Add SupplierCategory

This allows the categorization of suppliers. For a better reporting
it is necessary to split the expenses of the invoices.
E.g. we want to be able to generate independent sums of general cost
like the rent or electricity and the cost of the bought articles.

db/migrate/20181201000400_create_supplier_categories.rb

@@ -0,0 +1,25 @@
+class CreateSupplierCategories < ActiveRecord::Migration
+  class FinancialTransactionClass < ActiveRecord::Base; end
+  class SupplierCategory < ActiveRecord::Base; end
+  class Supplier < ActiveRecord::Base; end
+
+  def change
+    create_table :supplier_categories do |t|
+      t.string :name, null: false
+      t.string :description
+      t.references :financial_transaction_class, null: false
+    end
+
+    add_reference :suppliers, :supplier_category
+
+    reversible do |dir|
+      dir.up do
+        ftc = FinancialTransactionClass.first
+        sc = SupplierCategory.create name: 'Other', financial_transaction_class_id: ftc.id
+        Supplier.update_all supplier_category_id: sc.id
+      end
+    end
+
+    change_column_null :suppliers, :supplier_category_id, false
+  end
+end