add merge_organisations rake task

lib/tasks/merge_organisations.rake

@@ -0,0 +1,12 @@
+namespace :merge do
+  desc "Merge organisations into one"
+  task :merge_organisations, %i[absorbing_organisation_id merging_organisation_ids] => :environment do |_task, args|
+    absorbing_organisation_id = args[:absorbing_organisation_id]
+    merging_organisation_ids = args[:merging_organisation_ids]
+
+    raise "Usage: rake merge:merge_organisations[absorbing_organisation_id, merging_organisation_ids]" if merging_organisation_ids.blank? || absorbing_organisation_id.blank?
+
+    service = Merge::MergeOrganisationsService.new(absorbing_organisation_id:, merging_organisation_ids:)
+    service.call
+  end
+end

spec/lib/tasks/merge_organisations_spec.rb

@@ -0,0 +1,41 @@
+require "rails_helper"
+require "rake"
+
+RSpec.describe "emails" do
+  describe ":merge_organisations", type: :task do
+    subject(:task) { Rake::Task["merge:merge_organisations"] }
+
+    let(:organisation) { create(:organisation) }
+    let(:merging_organisation) { create(:organisation) }
+
+    let(:merge_organisations_service) { Merge::MergeOrganisationsService.new(absorbing_organisation_id: organisation.id, merging_organisation_ids: [merging_organisation.id]) }
+
+    before do
+      allow(Merge::MergeOrganisationsService).to receive(:new).and_return(merge_organisations_service)
+      allow(merge_organisations_service).to receive(:call).and_return(nil)
+      Rake.application.rake_require("tasks/merge_organisations")
+      Rake::Task.define_task(:environment)
+      task.reenable
+    end
+
+    context "when the rake task is run" do
+      it "raises an error when no parameters are given" do
+        expect { task.invoke(nil) }.to raise_error(RuntimeError, "Usage: rake merge:merge_organisations[absorbing_organisation_id, merging_organisation_ids]")
+      end
+
+      it "raises an error when only absorbing organisation is given" do
+        expect { task.invoke(1, nil) }.to raise_error(RuntimeError, "Usage: rake merge:merge_organisations[absorbing_organisation_id, merging_organisation_ids]")
+      end
+
+      it "raises an error when only merging organisations are given" do
+        expect { task.invoke(nil, [1, 2]) }.to raise_error(RuntimeError, "Usage: rake merge:merge_organisations[absorbing_organisation_id, merging_organisation_ids]")
+      end
+
+      it "raises runs the service with correct organisation IDs" do
+        expect(Merge::MergeOrganisationsService).to receive(:new).with(absorbing_organisation_id: 1, merging_organisation_ids: [2, 3]).once
+        expect(merge_organisations_service).to receive(:call).once
+        task.invoke(1, [2, 3])
+      end
+    end
+  end
+end