An ActiveModel::EachValidator to check local attribute value against "project" value.

en.yml

en:
  errors:
    messages:
      project_effort_exceeded: 'cannot be larger than project effort'

task.rb

class Task < ActiveRecord::Base
  belongs_to :project
  validates :effort => true, :numericality => {:greater_than_or_equal_to => 1}
end

task_effort_validator.rb

class TaskEffortValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    if record.project && record.project.effort > value
      record.errors.add(attribute, :project_effort_exceeded, options)
    end
  end
end