check columns (formerly `validate!`) on ron

don't execute anything from the constructor

lib/excelsior/import.rb

@@ -30,19 +30,15 @@ module Excelsior
       @rows = @sheet.rows
 
       @result = Result.new(@rows.length)
-
-      validate!
     end
 
     def run(&block)
+      check_columns!
+
       return unless valid?
 
       if self.class.use_transaction
-        model_class.transaction do
-          insert_rows(&block)
-
-          raise ActiveRecord::Rollback if report.failed.positive?
-        end
+        run_with_transaction(&block)
       else
         insert_rows(&block)
       end
@@ -50,22 +46,30 @@ module Excelsior
       result
     end
 
-    def validate!
-      fields.to_a.each do |f|
-        errors[:missing_column] << { missing: f[:header] } unless @columns.include?(f[:header])
-      end
-    end
-
     def valid?
       result.status != Result::Statuses::FAILED
     end
 
     private
 
+    def check_columns!
+      fields.to_a.each do |f|
+        errors[:missing_column] << { missing: f[:header] } unless @columns.include?(f[:header])
+      end
+    end
+
     def model_class
       self.class.name.gsub('Import', '').constantize
     end
 
+    def run_with_transaction(&block)
+      model_class.transaction do
+        insert_rows(&block)
+
+        raise ActiveRecord::Rollback if report.failed.positive?
+      end
+    end
+
     def add_model_errors(record, index)
       if record.errors.empty?
         report_insert

test/excelsior_test.rb

@@ -167,7 +167,7 @@ describe Excelsior do
 
   describe '#errors' do
     it 'returns an error when a column is missing in the excel' do
-      import = UserImport.new('test/files/missing-column.xlsx')
+      import = UserImport.new('test/files/missing-column.xlsx').tap(&:run)
       assert import.errors[:missing_column].any?
     end