ruby on rails - how to remove this error : undefined method `inc' for nil:NilClass -

i wrote in browser " http://localhost:3000/items/create?name=car1&description=good+car&price=500000&weight=0&real=1 " , , got error

undefined method `inc' nil:nilclass

extracted source (around line #11):

9 after_initialize { puts 'initialize' }  10 after_save { puts 'saved' }  11 after_create { category.inc(:items_count, 1) }   12 after_update { puts 'updated' }  13 after_destroy { category.inc(:items_count, -1) }  14 

rails.root: e:/work/my_store

application trace | framework trace | full trace

app/models/item.rb:11:in `block in <class:item>' app/controllers/items_controller.rb:9:in `create' 

request

parameters:

{"name"=>"car1",  "description"=>"good car",  "price"=>"500000",  "weight"=>"0",  "real"=>"1"} 

in line 11 you're doing category.inc(:items_count, 1) means category nil @ moment. don't see controller code, guess either did not create category yet, or did not set category variable proper value. , there no need things

   category.inc(:items_count, 1) 

because if set properly, can do

   category.items.count 

to count items in category.

did try complete tutorial yet? recommend michael hartl's tutorial. it's simple, can learn quite complicated things it.