Trouble on updating record using an 'has_many :through' Record Association

I am using Ruby on Rails 3.0.7 and I have a problem on an associated model validation for an updating controller action. I am using an "has_many :through" association so that I can handle user-category relationships.

In the User model I have:

has_many :user_category_relationships,
  :autosave    => true,
  :dependent   => :destroy
has_many :user_categories,
  :through     => :user_category_relationships,
  :source      => :user_category,
  :dependent   => :destroy

In the _form view file I have:

<% @current_user.user_categories.each do |user_category| %>
  <%= check_box_tag :user_category_ids, user_category.id, @user.user_categories.include?(user_category), :name => 'user_relationships[user_category_ids][]' %>
  <%= label_tag "user_category_#{user_category.id}", user_category.name %>
<% end %>

In the UserCategoryRelationship model I have:

belongs_to :users
belongs_to :categories

# Validation
validates :name_id,
  :presence   => true,
  :uniqueness => {
    :scope          => [:created_by_user_id, :category_id]
  }
validates :category_id,
  :presence   => true,
  :uniqueness => {
    :scope          => [:created_by_user_id, :name_id]
  }
validates :created_by_user_id,
  :presence   => true,
  :uniqueness => {
    :scope          => [:category_id, :name_id]
  }

In the UsersController I have:

def create
  ...

  # `params[:user_relationships][:user_category_ids]` used below are id values 
  # related to user-category relationships.

  unless params[:user_relationships][:user_category_ids]
    # Set default user category if not selected.
    @user.user_category_relationships.build(
      :category_id        => '1',
      :created_by_user_id => @current_user.id,
      :name_id            => @name.id
    )
  else
    params[:user_relationships][:user_category_ids].each { |user_category_id|
      @user.user_category_relationships.build(
        :category_id        => user_category_id,
        :created_by_user_id => @current_user.id,
        :name_id            => @name.id
      )
    }
  end

  if @user.save
    ...
  end
end

def update
  ...

  # Note: The following code is equal to that in the controller 'create' 
  # action. However here is my problem because validation (read after 
  # for more information about).

  unless params[:user_relationships][:user_category_ids]
    # Set default user category if not selected.
    @user.user_category_relationships.build(
      :category_id        => '1',
      :created_by_user_id => @current_user.id,
      :name_id            => @name.id
    )
  else
    params[:user_relationships][:user_category_ids].each { |user_category_id|
      @user.user_category_relationships.build(
        :category_id        => user_category_id,
        :created_by_user_id => @current_user.id,
        :name_id            => @name.id
      )
    }
  end

  if @user.update_attributes(params[:user])
    ...
  end
end

The above code works except for the update action: when I try to run that I get always the same error, as well.

# Validation errors on updating the 'user_category_relationships' associated model
:"user_category_relationships.category_id"        =>["has already been taken"]
:"user_category_relationships.name_id"            =>["has already been taken"]
:"user_category_开发者_JAVA技巧relationships.created_by_user_id" =>["has already been taken"]

What I would like to do, since I am not using the "RoR magical\automatic way" at all (that is, I don't use the collection_singular_ids method - see also this and this), is to make properly work the update controller action so that I can still use the "automatic" creation of user-category relationships (triggered on save and update_attributes(...) methods) but avoiding to generate above validation errors (that is, create valid data in the database table). How can I do? What do you advice about this issue?

P.S.: Where I am in trouble is in the update action, precisely figuring out how to "choose"\"filter" and "coding" records to update, to create and to delete in the database having params[:user_relationships][:user_category_ids] as input data.