#434

backend/app/services/counts_exporter.rb

@@ -6,6 +6,13 @@ class CountsExporter
                 'summary_columns', 'nested_json', 'child_sort', 'labels'].freeze
   attr_reader :format, :resource
 
+  def self.with_read_snapshot(&block)
+    # A caller-owned transaction already owns the read view (including fixtures).
+    # MySQL cannot change isolation inside it. Standalone workers always start RR.
+    options = ApplicationRecord.connection.transaction_open? ? {} : { isolation: :repeatable_read }
+    ApplicationRecord.transaction(**options, &block)
+  end
+
   def initialize(params, language_code:)
     params = CountsResourceDefinition.normalise_params(params)
     if params.key?('definition')
@@ -52,7 +59,7 @@ class CountsExporter
     return @table_export.body(progress:, phase:) if @table_export
 
     Enumerator.new do |output|
-      ApplicationRecord.transaction(isolation: :repeatable_read) do
+      self.class.with_read_snapshot do
         processed = 0
         first_row = true
         started = false