Dependant select boxes - sending no value when mutliple select box is empty

[zorec]

Given the following schema project has_many tasks. Employee has_many projects and has_many tasks through projects.

class Employee < ActiveRecord::Base
  has_many :projects, inverse_of: :employee
  has_many :tasks, through: :projects

  rails_admin do
    edit do
      field :projects
      field :tasks do
        options_source_params do
          { "f[project_id_in]" => bindings[:object].try(:project_ids) || [-1] }
        end
        html_attributes do
        { data: { 
          # set project's field as dependent select box
          "dependant-filteringselect" => "field=project_ids",

          # post parameters
          "dependant-param" => "f[project_id_in]" }
        }
        end
      end
      field :name
      # other fields
    end
  end

The problem is when project field is empty. Then no project_id is sent in options request to TasksController. This means that all tasks are shown