De-normalized table associations

Hi,

Working with legacy DB here…

2 tables:
Projects
Workers

A worker record has many fields in it to hold project codes as in:
project1, project2, project3, … project30 (great, isn’t it?..)

Normalizing the DB is out of the question.

I’ve been thinking about this for a while and I can’t find a way to
associate both tables as in:

“Project has_many :workers”
“Worker.project1 belongs_to :project”
“Worker.project2 belongs_to :project”
etc.

Any ideas?

Pepe

you need has_and_belongs_to_many
http://railsbrain.com/api/rails-2.3.2/doc/index.html?a=M001888&name=has_and_belongs_to_many

Thanks Ben, but I don’t believe so. As I said in the OP, the table is
‘de-normalized’. I don’t have 1 record per worker and project to be
able to associate the tables the usual way. I have many fields holding
project code values in the same record, obviously all of them with
different names. In other words, I have many fields to “hook into” for
the association to happen, not just one neat field I can associate
with.

On 23 February 2010 19:40, ben wiseley [email protected] wrote:

you need
has_and_belongs_to_many http://railsbrain.com/api/rails-2.3.2/doc/index.html?a=M001888&name=has_and_belongs_to_many

How, precisely will a join-table help, when he says that “Normalizing
the DB is out of the question”?
:-/

On 23 February 2010 19:32, pepe [email protected] wrote:

Normalizing the DB is out of the question.

I’ve been thinking about this for a while and I can’t find a way to
associate both tables as in:

“Project has_many :workers”
“Worker.project1 belongs_to :project”
“Worker.project2 belongs_to :project”
etc.

You are highly limited given your inability to re-structure the
database, and I wonder whether you want to reconsider negotiation with
whatever powers that be to refactor the database a little (or a lot!
:wink:

But working with the tables you have, you can at least use AR as much
as possible…

worker model

belongs_to :project1, :class_name => “Project”, :foreign_key =>
“project1_foreign_key_name_in_workers_table”
belongs_to :project2, :class_name => “Project”, :foreign_key =>
“project2_foreign_key_name_in_workers_table”
belongs_to :project3, :class_name => “Project”, :foreign_key =>
“project3_foreign_key_name_in_workers_table”

The project model is going to be harder, and you’re not going to be
able to intuitively assign workers to projects, but you can at least
fudge-together some helper methods to make it look like you’re
leveraging the power of Rails! :slight_smile:

#project model
has_many :project1_keyed_workers, :class_name => “Worker”,
:foreign_key => “project1_foreign_key_name_in_workers_table”
has_many :project2_keyed_workers, :class_name => “Worker”,
:foreign_key => “project2_foreign_key_name_in_workers_table”
has_many :project3_keyed_workers, :class_name => “Worker”,
:foreign_key => “project3_foreign_key_name_in_workers_table”

def workers

build and array of all the arrays of workers, flatten it and get

rid of nil values
([] << project1_keyed_workers << project2_keyed_workers <<
project3_keyed_workers).flatten.compact
end

Hope this helps…
Michael

On 23 February 2010 19:54, Michael P. [email protected] wrote:

whoa! that doesn’t look very legible once posted.
If you can’t make sense of my code snippet, let me know and I’ll post
a Gist link.

In fact, I might as well do that anyway:
http://gist.github.com/312634

Thanks a lot Michael for your explanation and efforts. :slight_smile:

I had thought about going the way you propose but it seemed to me like
too much work (I think the total amount of columns I would need to
write code for is 32) for what I actually need. If there is an easy
way of making the association work both ways I surely would like to
know, however, while you were trying to help me I think I found a neat
solution for my current need, which is not having the association work
both ways but accessing the project information for a given worker.

The problem I’m trying to solve is a little bit more complicated than
I originally posted. I am not really dealing with ID values. The
Projects table has an ID and also a project code, which is also
supposed to be unique “when it has a value” (this is enforced at
application level). The project ID is not what is stored in the
‘project’ columns in the Worker record, it is the project code
instead.

The solution I have so far works quite well and uses virtual
attirbutes (below). The first virtual attribute is a list of all
project codes actually stored in the record. If a ‘project*’ column is
empty it does not provide a value to the list. The second one is the
projects information based on those project codes the first virtual
attribute provides.

class Worker < ActiveRecord::Base
attr_reader :project_codes, :projects

def project_codes
self.attributes.delete_if {|key, value| key.downcase !~ /project/
|| value.blank? }.values.map{|v| v.strip}.uniq
end

def projects
Project.find_all_by_code self.project_codes
end
end

On 23 February 2010 19:52, Michael P. [email protected] wrote:

has_many :project1_keyed_workers, :class_name => “Worker”,
:foreign_key => “project1_foreign_key_name_in_workers_table”
has_many :project2_keyed_workers, :class_name => “Worker”,
:foreign_key => “project2_foreign_key_name_in_workers_table”
has_many :project3_keyed_workers, :class_name => “Worker”,
:foreign_key => “project3_foreign_key_name_in_workers_table”

whoa! that doesn’t look very legible once posted.
If you can’t make sense of my code snippet, let me know and I’ll post
a Gist link.

Michael,

If it were me who had created that monstrosity I would have apologized
in advance, believe me. :slight_smile:

Funny enough, I might very probably be on the phone talking to the
“owner of the beast” in a few minutes and believe it or not, instead
of starting to keep things ‘sane’ and add additional information in
new tables he has already suggested adding a brand new set of 32
columns for the related values we need to store now! :smiley:

The sad thing is that the way things are now we might just need to put
up with it! This guy is obviously on his way out and the application
will be rewritten and the DB redesigned at some point in the near
future. The owner of the application is trying to find the “path of
least resistance” right now to make things work so we can meet the
deadlines we have and start working on the replacement software asap
so we don’t need have this other person involved any longer.

I’ll remember to bring my Louisville Slugger if I ever meet him in
person. :wink:

On 23 February 2010 20:20, pepe [email protected] wrote:

instead.
Well, the first thing you have to do is creep up behind the original
developer of that table with your Louisville Slugger in hand…

But seriously, whoever produced that table should be fired. If my
plumber connected the taps that badly I’d get wet every time I change
channel on the TV. [1]

If there is an easy
way of making the association work both ways I surely would like to
know

Given your table structure, I think you’ve done well to get as much
use out of it as you have; your “projects” method is another little
fudge I might have considered.
You might get something else working (both ways with a little hassle)
using named scopes rather than associations; defining them (all 32+!)
in a loop to be DRY, and concatenating their results like I suggested
with the associations. But it’s a lot of hard work, and not really
what Rails is best at.

Honestly, I think you’d be better off spending the time scouring the
job sites for a new employer; somewhere where databases are free to
enjoy the benefits of friendly ORMs :slight_smile:

Good luck,
Michael

[1] If it was you that created the current tables, but “years ago,
before you knew what you were doing”… well, it’s fair enough that
you’re trying to sort it out now… as long as you’re suitably
apologetic :wink:

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