Allow tenant to be set if it is presently nil.

This is helpful, for instance, during a multi-step user/account registration process.

lib/acts_as_tenant/model_extensions.rb

@@ -76,13 +76,13 @@ def acts_as_tenant(association = :account)
         # - Add a helper method to verify if a model has been scoped by AaT
         #
         define_method "#{ActsAsTenant.fkey}=" do |integer|
-          raise ActsAsTenant::Errors::TenantIsImmutable unless new_record?
-          write_attribute("#{ActsAsTenant.fkey}", integer)  
+          raise ActsAsTenant::Errors::TenantIsImmutable unless new_record? || send(ActsAsTenant.fkey).nil?
+          write_attribute("#{ActsAsTenant.fkey}", integer)
         end
 
-        define_method "#{ActsAsTenant.tenant_klass.to_s}=" do |model|  
-          raise ActsAsTenant::Errors::TenantIsImmutable unless new_record?
-          super(model) 
+        define_method "#{ActsAsTenant.tenant_klass.to_s}=" do |model|
+          raise ActsAsTenant::Errors::TenantIsImmutable unless new_record? || send(ActsAsTenant.fkey).nil?
+          super(model)
         end
 
         def scoped_by_tenant?

spec/acts_as_tenant/model_extensions_spec.rb

@@ -165,7 +165,7 @@ class SubTask < ActiveRecord::Base
     end
   end
 
-  describe 'tenant_id should be immutable' do
+  describe 'tenant_id should be immutable, if already set' do
     before do
       @account = Account.create!(:name => 'foo')
       @project = @account.projects.create!(:name => 'bar')
@@ -174,6 +174,16 @@ class SubTask < ActiveRecord::Base
     it { lambda {@project.account_id = @account.id + 1}.should raise_error }
   end
 
+  describe 'tenant_id should be mutable, if not already set' do
+    before do
+      @account = Account.create!(:name => 'foo')
+      @project = Project.create!(:name => 'bar')
+    end
+
+    it { @project.account_id.should be_nil }
+    it { lambda { @project.account = @account }.should_not raise_error }
+  end
+
   describe 'Associations can only be made with in-scope objects' do
     before do
       @account = Account.create!(:name => 'foo')
@@ -275,7 +285,7 @@ class SubTask < ActiveRecord::Base
         @project1 = @account1.projects.create!(:name => 'foobar')
         ActsAsTenant.configuration.stub(require_tenant: true)
       end
-      
+
       it "should raise an error when no tenant is provided" do
         expect { Project.all.load }.to raise_error(ActsAsTenant::Errors::NoTenantSet)
       end