Belongs_to and has_many

Rails
1 
I know this has been rehashed many times, but I don't get it.

I can’t find it documented to were I understand all the parts. I can
make this work using Rails database design conventions, but in this case
I am not able to do that.
I find examples, but the ones I find do not follow the conventions.
They also do not give enough information to understand what is going on.

In the model:
What is the “association”? Is it the field name minus the _id?
:foreign_key is the key in the other table.
:class_name is the name of the model of the other table.
------createtables.sql
CREATE TABLE admins(
id INTEGER PRIMARY KEY,
admin VARCHAR(32) NOT NULL,
);
CREATE TABLE machines(
id INTEGER PRIMARY KEY,
padmin_id REFERENCES admins(admin),
sadmin_id REFERENCES admins(admin)
)
-------models/machine.rb------------
class Machine < ActiveRecord::Base
belongs_to :padmin, :class_name => “admin”, :foreign_key => “id”
belongs_to :sadmin, :class_name => “admin”, :foreign_key => “id”
end
-------models/admin.rb--------------
class Admin < ActiveRecord::Base
has_many :machines, :foreign_key => ‘padmin_id’
has_many :machines, :foreign_key => ‘sadmin_id’
end
-------views/_list_stripes.rhtml-----
If it were a straight association machine.admin_id and admin.id, then
this works

<%= list_stripes.admin ? list_stripes.admin[“admin”] : “UnAssigned”
%>

I would be happy if I could understand it well enough that I could use
it without the compilcation of “sadmin_id”.

Blake

2

I’m not sure i quite understand your data model…

If it’s: “One admin can have many machines, One machine belongs to many
admins” then you should have a joiner table:

administrators administrators_machines machines

id <---- administrator_id |–> id
admin machine_id – machine

Your models will then reflect this relationship:

class Machine < ActiveRecord::Base
has_and_belongs_to_many :administrators
end

class Administrator < ActiveRecord::Base
has_and_belongs_to_many :machines
end

Rails will automatically map everything together for you. The
associations should be avaliable as a collection in the models:

@administrator.machines.find(:all)

Hopefully this will be helpful - i apologise if i’ve mis-understood your
problem :0)

Steve

3

Steve,
I think you understand my data. I was trying to avoid habtm. I
haven’t found a way to associate data with each join. We have primary
and secondary admins for each machine.

habtm does not deal with this:

administrators administrators_machines machines

id <---- administrator_id |–> id
admin machine_id – machine
[primary | secondary]

In the mean time I would like to figure out the correct syntax for

describing the models and listing the data with just belongs_to and
has_many. I would drop the secondary admin out of the model. But instead
of renameing things I would like to know how to do it without following
rails naming conventions.

Does join through allow this type of model?

Blake

4

Blake Lewis wrote:

Does join through allow this type of model?

Blake

The :through approach requires creating a new Model in ghe middle, say
‘adminship’,

so administrator has_many :adminships and has_many :machines :though
=> ‘adminships’
and machine has_many :adminships and has_many :administrators :through
=> ‘adminships’.

The adminships table, in addition to administrayor_id and machine_id
will have, say a priority column indicating primary or secondary

The Adminship model would belongs_to :machine and belongs_to
:administrator.

More blurb here:
http://blog.hasmanythrough.com/articles/2006/04/20/many-to-many-dance-off

Does this help ?

Alan