Relationship Advice - who do I belong to & can I have many?

Rails
1

I am trying to figure out the has / belongs_to etc relationship for
rails but everyone always uses very abstract examples. Could someone
fill in the blanks on the following example I came up with over lunch?

In my actual program I never bothered filling out these relationships
but thought it might be a good idea to fix that. Do you always fill
these out and what sort of benifits do you get from it?

Say we have a school with students, teachers, classes and books.

A teacher can teach 1 to 3 classes
A student takes 6 of 10 offered classes
Each class has exactly 1 unique book
[A student:
Takes some but not all classes,
Has different teachers, but may have the same teacher multiple times
Has one book per class or 6 books]

The students could print out their booklist, schedule. The teacher
could print out a list of their classes and the students in them. etc.

A rough idea for the tables might look like so

Teachers
id, name…

Student
id, name…

Class
id, description…

schedule
id, teacher_id, student_id, class_id

Book
id, class_id

How would the models look? Here is what I came up with

Teacher
belongs_to :schedule

Student
belongs_to :schedule

Class
belongs_to :schedule
has_one :book

Schedule
has_many :teacher
has_many :student
has_many :class

Book
belongs_to :class

2

On Thu, 19 Oct 2006, Mic wrote:

I am trying to figure out the has / belongs_to etc relationship for
rails but everyone always uses very abstract examples. Could someone
fill in the blanks on the following example I came up with over lunch?

Not your example, but this will probably help…

http://mboffin.com/stuff/ruby-on-rails-data-relationships.png

3

That was just the sort of thing I was looking for!

Philip H. wrote:

Not your example, but this will probably help…

http://mboffin.com/stuff/ruby-on-rails-data-relationships.png

4

Mic wrote:

Relationship Advice - who do I belong to & can I have many?

First off, I should point out that ‘belong to’ is a strong term. In any
good relationship, there should certainly be a sense of belonging
towards each other, but you shouldn’t ever feel that you merely ‘belong
to’ somebody, as if you were their property. Otherwise you’ll be
dependent on them, and what will happen when they’re gone?

With that out of the way, the simple answer is that you belong to your
‘significant other’, although I admit that that’s something of a
tautology. Do you have someone locally who responds to ‘girlfriend’,
‘boyfriend’, ‘wife’, or ‘husband’? ‘Honey’? ‘Snookie-pie’? If you do,
chances are that you belong to them.

And to get the full benefits of the relationship, you should always
make sure the association is set up in the other direction as well.

You should note that in the case of ‘wife’ and ‘husband’, a slightly
more formal API applies:

As for the question of whether you can have many, the answer is
generally no, particularly in the case of the formal API (although, as
usual, there are exceptions depending on what environment you’re in).
Most people just have one.

If you insist on having many, it is possible, and you’ll find that each
relationship basically works the same as usual. And you can still keep
your original ‘has one’ relationship – that way, you can refer to one
of your many (by default, the earliest one) as a special case.

Just make sure you don’t confuse their names.

And you certainly don’t want to find all of them in the same place.
It’s probably best if you keep them within different scopes. Of course,
I don’t recommend this kind of chicanery. It is likely to be deprecated
soon, anyway.

Chris

5

From my dating experiences, I can also confirm that :has_many from my
end results in cache misses. :slight_smile:

Yeah, I know. It’s lame.

Chris M. [email protected] wrote:
Mic wrote:

Relationship Advice - who do I belong to & can I have many?

First off, I should point out that ‘belong to’ is a strong term. In any
good relationship, there should certainly be a sense of belonging
towards each other, but you shouldn’t ever feel that you merely ‘belong
to’ somebody, as if you were their property. Otherwise you’ll be
dependent on them, and what will happen when they’re gone?

With that out of the way, the simple answer is that you belong to your
‘significant other’, although I admit that that’s something of a
tautology. Do you have someone locally who responds to ‘girlfriend’,
‘boyfriend’, ‘wife’, or ‘husband’? ‘Honey’? ‘Snookie-pie’? If you do,
chances are that you belong to them.

And to get the full benefits of the relationship, you should always
make sure the association is set up in the other direction as well.

You should note that in the case of ‘wife’ and ‘husband’, a slightly
more formal API applies:

As for the question of whether you can have many, the answer is
generally no, particularly in the case of the formal API (although, as
usual, there are exceptions depending on what environment you’re in).
Most people just have one.

If you insist on having many, it is possible, and you’ll find that each
relationship basically works the same as usual. And you can still keep
your original ‘has one’ relationship – that way, you can refer to one
of your many (by default, the earliest one) as a special case.

Just make sure you don’t confuse their names.

And you certainly don’t want to find all of them in the same place.
It’s probably best if you keep them within different scopes. Of course,
I don’t recommend this kind of chicanery. It is likely to be deprecated
soon, anyway.

Chris

Do you Yahoo!?
Everyone is raving about the all-new Yahoo! Mail.

6

Great post Chris! It was just what I need on a Friday!