Move default task build_enumerator logic back into the job

README.md

@@ -436,7 +436,7 @@ module Maintenance
     def collection
       Post.where(created_at: 2.days.ago...1.hour.ago)
     end
-    
+
     def process(post)
       post.update!(content: "updated content")
     end
@@ -626,6 +626,42 @@ module Maintenance
 end
 ```
 
+### Writing tests for a Task that uses a custom enumerator
+
+Tests for tasks that use custom enumerators need to instantiate the task class
+in order to call `#build_enumerator`. Once the task instance is set up, validate
+that `#build_enumerator` returns an enumerator yielding pairs of [item, cursor]
+as expected.
+
+```ruby
+# test/tasks/maintenance/custom_enumerating_task.rb
+
+require "test_helper"
+
+module Maintenance
+  class CustomEnumeratingTaskTest < ActiveSupport::TestCase
+    setup do
+      @task = CustomEnumeratingTask.new
+    end
+
+    test "#build_enumerator returns enumerator yielding pairs of [item, cursor]" do
+      enum = @task.build_enumerator(cursor: 0)
+      expected_items = [:b, :c]
+
+      assert_equal 2, enum.size
+
+      enum.each_with_index do |item, cursor|
+        assert_equal expected_items[cursor], item
+      end
+    end
+
+    test "#process performs a task iteration" do
+      ...
+    end
+  end
+end
+```
+
 ### Running a Task
 
 #### Running a Task from the Web UI

app/jobs/concerns/maintenance_tasks/task_job_concern.rb

@@ -33,6 +33,45 @@ def retry_on(*, **)
     def build_enumerator(_run, cursor:)
       cursor ||= @run.cursor
       @collection_enum = @task.enumerator_builder(cursor: cursor)
+
+      @collection_enum ||= case (collection = @task.collection)
+      when :no_collection
+        enumerator_builder.build_once_enumerator(cursor: nil)
+      when ActiveRecord::Relation
+        enumerator_builder.active_record_on_records(collection, cursor: cursor, columns: @task.cursor_columns)
+      when ActiveRecord::Batches::BatchEnumerator
+        if collection.start || collection.finish
+          raise ArgumentError, <<~MSG.squish
+            #{@task.class.name}#collection cannot support
+            a batch enumerator with the "start" or "finish" options.
+          MSG
+        end
+
+        # For now, only support automatic count based on the enumerator for
+        # batches
+        enumerator_builder.active_record_on_batch_relations(
+          collection.relation,
+          cursor: cursor,
+          batch_size: collection.batch_size,
+          columns: @task.cursor_columns,
+        )
+      when Array
+        enumerator_builder.build_array_enumerator(collection, cursor: cursor&.to_i)
+      when BatchCsvCollectionBuilder::BatchCsv
+        JobIteration::CsvEnumerator.new(collection.csv).batches(
+          batch_size: collection.batch_size,
+          cursor: cursor&.to_i,
+        )
+      when CSV
+        JobIteration::CsvEnumerator.new(collection).rows(cursor: cursor&.to_i)
+      else
+        raise ArgumentError, <<~MSG.squish
+          #{@task.class.name}#collection must be either an
+          Active Record Relation, ActiveRecord::Batches::BatchEnumerator,
+          Array, or CSV.
+        MSG
+      end
+
       throttle_enumerator(@collection_enum)
     end
 

app/models/maintenance_tasks/task.rb

@@ -260,55 +260,15 @@ def count
       self.class.collection_builder_strategy.count(self)
     end
 
-    # Default enumeration builder. You may override this method to return any
+    # Default enumerator builder. You may override this method to return any
     # Enumerator yielding pairs of `[item, item_cursor]`.
     #
     # @param cursor [String, nil] cursor position to resume from, or nil on
     #   initial call.
     #
     # @return [Enumerator]
     def enumerator_builder(cursor:)
-      collection = self.collection
-
-      job_iteration_builder = JobIteration::EnumeratorBuilder.new(nil)
-
-      case collection
-      when :no_collection
-        job_iteration_builder.build_once_enumerator(cursor: nil)
-      when ActiveRecord::Relation
-        job_iteration_builder.active_record_on_records(collection, cursor: cursor, columns: cursor_columns)
-      when ActiveRecord::Batches::BatchEnumerator
-        if collection.start || collection.finish
-          raise ArgumentError, <<~MSG.squish
-            #{self.class.name}#collection cannot support
-            a batch enumerator with the "start" or "finish" options.
-          MSG
-        end
-
-        # For now, only support automatic count based on the enumerator for
-        # batches
-        job_iteration_builder.active_record_on_batch_relations(
-          collection.relation,
-          cursor: cursor,
-          batch_size: collection.batch_size,
-          columns: cursor_columns,
-        )
-      when Array
-        job_iteration_builder.build_array_enumerator(collection, cursor: cursor&.to_i)
-      when BatchCsvCollectionBuilder::BatchCsv
-        JobIteration::CsvEnumerator.new(collection.csv).batches(
-          batch_size: collection.batch_size,
-          cursor: cursor&.to_i,
-        )
-      when CSV
-        JobIteration::CsvEnumerator.new(collection).rows(cursor: cursor&.to_i)
-      else
-        raise ArgumentError, <<~MSG.squish
-          #{self.class.name}#collection must be either an
-          Active Record Relation, ActiveRecord::Batches::BatchEnumerator,
-          Array, or CSV.
-        MSG
-      end
+      nil
     end
   end
 end

test/dummy/app/tasks/maintenance/custom_enumerating_task.rb

@@ -12,7 +12,8 @@ def count
       3
     end
 
-    def process(_)
+    def process(letter)
+      Rails.logger.debug("letter: #{letter}")
     end
   end
 end