using a paginate method to display the second group of 25 records on page 2

rubyonrails-talk
1

Hey all,

I'm trying to fully understand what this method in Ruby does. Basically it passes a collection of records from the database (using the Rails find method) into the argument list as a local variable called resources. It assigns the local variable to an instance variable called @all_resources. Then it checks the query string in the url to see if the limit key has a value and if so grab it(e.g. ?limit=10), otherwise default to 25. Then it checks the query string for a page key and if none is there, it defaults to 1 and assigns it to instance variable @page. Then we subtract 1 from @page, presumably because if we are on page 1, then 1 -1 = 0, so 0 is assigned to @page_index. This means when we multiply page_index by 25 (limit), we get 0, and so when we slice the resources we slice the limit (25) by offset (0), and hence we return the first 25 records, making sense since we want to display first 35 records on page 1. However, if $page is equal to 2 and limit 25, then page_index is equal to 1, and offset equal to 25. But I dont see how we are getting the next 25 records on page 2.

I think what's confusing me is this line of code:

((@all_resources.size - 1) / @limit).to_i + 1

What does the size method do and why add one at the end?

Here's the method:

def paginate(resources)    @all_resources = resources   @limit = (params[:limit] && params[:limit].to_i) || 25   @page = (params[:page] && params[:page].to_i) || 1   @page_index = @page - 1   @offset = @page_index * @limit   @num_pages = if @limit == 0     1   elsif @all_resources.size == 0     1   else     ((@all_resources.size - 1) / @limit).to_i + 1   end   @last_page = @num_pages   @limit == 0 ? @all_resources : @all_resources.slice(@offset, @limit) end

Thanks for response

2

Hey all,

I'm trying to fully understand what this method in Ruby does. Basically it passes a collection of records from the database (using the Rails find method) into the argument list as a local variable called resources. It assigns the local variable to an instance variable called @all_resources. Then it checks the query string in the url to see if the limit key has a value and if so grab it(e.g. ?limit=10), otherwise default to 25. Then it checks the query string for a page key and if none is there, it defaults to 1 and assigns it to instance variable @page. Then we subtract 1 from @page, presumably because if we are on page 1, then 1 -1 = 0, so 0 is assigned to @page_index. This means when we multiply page_index by 25 (limit), we get 0, and so when we slice the resources we slice the limit (25) by offset (0), and hence we return the first 25 records, making sense since we want to display first 35 records on page 1. However, if $page is equal to 2 and limit 25, then page_index is equal to 1, and offset equal to 25. But I dont see how we are getting the next 25 records on page 2.

Have a look at the Rails Guide on debugging and find out how to use ruby-debug to break into the code and inspect data. Then break in at the appropriate points and see what is happening. By a combination of examining variables, entering expressions for evaluation to see what the results are and stepping through the code you should see how (if) it works.

I think what's confusing me is this line of code:

((@all_resources.size - 1) / @limit).to_i + 1

What does the size method do and why add one at the end?

The size method is Array#size and returns the number of elements in the array, ie the number of records in @all_resources in this case.

Colin

3

John Merlino wrote in post #989291:

((@all_resources.size - 1) / @limit).to_i + 1

What does the size method do and why add one at the end?

Suppose 100 records, with a page size of 30

num_pages = ((100-1)/30) num_pages = 99/30 num_pages = 3.3 num_pages = 3 (to_i) num_pages = 4

3 full pages of 30 records, and 1 page with 9 records

Suppose 90 records... ((90-1)/30).to_i + 1 = 3, 3 full pages

It's just some math hijinks to make sure any partial page isn't missed in the page count.

The slice(offset,limit) is the elegant part. Start at the "offset", and return "limit" items.

On page 2, the offset is 1*25, skipping the records that would be returned on page 1, and returns the next page's records.

4

Suppose 90 records... ((90-1)/30).to_i + 1 = 3, 3 full pages

It's just some math hijinks to make sure any partial page isn't missed in the page count.

Thanks that was very helpful.

5

BTW, if you found this in production code, I'd highly recommend KILLING IT WITH FIRE and using something like will_paginate if at all possible - this is the quite-ancient original pagination code included with Rails (stripped out to the classic_pagination plugin in 1.2.4!) and is immensely inefficient. The Rails version of your code really doesn't matter; I just got will_paginate up and running on an old 1.2.3 app without any major issues.

--Matt Jones