Add batch_transaction to make transaction work on model validation*

This is the core part when importing from csv.

```ruby
def batch_import
  @resource.transaction do
    run_callback(:before_batch_import)
    batch_result = resource.import(headers.values, csv_lines, import_options)
    run_callback(:after_batch_import)
    batch_result
  end
end
```

What confusing me is that the transaction won't rollback, when some records fail on validation. It means that now the csv file is partially imported, and I need to modify the failed records and delete the succeed records to import again, which seems weird to me.

The transaction relies on the `import` method defined by *activerecord-import*. The `import` method does the validation and saves the result to `failed_instances`, which will never raise an exception on validation. Check the code for details https://github.com/zdennis/activerecord-import/blob/6c06b3ceb53f83e1ce930eab35cdd4b6375c57ca/lib/activerecord-import/import.rb#L340-L365

So I made this update to manually trigger a rollback:

```ruby
raise ActiveRecord::Rollback if import_options[:batch_transaction] &&
batch_result.failed_instances.any?
```

P.S.

The validation I'm talking about is on model level, not on DB
level. The `import` method will raise the DB level exception to tirgger
the rollback, like add a record with a duiplicate field which has a uniq index.

Author: ifyouseewendy

README.md

@@ -65,6 +65,7 @@ Tool                    | Description
 :csv_options			|hash with column separator, row separator, etc 
 :validate				|bool means perform validations or not
 :batch_size				|integer value of max  record count inserted by 1 query/transaction
+:batch_transaction    |bool (false by default), if transaction is used when batch importing and works when :validate is set to true
 :before_import			|proc for before import action, hook called with  importer object
 :after_import			|proc for after import action, hook called with  importer object
 :before_batch_import	|proc for before each batch action, called with  importer object

lib/active_admin_import/dsl.rb

@@ -8,6 +8,7 @@ module DSL
     # +back+:: resource action to redirect after processing
     # +csv_options+:: hash to override default CSV options
     # +batch_size+:: integer value of max  record count inserted by 1 query/transaction
+    # +batch_transaction+:: bool (false by default), if transaction is used when batch importing and works when :validate is set to true
     # +before_import+:: proc for before import action, hook called with  importer object
     # +after_import+:: proc for after import action, hook called with  importer object
     # +before_batch_import+:: proc for before each batch action, called with  importer object
@@ -31,11 +32,12 @@ module DSL
       if result.empty?
         flash[:warning] = I18n.t('active_admin_import.file_empty_error')
       else
-        if result.has_imported?
-          flash[:notice] = I18n.t('active_admin_import.imported', count: result.imported_qty, model: model_name, plural_model: plural_model_name)
-        end
         if result.has_failed?
           flash[:error] = I18n.t('active_admin_import.failed', count: result.failed.count, model: model_name, plural_model: plural_model_name)
+          return if options[:batch_transaction]
+        end
+        if result.has_imported?
+          flash[:notice] = I18n.t('active_admin_import.imported', count: result.imported_qty, model: model_name, plural_model: plural_model_name)
         end
       end
     end

lib/active_admin_import/importer.rb

@@ -16,6 +16,7 @@ class Importer
         :after_batch_import,
         :headers_rewrites,
         :batch_size,
+        :batch_transaction,
         :csv_options
     ].freeze
 
@@ -47,7 +48,7 @@ def import
     end
 
     def import_options
-      @import_options ||= options.slice(:validate, :on_duplicate_key_update, :ignore, :timestamps)
+      @import_options ||= options.slice(:validate, :on_duplicate_key_update, :ignore, :timestamps, :batch_transaction)
     end
 
     def batch_replace(header_key, options)
@@ -97,12 +98,14 @@ def run_callback(name)
     end
 
     def batch_import
+      batch_result = nil
       @resource.transaction do
         run_callback(:before_batch_import)
         batch_result = resource.import(headers.values, csv_lines, import_options)
+        raise ActiveRecord::Rollback if import_options[:batch_transaction] && batch_result.failed_instances.any?
         run_callback(:after_batch_import)
-        batch_result
       end
+      batch_result
     end
 
     def assign_options(options)

lib/active_admin_import/options.rb

@@ -6,6 +6,7 @@ module Options
         :csv_options,
         :validate,
         :batch_size,
+        :batch_transaction,
         :before_import,
         :after_import,
         :before_batch_import,
@@ -40,4 +41,4 @@ def self.options_for(config, options= {})
     end
 
   end
-end
+end