Composing Data from THREE models with Aggregation

[Demetrius]

Hello! I'm looking for assistance EXTENDING an example from the Agile
Web Development With Rails book: Composing Data with Aggregation (page
324).

I'm trying to map three columns to a single Ruby object. However,
unlike the example in the Agile Web Development book, the three
columns I want to map into one object come from THREE DIFFERENT
models.

I want to map the following three columns into one object:
grade.grade_abbr, strand.strand_abbr, standard.number (ex. 5 NS 2.3).
Also, I only need to read the new object--not edit the original
columns.

Here are the models and their relationships:

class Standard
belongs_to :strand
belongs_to :grade

class Strand
has_many :standards

class Grade
has_many :standards

When I originally setup my Standard table with grade_abbr and
strand_abbr, it easy to follow the example in the Agile Web
Development because each column was contained within the Standard
model. However, I had to move Grade and Strand into their own tables,
so the Standard table contains grade_id and strand_id--not the actual
objects themselves.

When I played with the example from the Agile Web Development book, I
couldn't get :grade.grade_abbr and strand.strand_abbr to work. The
code below was the best that I could come up with but it shows
grade_id and strand_id. Does anyone have a suggestion for how to make
this work or for a new strategy?

class Standard < ActiveRecord::Base
composed_of :full_standard,
:mapping =>
[
%w[grade_id grade_id],
%w[strand_id strand_id]
%w[number number]
]

class FullStandard < ActiveRecord::Base
attr_reader :grade_id, :strand_id, :number

def initialize(grade_id, strand, number)
@grade_id = grade_id
@strand_id = strand_id
@number = number
end

def to_s
[ @grade_id, @strand_id, @number ].compact.join(" ")
end
end

[Frederick Cheung]

On Nov 18, 2:07 pm, Demetrius <demetri...@hotmail.com> wrote:
> Hello! I'm looking for assistance EXTENDING an example from the Agile
> Web Development With Rails book: Composing Data with Aggregation (page
> 324).
>
> I'm trying to map three columns to a single Ruby object. However,
> unlike the example in the Agile Web Development book, the three
> columns I want to map into one object come from THREE DIFFERENT
> models.
>

Composed_of is not normally used (or probably designed to be used) in
this way). The basic idea is that you have some raw attributes and you
turn them into a object, for example you might have attributes on your
model called start and finish and use composed_of to create a magic
attribute called Range whose endpoints are those attributes.

If you want your standard object to have the name from the Grade
object, could you just do

def grade_abbr
grade.grade_abbr
end

(and when you're happy with this, have a look at delegate)

Fred

[Demetrius]

Thank you so much for your insights! I checked out delegate and that
really helped to simplify things (especially because I have multiple
related classes with the same name attributes).

In the end, what solved my naming problem was using the following:

<%= link_to "#{course.order_taught} #{course.type_name} #
{course.name}", {:action => 'show', :id => course } %>

I had to take some time to go back and review some of the basics of
Ruby, and in the end, the solution was really simple.

Thanks again for your help!!

On Nov 18, 7:16 am, Frederick Cheung <frederick.che...@gmail.com>
wrote:

> On Nov 18, 2:07 pm, Demetrius <demetri...@hotmail.com> wrote:
>
> > Hello! I'm looking for assistance EXTENDING an example from the Agile

> > Web Development With Rails book:ComposingDatawith Aggregation (page
> > 324).
>
> > I'm trying to mapthreecolumns to a single Ruby object. However,

> > unlike the example in the Agile Web Development book, thethree
> > columns I want to map into one object come fromTHREEDIFFERENT
> >models.
>
> Composed_of is not normally used (or probably designed to be used) in
> this way). The basic idea is that you have some raw attributes and you
> turn them into a object, for example you might have attributes on your
> model called start and finish and use composed_of to create a magic
> attribute called Range whose endpoints are those attributes.
>
> If you want your standard object to have the name from the Grade
> object, could you just do
>
> def grade_abbr
>   grade.grade_abbr
> end
>
> (and when you're happy with this, have a look at delegate)
>
> Fred
>

> > I want to map the followingthreecolumns into one object:

> > grade.grade_abbr, strand.strand_abbr, standard.number (ex. 5 NS 2.3).
> > Also, I only need to read the new object--not edit the original
> > columns.
>

> > Here are themodelsand their relationships: