add methods for prepare

lib/embulk/output/gsheets.rb

@@ -22,9 +22,10 @@ def self.transaction(config, schema, count, &control)
           'application_name' => config.param('application_name', :string, default: 'embulk-output-gsheets'),
           'bulk_num' => config.param('bulk_num', :integer, default: 200),
           'with_header' => config.param('with_header', :bool, default: true),
+          'mode' => config.param('mode', :string, default: 'append'),
         }
 
-        Embulk.logger.info "Writing google sheets: " +
+        Embulk.logger.info 'Writing google sheets: ' +
                                "spreadsheet id = [#{task['spreadsheet_id']}], sheet name = [#{task['sheet_name']}]"
 
         service = Service.new(task)
@@ -35,6 +36,15 @@ def self.transaction(config, schema, count, &control)
           service.write([header_record])
         end
 
+        case task['mode']
+        when 'truncate'
+          service.clear
+        when 'append'
+          # nothing to do
+        else
+          raise ConfigError.new "Invalid write mode detected. mode = #{task['mode']}"
+        end
+
         task_reports = yield(task)
         next_config_diff = {}
         return next_config_diff

lib/embulk/output/gsheets/service.rb

@@ -96,6 +96,18 @@ def get_header
           header = get_values(range)
           header
         end
+
+        def clear
+          begin
+            response = @service.batch_clear_values(
+              @spreadsheet_id,
+              Google::Apis::SheetsV4::BatchClearValuesRequest.new)
+          rescue => e
+            # TODO: more appropriate error handling
+            raise "Could not clear values from Google Sheets #{e.message}" +
+                      "spreadsheet_id = #{@spreadsheet_id}"
+          end
+        end
       end
     end
   end