Just a matter of taste or the need of a formal approach?

Rails
1

Hi,

thanks for reading this post. At the moment, I am thinking about the
following problem:

Let’s assume the following situation:

I’ve got two entity types called “User” and “Group”. Each entity type
got some attributes, let’s think about “name” and “password” for
“User” and “name” as well as “type” for “Group”. Those attributes are
not really essential for my problem description.

So now we have:

User

  • name
  • password

Group

  • name
  • type

We can also create relations between those entities, for example:
membership(Gx, Uy), which tells us that user y is a member of group x.
As all users can join multiple groups, we could just create an
intermediate join table to model this for our relational database, so
far, that’s all right, there are no other reasonable solutions, as far
as I know.

Now, let’s assume you could dynamically add an arbitrary number of
attributes, which can of course also be just foreign keys, to each
entity and that this is also supported by your database system. This
is the case, I am dealing with and it brings in some new thoughts
about modeling the relation between “User” and “Group”.

If we look at the relation “membership” again, you could also think:
Well, somehow, “member” could be an attribute of “Group”, so we would
say:

Group

  • name
  • type
  • member_fk_1 (or user_fk, that’s not essential here)
  • member_fk_2 (also the naming is not essential)
    …

And you could say, well: the fact, that a user is a member of a group,
could also be represented by further “User” attributes:

User

  • name
  • password
  • group_fk_1 (the naming again is not essential here)
  • group_fk_2
    …

The fact, that you can add as many fields as you want, makes both
solutions possible. In fact, only one entity should save the
membership information, as we are following the dry principle.

What to do now? How would you argue? Should I use a join model anyhow
or should I really decide to model the membership in the second way?
Maybe: Is there a formal approach to decide this?

There are other examples, where I have got to model some kind of
composition: in this case I have got entities which are components of
other entities, which only can exist through out these associations.
In this case, it is somehow more intuitive to model the foreign keys
directly as attributes not using a join model.

Thanks for you suggestions!

Cheers,
ms

2

Investigate polymorphic join relationships… you can join arbitrary
numbers of other ‘things’ without adding foreign_keys to your base table
(User). A new ‘thing’ just has to participate in the poly join
relationship.

Requirement (these can be related to any number of other entities,
depending on how that project team manages their data)
has_many :reqlinks, :dependent => :destroy
has_many :unittests, :through => :reqlinks, :source => :unittest,
:conditions => “reqlinks.reqlinkable_type = ‘unittest’”
has_many :scenarios, :through => :reqlinks, :source => :scenario,
:conditions => “reqlinks.reqlinkable_type = ‘scenario’”
has_many :projects, :through => :reqlinks, :source => :project,
:conditions => “reqlinks.reqlinkable_type = ‘project’”

Reqlink (the polymorphic join table)
belongs_to :requirement
belongs_to :reqlinkable, :polymorphic => true
belongs_to :unittest, :class_name => ‘Unittest’, :foreign_key =>
‘reqlinkable_id’
belongs_to :scenario, :class_name => ‘Scenario’, :foreign_key =>
‘reqlinkable_id’
belongs_to :project, :class_name => ‘Project’, :foreign_key =>
‘reqlinkable_id’

Project
has_many :reqlinks, :as => :reqlinkable, :dependent => :destroy
has_many :requirements, :through => :reqlinks

Scenario
has_many :reqlinks, :as => :reqlinkable, :dependent => :destroy
has_many :requirements, :through => :reqlinks

Unittest
has_many :reqlinks, :as => :reqlinkable, :dependent => :destroy
has_many :requirements, :through => :reqlinks

3

ms wrote:
[…]

What to do now? How would you argue? Should I use a join model anyhow
or should I really decide to model the membership in the second way?

You should use a junction table and habtm or has_many :through.
Repeating fields may seem more intuitively appealing, but they are a
terrible DB design technique. Please read some articles on DB
normalization (the ones on Wikipedia are excellent), and feel free to
ask if you have any further questions. Generally speaking, most
applications should have their tables at least in Third Normal Form
(3NF).

Maybe: Is there a formal approach to decide this?

Yes. Never use repeating fields. :slight_smile:

There are other examples, where I have got to model some kind of
composition: in this case I have got entities which are components of
other entities, which only can exist through out these associations.

Can you post an example?

In this case, it is somehow more intuitive to model the foreign keys
directly as attributes not using a join model.

But the intuitive solution is not always the best (although it’s often
a good place to start).

Thanks for you suggestions!

Cheers,
ms

Best,

Marnen Laibow-Koser
http://www.marnen.org
[email protected]