using acts_as_list and transferring floats to int value

Question

I would like to use acts_as_list in an app that was originally written in php and is being moved to rails. We used a 'position' value that was a float such that if a user wanted to put something between position 1 and 2, they would just enter 1.5 in the form. It looks like acts_as_list just uses integers. Is there a way in acts_as_list to make it use floats rather than integers? Or possibly convert a set of floats to an integer for insert?

thx

Answer 1

You could modify it, but it's easier to user integers and just reorder all the items in the list that appear after the one you're moving.

Using floats you're forced to split numbers into higher and higher precision every time you move a list around, and if a list gets enough reordering in it, you're likely to eventually run into problems related to how floating point numbers are stored, and then you'll have a list whose ordering breaks in subtle ways that won't be immediately obvious. The other issue with using floats really has to do with storing position (which is inherently a whole number) as a floating point number. When you're standing in a line, you don't think of yourself as being in position 1.5 - you're either in position 1 or 2. The only case where a measurement like 1.5 makes sense in something like people standing in a line is if you're measuring distance (like physical distance in feet/meters) from, say, the front of the line. However, at that point the 1.5 measurement has a very different meaning - it's no longer position, it's distance.

If you're trying to save the on the number of queries/DB changes that are required (floats would allow you to update just one column on one record instead of one column on multiple records), then you're probably missing out on the convenience of the gem doing it for you, and you might want to roll your own if there's some reason you really need to support floats.

However, given the points above about position inherently being a whole number, I'd recommend against doing floats just to save on DB time. How often are people really going to re-sort a list, and how much real load would it put on your app?

If instead, you have to support floats because of some integration point with the old system, then please tell us more about that.