Rails / ActiveRecord has_one and has_many to same model

organization.rb

# From https://github.com/rails/rails/issues/20606#issuecomment-113323102
# Almost good, but I don't think it handles the case of promoting an
# existing location to being the primary location correctly in some cases

class Organization < ActiveRecord::Base
  has_many :locations, dependent: :destroy, autosave: true # autosave necessary for the importer
  has_one :primary_location, -> { where(locations: { primary: true }) }, class_name: "Location", autosave: true

  # Override getter to fix issue with Rails not reloading the primary_location after resetting it to nil
  def primary_location
    @primary_location || locations.detect{ |l| l.primary? }
  end

  # Override the builder to handle removale of existing primary locations and building a new one with the proper attributes
  def build_primary_location(attributes = {})
    handle_existing_primary_location
    @primary_location = self.locations.build(attributes.merge({primary: true}))
  end

  # Override the primary_location setter to work around Rails' conflicting behaviour between the has_one :primary_location and has_many :locations
  # Rails is unable to re-assign a primary_location as it tries to clear the organization_id foreign_key on the location, which isn't possible because of NOT NULL
  # +location_or_id+ supports passing a location object OR a location ID (workaround for inline-creation using select_or_create which was designed to work with belongs_to)
  def primary_location=(location_or_id)
    if location_or_id
      new_primary_location = location_or_id.is_a?(Location) ? location_or_id : self.locations.find(location_or_id)
      new_primary_location.try(:assign_attributes, primary: true, organization: self)
    end
    handle_existing_primary_location

    # if this is a new location, make sure it's added to self.locations so that it gets saved alongside the organization
    # use the target method here, as it allows us to add to the locations (and have it saved on organization save) w/o saving the location immediately
    self.locations.target << new_primary_location if new_primary_location && new_primary_location.new_record?
    @primary_location = new_primary_location
  end


  private

  def handle_existing_primary_location
    if self.primary_location
      if self.primary_location.new_record?
        self.locations.detect{ |l| l.primary? }.mark_for_destruction
      end
      self.locations.detect{ |l| l.primary? }.try(:assign_attributes, {primary: false})
    end
  end
end

spec.rb

describe Organization do

  describe "#primary_location=" do
    let(:new_location) { Location.new(name: "Test Location") }
    context "new organization" do
      let(:organization) { build(:organization) }
      it "creates a new primary via attributes when there is no primary_location" do
        o = organization
        o.assign_attributes primary_location_attributes: { name: "New Location", address: "123 W Pender St" }
        o.save
        expect(o.primary_location.name).to eq "New Location"
      end

      it "creates a new primary via attributes when a primary_location exists" do
        o = organization
        o.primary_location = new_location
        o.save
        o.reload
        o.assign_attributes primary_location_attributes: { name: "New Location", address: "123 W Pender St" }
        o.save
        expect(o.primary_location.name).to eq "New Location"
      end

      it "the built primary_location is accessible after being set but before save" do
        o = organization
        o.assign_attributes primary_location_attributes: { name: "New Location", address: "123 W Pender St" }
        expect(o.primary_location.name).to eq "New Location"
      end

      it "the primary_location is accessible after being set but before save" do
        o = organization
        o.primary_location = new_location
        expect(o.primary_location).to eq new_location
        expect(o.primary_location).to_not be_persisted
      end

      it "allows the primary_location to be reset after being set but before the organization is persisted" do
        o = organization
        o.primary_location = new_location
        expect(o.primary_location).to eq new_location
        o.primary_location = nil
        expect(o.primary_location).to be_nil
        o.save
        expect(o.reload.primary_location).to be_nil
      end

      it "persists the primary_location on save of the organization" do
        o = organization
        o.primary_location = new_location
        o.save
        o.reload
        expect(o.primary_location.name).to eq new_location.name
        expect(o.locations.first.primary?).to be_true
      end
    end

    context "existing organization" do
      let(:organization) { create(:organization) }
      context "new location" do
        it "the primary_location is accessible after being set but before save" do
          o = organization
          expect(o.primary_location).to be_nil
          o.primary_location = new_location
          expect(o.primary_location).to eq new_location
        end

        it "assigns the primary_location immediately" do
          o = organization
          expect(o.primary_location).to be_nil
          o.primary_location = new_location
          expect(o.primary_location.name).to eq new_location.name
        end

        it "doesn't save new locations on assignment" do
          o = organization
          o.primary_location = new_location
          expect(o.primary_location.name).to eq new_location.name
          expect(o.primary_location.id).to eq nil
        end

        it "doesn't save extra locations after subsequent primary_location sets with new locations" do
          o = organization
          o.primary_location = Location.new name: "test"
          o.primary_location = new_location
          o.save
          expect(o.reload.primary_location).to eq new_location.reload
          expect(o.locations.count).to eq 1
        end

        it "doesn't save extra locations after subsequent build_primary_location calls" do
          o = organization
          o.build_primary_location name: "test"
          o.build_primary_location name: "test part two"
          o.save
          expect(o.reload.primary_location.name).to eq "test part two"
          expect(o.locations.count).to eq 1
        end
      end

      context "existing location" do
        let(:location1) { create(:location, organization: organization) }
        let(:location2) { create(:location, organization: organization) }
        context "primary_location first assignment" do
          it "assigns the primary_location immediately when given a location" do
            o = organization
            expect(o.primary_location).to be_nil
            o.primary_location = location1
            expect(o.reload.primary_location).to eq location1
          end

          it "assigns the primary_location immediately when given a location ID" do
            o = organization
            expect(o.primary_location).to be_nil
            o.primary_location = location1.id
            expect(o.reload.primary_location).to eq location1
          end
        end

        context "primary_location reassignment" do
          it "resets the primary location to nil after a new one is built and an old one exists" do
            o = organization
            o.primary_location = location1
            o.save
            o.build_primary_location name: "test built"
            o.primary_location = nil
            o.save
            expect(o.reload.primary_location).to eq nil
          end

          it "reassigns the primary_location immediately when given a location" do
            o = organization
            o.primary_location = location1
            expect(o.reload.primary_location).to eq location1
            o.primary_location = location2
            expect(o.reload.primary_location).to eq location2
          end

          it "reassigns the primary_location immediately when given a location ID" do
            o = organization
            o.primary_location = location1.id
            expect(o.reload.primary_location).to eq location1
            o.primary_location = location2.id
            expect(o.reload.primary_location).to eq location2
          end
        end

        context "primary_location reset" do
          it "resets the primary_location immediately" do
            o = organization
            o.primary_location = location1
            expect(o.reload.primary_location).to eq location1
            o.primary_location = nil
            expect(o.primary_location).to be_nil
            expect(o.reload.primary_location).to be_nil
          end
        end
      end
    end
  end
end