Question

I cannot seem to find the answer to this simple need: how to tell Rails, and hence DataTables, about related data tables? This is based on Railscast 340. (Where Ryan Bates warns that the server side processing is more complicated; indeed it is!)

I am not too experienced in Rails yet, so am still learning about the various find() methods for telling ROR about related data using Active Record; but the essence of my task is to display in the index.html.erb view file for genotypes, where the tricky bit in the view is:

  def index
     respond_to do |format|
      format.html
      format.json { render json: GenotypesDatatable.new(view_context) }
    end
  end

See below for the full class of GenotypesDatatables; this goes in a new class directly under the /app folder.

I need to display/edit data from three models: genotypes, gmarkers and gvision. I need to also display data two models related to genotypes, from gsamples and gmarkers.

The models are constructed like:

class Gmarker < ActiveRecord::Base
attr_accessible :marker
has_many :genotypes, :dependent => :delete_all

...
class Genotype < ActiveRecord::Base
attr_accessible :allele1, :allele2, :run_date
belongs_to :gmarkers
belongs_to :gsamples
...
class Gsample < ActiveRecord::Base
belongs_to :gupload
has_many :genotypes, :dependent => :delete_all
attr_accessible :box, :labid, :subjectid, :well

As can be seen in this error message output from the webserver, the problem lies in not getting the correct data associations:
…

CACHE (0.0ms) SELECT COUNT(*) FROM "genotypes"
Genotype Load (10.5ms) SELECT "genotypes".* FROM "genotypes" ORDER BY allele1 asc LIMIT 10 OFFSET 0
Completed 500 Internal Server Error in 255ms

NameError (undefined local variable or method `f' for #<GenotypesDatatable:0x9833ad4>):
app/datatables/genotypes_datatable.rb:24:in `block in data'
app/datatables/genotypes_datatable.rb:21:in `data'
app/datatables/genotypes_datatable.rb:14:in `as_json'
app/controllers/genotypes_controller.rb:7:in `block (2 levels) in index'
app/controllers/genotypes_controller.rb:5:in `index'
...

The data is supposed to be prepared using server-side processing, but which results in a JSON array that is passed to the jQuery in DataTables. The JSON array gets prepared in the class DataTables:

class GenotypesDatatable
  delegate :params, :h, :link_to, to: :@view

  def initialize(view)
    @view = view
  end

  def as_json(options = {})
    # This is what feeds directly into DataTables
    {
      sEcho: params[:sEcho].to_i,
      iTotalRecords: Genotype.count,
      iTotalDisplayRecords: genotypes.total_entries,
      aaData: data
    }
  end

private

  def data
    genotypes.map do |genotype|
      [
        # Note: h is shorthand for html_escape
        h(Gmarker.find(f.gmarkers_id).marker),
        h(Gsample.find(f.gsamples_id).labid),
        h(Gsample.find(f.gsamples_id).subjectid),
        h(Gsample.find(f.gsamples_id).box),
        h(Gsample.find(f.gsamples_id).well),
        h(genotype.allele1),
        h(genotype.allele2),
        h(genotype.run_date)
      ]
    end
  end

  def genotypes
    @genotypes ||= fetch_genotypes
  end

  def fetch_genotypes
    genotypes = Genotype.order("#{sort_column} #{sort_direction}")
    genotypes = genotypes.page(page).per_page(per_page)
    if params[:sSearch].present?
      genotypes = genotypes.where("labid like :search or category like :search", search: "%#{params[:sSearch]}%")
    end
    genotypes
  end
...

Would sure appreciate any pointers here; feel like I’m lost in the jungle without a map or a flashlight!

Thanks,
Rick Casey

Answer 1

As you know I am doing similar. Rather than try to change your code which would probably confuse the issue, I will post my code as an example. My table neighbours contains associations.

class Neighbour < ActiveRecord::Base

  belongs_to :locality, :class_name => "Locality"
  belongs_to :neighbour, :class_name => "Locality"
  belongs_to :highway,  :class_name => "Highway"

  default_scope joins(:locality, :highway).order('localities.name')

end

I think the default_scope is irrelevant in this discussion.

and what I think is the relevant code in the NeighboursDatatable class:

def data
  neighbours.map do |neighbour|
    [
     neighbour.locality.name,
     neighbour.neighbour.name,        
     neighbour.highway.name, 
     neighbour.distance,
     neighbour.id
    ]
 end

end

So I am not sure you need to do the explicit finds. My table displays properly using DataTables. Hope this helps from another ROR newbie.

John