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
        }
      )

fixed:

  def tasks_by_org_uuid(orguuid, offset, limit) do
    {:ok, org_uuid} = Ecto.UUID.dump(orguuid)

    query =
      from(task in Task,
      join: org in Org,
      join: rubric in Rubric,
      join: coursecode in CourseCode,
      where: org.uuid == ^org_uuid,
        where: org.id == task.org_id,
        where: rubric.id == task.rubric_id,
        where: coursecode.id == task.coursecode_id,
        preload: [:taskcategories, taskcategories: :taskitems],
        select: %{task: task, coursecode: coursecode,  rubric: rubric, org: org}
      )

    query
    |> Repo.paginate(page: offset, page_size: limit)
  end

What was the issue?