Using ruby methods map and zip along with Rails extended finds

Posted on 19 July 2007

This is a combination of techniques that I find useful when you need to output related data from various database tables in the same list.

In this example I have organisations, users and a linking table between the two called jobs; each user can only have one job with an organisation, but they can have many jobs.

I want to output a list of users, for a particular organisation, along with information about their job with that organisation.

In the controller I pull out all the information I need:

@organisation = Organisation.find(params[:id])
@users = @organisation.users
@jobs = @users.map { |user| user.jobs.organisation(@organisation) }
@users_with_jobs = @users.zip(@jobs)

@users contains all the users for the organisation. The clever bit is in utilising the ruby map method and an extended find to allow me to get the job information for each user I just pulled out into @users to create a second array @jobs.

The user.jobs.organisation(@organisation) method is an extended find in my User model:

has_many :jobs do
   def organisation(organisation)
     find(:first, :conditions => ["organisation_id = ?", organisation.id])
   end
end

That method allows me to search the jobs for a user and just pull out the job for a particular organisation.

Using the zip method I combine the @users and @jobs arrays into one instance variable that I can loop through. As I've created them using the map method I know they will match up.

Now back in my view I can do:

@users_with_jobs.each do |user, job|
  user.name
  job.name
end

I could have just used my extended find in the view like this:

@users.each do |user|
  user.name
  user.jobs.organisation(@organisation).name
end 

But I want to keep the view simple and do all the data collection in the controller.

As a note the zip method works like this:

a = [1,2,3]
=> [1, 2, 3]

b = [4,5,6]
=> [4, 5, 6]

a.zip b
=> [[1, 4], [2, 5], [3, 6]]

b.zip a
=> [[4, 1], [5, 2], [6, 3]]

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