Updating Object property in DataMapper

Question

Using DataMapper, I have noticed that unless I modify the entire Object, the property is not updated - updating fields of an object and then saving does not persist changes.

I'm updating the value attribute of my UnitValue class, which I store as an Object property in my ProductQuantity class.

require 'data_mapper'

class ProductQuantity
  include DataMapper::Resource
  property :id, Serial
  property :quantity, Object, :required => true
end

# RSpec:

# Create a UnitValue with value 300, save as the Object property of this ProductQuantity
prod_quantity1 = ProductQuantity.create(:quantity => UnitValue.new(300, 'kg'))
expect(prod_quantity1.quantity.value).to eql(300) # true

# Replace the entire UnitValue object
prod_quantity1.quantity = UnitValue.new(400, 'kg')
expect(prod_quantity1.quantity.value).to eql(400)
prod_quantity1.save

# Check save worked when modifying the entire object
expect(ProductQuantity.get(prod_quantity1.id).quantity.value).to eql(400) # true

# Modify only a single field
prod_quantity1.quantity.value = 500
prod_quantity1.save

# Check save worked when modifying a single object - this FAILS
expect(ProductQuantity.get(prod_quantity1.id).quantity.value).to eql(500) # false

Answer 1

After looking through docs and finding this outdated thread I believe I've found the solution. You can mark attributes as dirty by changing the persistence_state of a DataMapper::Resource instance.

DirtyState = DataMapper::Resource::PersistenceState::Dirty
quantity_property = ProductQuantity.properties[:quantity]

old_quantity = prod_quantity1.quantity
prod_quantity1.quantity.value = 500
dirty_state = DataMapper::Resource::PersistenceState::Dirty.new(prod_quantity1)
dirty_state.original_attributes[quantity_property] = old_quantity
prod_quantity1.persistence_state = dirty_state
expect(prod_quantity1.persistence_state.is_a? DirtyState).to eql(true)
expect(prod_quantity1.dirty?).to eql(true)
prod_quantity1.save
expect(prod_quantity1.dirty?).to eql(false)

Basically we create a DataMapper::Resource::PersistenceState::Dirty object and then using the "quantity" property of my ProductQuantity as the key, we set the value as the old object in original_attributes. Any non-empty map will return true for prod_quantity1.dirty?.

I've made it into a module below. Just call <resource_instance>.make_dirty(*attributes) with some attributes names.

require 'data_mapper'

module DataMapper
  module Resource

    # Make the give attributes dirty
    def make_dirty(*attributes)
      if attributes.empty?
        return
      end
      unless self.clean?
        self.save
      end
      dirty_state = DataMapper::Resource::PersistenceState::Dirty.new(self)
      attributes.each do |attribute|
        property = self.class.properties[attribute]
        # Any value will do here and return true for self.dirty?, but it expects the old version of this attribute.
        dirty_state.original_attributes[property] = nil
        self.persistence_state = dirty_state
      end
    end

  end
end

And I've tested it below:

describe DataMapper::Resource do

  before(:all) do
    DataMapper.auto_migrate!
    @prod1 = Product.create(:name => 'Snickers')
  end

  after(:all) do
    DataMapper.auto_migrate!
  end

  it 'should fail when an attribute is not dirty' do
    prod_quantity1 = ProductQuantity.create(:product => @prod1, :quantity => UnitValue.new(300, 'kg'))
    prod_quantity1.quantity.value = 400
    expect(prod_quantity1.dirty?).to eql(false)
    prod_quantity1.save
    prod_quantity1 = ProductQuantity.get(prod_quantity1.id)
    expect(prod_quantity1.quantity.value).to eql(300)
  end

  it 'should mark an attribute as dirty' do
    prod_quantity1 = ProductQuantity.create(:product => @prod1, :quantity => UnitValue.new(300, 'kg'))
    prod_quantity1.quantity.value = 400
    expect(prod_quantity1.dirty?).to eql(false)
    prod_quantity1.make_dirty(:quantity)
    expect(prod_quantity1.dirty?).to eql(true)
    prod_quantity1.save
    expect(prod_quantity1.quantity.value).to eql(400)
    expect(prod_quantity1.dirty?).to eql(false)
  end

end