KanBan Database architecture
<p>I'm trying to implement a kanban agile board like Trello in my application. I'm wondering how to do it the best way. I'm considering these entities:  </p>

<p><strong>Board</strong><br>
has many Lists</p>

<p><strong>List</strong><br>
has many Cards</p>

<p><strong>Card</strong><br>
contains some content</p>

<p>However I'm stuck with ordering of Cards. Each Card should have a sort position to sort Cards in particular order in a List. When dragging a Card, I should change it's position and save it in the database. What is the most efficient way of doing this?</p>

<p>Adding a <code>position</code> field to each Card seems redundant since I would have to recalculate positions of all Cards in a List (or two) after I drag a Card to a different position (there will be hundreds of Cards). I was thinking of storing an array of all Card ids in a List and sort Cards by this array. What are pros/cons of this solution? And are there any better solutions?</p>

<p>I'm using Ruby on Rails and PostgreSQL.</p>

<p><strong>UPDATE</strong></p>

<p>Using @cske answer I came up with the following solution:</p>

<pre><code>CREATE OR REPLACE FUNCTION move_buyer_card(
    new_list_id   INT
  , param_id      INT
  , new_position  INT
) RETURNS FLOAT4
LANGUAGE plpgsql SECURITY DEFINER
AS $$
DECLARE
  var_lower_bound FLOAT4;
  var_upper_bound FLOAT4;
  var_new_weight  FLOAT4; /*between 0 and 1*/
BEGIN
  IF new_position &lt; 2 THEN /*first position*/
    var_lower_bound := 0;
    SELECT MIN(weight) FROM Buyers
      WHERE board_list_id = new_list_id
      INTO var_upper_bound;
    IF var_upper_bound IS NULL THEN /*empty list*/
      var_upper_bound := 1;
    END IF;
  ELSE /*not first position*/
    WITH ordered_cards AS (
      SELECT id, RANK() OVER (ORDER BY weight ASC) AS rank, weight
      FROM Buyers WHERE board_list_id = new_list_id
    )
    SELECT cards0.weight, cards1.weight from ordered_cards cards0
      JOIN ordered_cards cards1
        ON cards0.rank = cards1.rank - 1
      WHERE cards1.rank = new_position
      INTO var_lower_bound, var_upper_bound;
    IF NOT FOUND THEN /*only 1 item in list OR last position*/
      SELECT MAX(weight) FROM Buyers WHERE board_list_id = new_list_id
        INTO var_lower_bound;
      IF var_lower_bound IS NULL THEN /*empty list*/
        var_lower_bound := 0;
      END IF;
      var_upper_bound := 1;
    END IF;
  END IF;
  var_new_weight := var_lower_bound + (var_upper_bound - var_lower_bound) / 2;
  UPDATE Buyers
    SET weight = var_new_weight,
      board_list_id = new_list_id
    WHERE id = param_id;
  RETURN var_new_weight;
END;
$$;
</code></pre>