Rails model validation which check if already exists object with some property

Question

I have following model:

class TestModel < ActiveRecord::Base
  enum some_column: [:prop1, :prop2, :prop3]
end

I want to ensure i will always have maximum one object with prop1 column value in database, while i can have multiple numbers of object with prop2 or prop3 objects or none of them. It would be great if it could be a model validation. I tried this way but not sure if this is good practice for a rails app:

      if ((id.nil? and TestModel.where(some_column: 'prop1')) or (TestModel.where(some_column: 'prop1').where.not(id: id)))

I tried with left side to cover create action, with right side to cover update. Is there any rails way to do this?

Thanks

UPDATED

This is solution for my problem

def only_one_prop1_record
  if(some_column == 'prop1')
    if new_record? #if create action
      if TestModel.where(some_column: 'prop1').any?
        errors.add(:base, 'You can only have one prop1 record')
      end
    elsif persisted? #if update action
      if TestModel.where(some_column: 'prop1').where.not(id: id).any?
        errors.add(:base, 'You can only have one prop1 record')
      end
    end
  end
end

And call validation like this: validate :only_one_prop1_record

Answer 1

This might help you:

class TestModel < ActiveRecord::Base
  enum some_column: [:prop1, :prop2, :prop3]

  validate :only_one_prop1_record, on: :create

  def only_one_prop1_record
    if TestModel.exists?(some_column: prop1)
      errors.add(:base, 'You can only have one prop1 record')
    end
  end
end

Or may be to run the validation on update and create both you can try this:

def only_one_prop1_record
  existing_record = TestModel.where(some_column: prop1).first
  if (new_record? && existing_record.present?) || (persisted? && existing_record != self)
    errors.add(:base, 'You can only have one prop1 record')
  end
end

And remove the on: :create from above.