Question

This happen in my app/controllers/categories_controller.rb:3:in `create’
I plan to develop a blog and each user can create fews categories for their micropost. Therefore each micropost can only has one category. I have 3 table :user, microposts and category. My intention is to have a user to add category at user profile page.

model/category.rb
class Category < ActiveRecord::Base
    belongs_to :user    
    attr_accessible :category
end

model/user.rb

has_many :category, :dependent => :destroy
accepts_nested_attributes_for :category, :reject_if =>lambda {|a| a[:category].blank?}  

categoriesController

class CategoriesController < ApplicationController
def create
    @category = current_user.categories.new(params[:category])
    if @category.save 
        flash[:success] = "Category created!"
        redirect_to @user
    else
        flash[:error] = "Category not created."
        render @user
    end
  end
end

usersController

def show
    @user = User.find(params[:id])
    @title = @user.name
    @category = @user.category.new
end

user show.html

<%= form_for @category do |f|%>
        <%= hidden_field_tag :user_id, @user.id  %>
        <%= f.label :category ,"Category:"%>
        <%=h f.text_field :category %><br />
        <%= f.submit "Add Category" %>  
 <% end %>

Answer 1

In User model you have relation

has_many :category

But in controller get from user categor ies

current_user.categories

Rename relation name to has_many :categories.