field `taskcategories` in `select` is a virtual field in schema Db.Schema.Task in query:

query.ex

    query =
      from(task in Task,
      join: org in Org,
      join: rubric in Rubric,
      join: coursecode in CourseCode,
      join: task_category in TaskCategory,
      where: org.uuid == ^org_uuid,
        where: org.id == task.org_id,
        where: rubric.id == task.rubric_id,
        where: coursecode.id == task.coursecode_id,
        where: task_category.task_id == task.id,
        preload: [taskcategories: :taskitems],
        select: %{
          title: task.task_title,
          uuid: task.uuid,
          org_uuid: org.uuid,
          archived: task.archived,
          course_task_type: task.course_task_type,
          task_id: task.id,
          rubric: rubric,
          coursecode: coursecode,
          task_category: task.taskcategories,
          inserted_at: task.inserted_at,
          updated_at: task.updated_at,
          hash_id: task.hash_id
        }
      )

What was the issue?