Pagy problem with changing collection in API

[alsted]

For an API endpoint index action that has a simple filter action as:

   def index
      # Filter @agreements if ext_ref param is present
      if params[:ext_ref].present?
        @agreements = @agreements.where(ext_ref: params[:ext_ref])
       end
    end

This causes a change to the collection that Pagy is initialized with in:
bullet_train-core/bullet_train-api/app/controllers/concerns/api/controllers/base.rb

where initialization takes place as:

  def apply_pagination
    collection = instance_variable_get collection_variable
    @pagy, collection = pagy_cursor collection, after: params[:after], order: {id: :asc}
    instance_variable_set collection_variable, collection
  end

Pagination headers are set later by:

def set_pagination_headers
    return unless @pagy

    if @pagy.has_more?
      if (collection = instance_variable_get(collection_variable))
        next_cursor = collection.last.id
        link_header = response.headers["Link"]
        link_value = "<#{modify_url_params(request.url, after: next_cursor)}>; rel=\"next\""
        response.headers["Link"] = link_header ? "#{link_header}, #{link_value}" : link_value
        response.headers["Pagination-Next"] = next_cursor
      end
    end
  end

The the simple filter shown is run in between apply_pagination and set_pagination_header. The filtering can cause an empty collection which leads to an error in next_cursor = collection.last.id as collection is has no members.