer exists!
Cheers,
Eric.
PS didn''t mean to bite.

Kevin Olbrich wrote:

> Eric Sloane wrote:
> 
>>Kevin Olbrich wrote:
>>
>>>Not to get all semantic or anything, but what do you mean when you
say a
>>>"project has many roles"?
>>>
>>>I''ve always considered a role to be an attribute of a
relationship, not
>>>a thing.
>>>
>>
>>Yeah - And, your point is? It''s expressed this way for
flexibility
> 
> 
> I didn''t have a point, I had a question.  I have no doubt there is
> reason for that setup, I just didn''t see the logic.
>