Rails：模型的自定义订购和优先级

Question

For a Rails project I am working on, I have a model that has an image and a priority attached to it. So it looks something like this:

card: 
  -id 
  -name
  -img_url
  -priority

The priority is a number where 1 is the highest priority (default) and it goes on to 2, 3, etc.

My question is what is the best way to assign these priorities on creation? Should I keep a variable in the controller that keeps track of the priorities assigned already and assigns the next one? Also, these priorities are like to change ie the card with priority 3 changes to 1 and so 1 must become 2, and 2 must become 3.

What's the best way to deal with this?

Answer 1

you can find last number and then add it and check

def create
  @card = Card.new(params...)
  @last_card = Card.last
  priority = @last_card.priority
  if priority == 3 
    next_priority = 1
  else
    next_priority = priority + 1
  end
  @card.priority = next_priority
end 

Answer 2

I would suggest to take priority management operation to DB and DB trigger is a good solution.

First set unique constraint the priority column to make sure another thread cannot insert same next priority number. You can use max function to get the next number.

And when priority changes, you change priority of all rows one by one which will be resource intensive operation if done in the rails.