Retrieving value in controller - NoMethodError (undefined method `35=' for #<TimesheetItem:0x8ddda08>):

How can I retrieve the values in my controller for the text field
below? The "35" is the id of the item I want to update.

<input type="text" id="timesheet_item_35_work_category"
name="timesheet_item[35][work_category]" value="PROJECTS" />

My controller is:

def update
    @timesheet = Timesheet.find(params[:id])
    @timesheet.timesheet_items.build(params[:timesheet_item])
    @timesheet.update_attributes(params[:timesheet])
end