Allow users to have additional organisations

This creates a `has_and_belongs_to_many` join table relationship between
organisations and users, allowing a user to belong to
`additional_organisations` and paves the way to allowing a user to set their
`organisation` to be a `Current.organisation` and thus view data for that
organisation instead of their own "real" organisation.

This should change nothing about the existing functionality of the site and
will be entirely invisible to end users. (The next phase will include
front-end changes which *will* allow selected users to switch their current
organisation.)

Helper methods on the `User` model to navigate this relationship include
`primary_organisation` (to retrieve the user's actual `organisation`, which
is necessarily overwritten), `all_organisations` (the union of
`primary_organisation` and `additional_organisations`) and
`additional_organisations?` (to determine whether a user has a non-zero
number of `additional_organisations`).

app/models/current.rb

@@ -0,0 +1,3 @@
+class Current < ActiveSupport::CurrentAttributes
+  attribute :organisation
+end

app/models/user.rb

@@ -3,6 +3,7 @@ class User < ApplicationRecord
     otp_secret_encryption_key: ENV["SECRET_KEY_BASE"]
 
   belongs_to :organisation
+  has_and_belongs_to_many :additional_organisations, class_name: "Organisation", join_table: "organisations_users"
   has_many :historical_events
   validates_presence_of :name, :email
   validates :email, with: :email_cannot_be_changed_after_create, on: :update
@@ -18,6 +19,25 @@ class User < ApplicationRecord
 
   delegate :service_owner?, :partner_organisation?, to: :organisation
 
+  def organisation
+    if Current.organisation
+      return Organisation.find(Current.organisation)
+    end
+    super
+  end
+
+  def primary_organisation
+    Organisation.find(organisation_id)
+  end
+
+  def all_organisations
+    Organisation.where(id: [organisation_id, additional_organisations.map(&:id)].flatten)
+  end
+
+  def additional_organisations?
+    additional_organisations.any?
+  end
+
   def active_for_authentication?
     active
   end

db/migrate/20241204220209_create_join_table_organisations_users.rb

@@ -0,0 +1,8 @@
+class CreateJoinTableOrganisationsUsers < ActiveRecord::Migration[6.1]
+  def change
+    create_join_table :organisations, :users, column_options: {type: :uuid} do |t|
+      t.index [:organisation_id, :user_id]
+      t.index [:user_id, :organisation_id]
+    end
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2024_11_14_114012) do
+ActiveRecord::Schema.define(version: 2024_12_04_220209) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pgcrypto"
@@ -286,6 +286,13 @@
     t.index ["iati_reference"], name: "index_organisations_on_iati_reference", unique: true
   end
 
+  create_table "organisations_users", id: false, force: :cascade do |t|
+    t.uuid "organisation_id", null: false
+    t.uuid "user_id", null: false
+    t.index ["organisation_id", "user_id"], name: "index_organisations_users_on_organisation_id_and_user_id"
+    t.index ["user_id", "organisation_id"], name: "index_organisations_users_on_user_id_and_organisation_id"
+  end
+
   create_table "outgoing_transfers", id: :uuid, default: -> { "public.gen_random_uuid()" }, force: :cascade do |t|
     t.uuid "source_id", null: false
     t.uuid "destination_id", null: false

spec/models/user_spec.rb

@@ -31,6 +31,70 @@
     # This also validates that the relationship is present
     it { is_expected.to belong_to(:organisation) }
     it { is_expected.to have_many(:historical_events) }
+    it { is_expected.to have_and_belong_to_many(:additional_organisations) }
+
+    it "has a primary organisation" do
+      user = create(:administrator)
+
+      expect(user.primary_organisation).to be_valid
+      expect(user.primary_organisation.id).to eq(user.organisation_id)
+    end
+
+    it "has additional organisations" do
+      org1 = create(:partner_organisation)
+      org2 = create(:partner_organisation)
+      org3 = create(:partner_organisation)
+
+      user = create(:administrator)
+      user.additional_organisations << [org1, org2, org3]
+
+      expect(user.additional_organisations.pluck(:id)).to include(org1.id)
+      expect(user.additional_organisations.count).to eq(3)
+    end
+
+    it "shows all organisations including the primary organisation" do
+      org = create(:partner_organisation)
+
+      user = create(:administrator)
+      user.additional_organisations << org
+
+      expect(user.all_organisations.size).to eq(2)
+      expect(user.all_organisations.pluck(:id)).to include(user.primary_organisation.id)
+      expect(user.all_organisations.pluck(:id)).to include(org.id)
+    end
+
+    it "determines whether there are additional organisations" do
+      org = create(:partner_organisation)
+
+      user = create(:administrator)
+      user.additional_organisations << org
+
+      expect(user.additional_organisations?).to eq(true)
+
+      user.additional_organisations = []
+      expect(user.additional_organisations?).to eq(false)
+    end
+
+    context "when the current organisation has been set" do
+      let(:current_organisation) do
+        create(:partner_organisation)
+      end
+
+      before do
+        Current.organisation = current_organisation.id
+      end
+
+      it "returns the current organisation instead of the primary organisation" do
+        user = create(:administrator)
+
+        expect(user.organisation).to eq(current_organisation)
+        expect(user.primary_organisation).not_to eq(current_organisation)
+      end
+
+      after do
+        Current.organisation = nil
+      end
+    end
   end
 
   describe "delegations" do