Meta Programming: instance_variable_set() and instance_variable_get()

digitize_step_model.rb

# /app/models/digitize_step

class DigitizeStep < ActiveRecord::Base
  belongs_to :process_step
end

final_tpop_map_model.rb

# /app/models/final_maps

class FinalTpopMap < ActiveRecord::Base
  has_many :process_steps
end

meta_programming.md

The TerraPop Metadata tracks and documents the full process of preparing the data that will be fed into the TerraPop application. part of this process involveds manipulating geographic data, namely maps. These maps may come from a variety of sources, then depending on the final map needed to match a set of variables, these maps are run through a series of processing steps. There are 10 different processing steps to be exact and no single complete process will look the same.

My task at hand was to construct a streamlined UI that would allow users to craft a full map process with any combination of map steps. The main models involved in this process look a bit like this (code samples were simplified for demonstration purposes):

And all of the process steps, 10 in total, were structured like this: /app/models/process_steps

The first tricky issue I'd like to tackle is to discuss how I created the methods in the ProcessStepsController to handle a process_step with an association to to 1 of potentially 10 different step types.

To faciliate this connection, I created a field in the step_type_id field in the ProcessStep model forming a has_one/belongs_to relationship with a domain model consisting of the 10 different steps. Here is how this domain model was seeded: /db/seeds.rb

To make it possible to write simple crud methods in the ProcessStepsController required heavy use of meta programming, including the send(), instance_variable_set(), and instance_variable_get() methods. For example, to set instance variables for the set step_type = StepType.find(@process_step.step_type_id).name.downcase + "_step" instance_variable_set('@'+ step_type, @process_step.send(step_type))

Put it all together, and the create method looks like this: /app/controllers/process_steps_controller.rb

(See bottom of post for full ProcessStepsController code.)

process_step_controller.rb

# coming

process_steps_class.rb

# /app/models/process_steps
class ProcessStep < ActiveRecord::Base
  belongs_to :user
  belongs_to :step_type
  belongs_to :final_tpop_map
  
  has_one :digitize_step, dependent: :destroy
  accepts_nested_attributes_for :digitize_step, allow_destroy: true
  has_one :disaggregate_step, dependent: :destroy
  accepts_nested_attributes_for :disaggregate_step, allow_destroy: true
  has_one :regionalize_step, dependent: :destroy
  accepts_nested_attributes_for :regionalize_step, allow_destroy: true
  has_one :subset_map_step, dependent: :destroy
  accepts_nested_attributes_for :subset_map_step, allow_destroy: true
  has_one :dissolve_step, dependent: :destroy
  accepts_nested_attributes_for :dissolve_step, allow_destroy: true
  has_one :transfer_poly_step, dependent: :destroy
  accepts_nested_attributes_for :transfer_poly_step, allow_destroy: true
  has_one :code_match_step, dependent: :destroy
  accepts_nested_attributes_for :code_match_step, allow_destroy: true
  has_one :verify_step, dependent: :destroy
  accepts_nested_attributes_for :verify_step, allow_destroy: true
  has_one :lsib_align_step, dependent: :destroy
  accepts_nested_attributes_for :lsib_align_step, allow_destroy: true
  has_one :harmonize_step, dependent: :destroy
  accepts_nested_attributes_for :harmonize_step, allow_destroy: true
  
  validates :step_type_id, :presence => true
end

process_steps_controller_create.rb

# /app/controllers/process_steps_controller.rb

def create
  @process_step = ProcessStep.new(process_step_params)
  respond_to do |format|
    if @process_step.save
      step_type = StepType.find(@process_step.step_type_id).name.downcase + "_step"
      instance_variable_set('@'+ step_type, @process_step.send('create_' + step_type))
      if instance_variable_get('@'+ step_type).update(process_step_params[step_type + "_attributes"])
        format.html { redirect_to country_final_tpop_map_process_steps_path(country_id: @country.id, final_tpop_map_id: @final_tpop_map.id), notice: 'Process step was successfully created.' }
        format.json { render :show, status: :created, location: @process_step }
      end
    else
      format.html { render :new }
      format.json { render json: @process_step.errors, status: :unprocessable_entity }
      format.js
    end
  end
end

step_types_seeds.rb

# /db/seeds.rb

step_types = StepType.create([{ name: 'Digitize'}, 
  { name: 'Subset Map'}, 
  { name: 'Dissolve'}, 
  { name: 'Transfer Poly'}, 
  { name: 'Code Match'}, 
  { name: 'Harmonize'}, 
  { name: 'Regionalize'}, 
  { name: 'Disaggregate'},
  { name: 'Verify'},
  { name: 'LSIB Align'}
  ]