rails static model validations

Question

I have a Subject model which represents a treeview with parent children nodes.

To move a subject to another branch/node, I need to have a 2 subject id's that represent from and to values.

I have started by putting all of the logic into the controller but am now wanting to reuse the copy method and would like to set it up in the model.

Here is some of my controller code.

def copy
    from = Subject.find(params[:from])
    to = Subject.find(params[:to])

    if to.is_descendant_of? from
        render :json => {:error => ["Can't move branch because the target is a descendant."]}.to_json,  :status => :bad_request
        return
    end

    if to.read_only?
        render :json => {:error => ["Can't copy to this branch as it is read only."    ]}.to_json,  :status => :bad_request
        return
    end

    if params[:subjects] == 'copy'
      subject = Subject.create(:name => from.name, :description => from.description, :parent_id => to.id)

      #recursively walk the tree
      copy_tree(from, subject)
    else
      #move the tree
      if !(from.read_only or to.read_only)
        to.children << from
      end
    end

end

Here is what I started doing in my model

class Subject < ActiveRecord::Base



    def self.copy(from, to, operations)

        from = Subject.find(from)
        to = Subject.find(to)

        if to.is_descendant_of? from
           #how do I add validation errors on this static method?

        end

    end
end

My first concern is how to add errors to a static method in a model?

I'm not sure if I'm going about it the right way by using a static method or an instance method.

Anyone able to give me a bit of help in refactoring this code?

Answer 1

You have three possible solutions. ( I prefer the 3rd approach)

1) Return nil on success, error string on failure

# model code
def self.copy(from, to, operations)
  if  to.is_descendant_of? from
    return "Can't move branch because the target is a descendant."
  end
end

# controller code
error = Subject.copy(params[:from], params[:to], ..) 
if (error)
  return render(:json => {:error => [error]},  :status => :bad_request)
end

2) Throw exception on error

def self.copy(from, to, operations)
  if  to.is_descendant_of? from
    throw "Can't move branch because the target is a descendant."
  end
end

# controller code
begin
  Subject.copy(params[:from], params[:to], ..) 
rescue Exception => ex
  return render(:json => {:error => [ex.to_s]},  :status => :bad_request)
end

3) Add an instance method on Subject class.

def copy_from(from, operations)
  if  is_descendant_of? from
    errors.add_to_base("Can't move branch because the target is a descendant.")
    return false
  end

  return true #upon success
end

# controller code
from = Subject.find(params[:from]) #resolve from and to
to = Subject.find(params[:to])

if to.copy_from(from)
  # success
else
  # errors
  return render(:json => {:error => [to.errors]},  :status => :bad_request)
end