Introduce a configuration value for migration path

Since Rails 6 multiple database support is a native feature, which
allows users to configure separate read/write databases. This change
allows users to configure the path to the migrations folder from the
default "db/migrate" if they are using the multiple database support and
have specified a different database for the Solidus engine. This will
ensure the check that all migrations are installed can look at a user
configurable folder.

One change required to make the folder path configurable is to run the
migration check after 'load_config_initializers' instead of before. This
will allow the value of `migration_path` configuration to be set in an
initializer.

Co-authored-by: Adam Mueller <adam@super.gd>
Co-authored-by: Alex Blackie <alex@super.gd>
Co-authored-by: Benjamin Willems <benjamin@super.gd>
Co-authored-by: Jared Norman <jared@super.gd>

core/lib/spree/app_configuration.rb

@@ -263,7 +263,6 @@ class AppConfiguration < Preferences::Configuration
     #   @return [] Track on_hand values for variants / products. (default: true)
     preference :track_inventory_levels, :boolean, default: true
 
-
     # Other configurations
 
     # Allows restricting what currencies will be available.
@@ -525,6 +524,17 @@ def payment_canceller
     # Enumerable of taxons adhering to the present_taxon_class interface
     class_name_attribute :taxon_attachment_module, default: 'Spree::Taxon::ActiveStorageAttachment'
 
+    # Configures the absolute path that contains the Solidus engine
+    # migrations. This will be checked at app boot to confirm that all Solidus
+    # migrations are installed.
+    #
+    # @!attribute [rw] migration_path
+    # @return [String] the configured path. (default: "#{Rails.root}/db/migrate")
+    attr_writer :migration_path
+    def migration_path
+      @migration_path ||= ::Rails.root.join('db', 'migrate')
+    end
+
     # Allows providing your own class instance for generating order numbers.
     #
     # @!attribute [rw] order_number_generator

core/lib/spree/core/engine.rb

@@ -40,7 +40,7 @@ class Engine < ::Rails::Engine
         ]
       end
 
-      initializer "spree.core.checking_migrations", before: :load_config_initializers do |_app|
+      initializer "spree.core.checking_migrations", after: :load_config_initializers do |_app|
         Migrations.new(config, engine_name).check
       end
 

core/lib/spree/migrations.rb

@@ -66,7 +66,7 @@ def app_migrations
     end
 
     def app_dir
-      "#{Rails.root}/db/migrate"
+      Spree::Config.migration_path
     end
 
     def engine_dir

core/spec/lib/spree/app_configuration_spec.rb

@@ -129,6 +129,26 @@ class DummyClass; end;
     end
   end
 
+  describe "#migration_path" do
+    subject { config_instance.migration_path }
+
+    let(:config_instance) { described_class.new }
+
+    it "has a default value" do
+      expect(subject.to_s).to end_with "db/migrate"
+    end
+
+    context "with a custom value" do
+      before do
+        config_instance.migration_path = "db/secondary_database"
+      end
+
+      it "returns the configured value" do
+        expect(subject).to eq "db/secondary_database"
+      end
+    end
+  end
+
   it 'has a default admin VAT location with nil values by default' do
     expect(prefs.admin_vat_location).to eq(Spree::Tax::TaxLocation.new)
     expect(prefs.admin_vat_location.state_id).to eq(nil)

core/spec/lib/spree/migrations_spec.rb

@@ -10,10 +10,17 @@ module Spree
     let(:config) { double("Config", root: "dir") }
 
     let(:engine_dir) { "dir/db/migrate" }
-    let(:app_dir) { "#{Rails.root}/db/migrate" }
+    let(:app_dir) { 'app/db/migrate' }
 
     subject { described_class.new(config, "spree") }
 
+    around do |example|
+      original_path = Spree::Config.migration_path
+      Spree::Config.migration_path = app_dir
+      example.run
+      Spree::Config.migration_path = original_path
+    end
+
     it "detects missing migrations" do
       expect(Dir).to receive(:entries).with(app_dir).and_return app_migrations
       expect(Dir).to receive(:entries).with(engine_dir).and_return engine_migrations