Need help with Querying select relationship on Repeater form fields

I have Product and BranchProduct model, I have defined the hasMany and BelongsTo relationship for Product and BranchProduct respectively. Product contains name, buy_price, retail_price and whole_price and BranchProduct contains product_id,stock, stock_limit. Now how can i live update the quantity(stock) field, checkout my sample codes if you can understand mean. I need help