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


#1

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 S. 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 S.”

all projects assocated with “John S.”

person.projects

all roles assocated with “John S.”

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.


#2

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.


#3

Kevin O. 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


#4

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.


#5

Eric S. wrote:

Kevin O. 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.