Rails: Creating new model based on a parameter in another model's controller

I'm trying to create a model where every blog post has a number of tags (a separate model) associated to it. In my form, I pass back a separated string of tags along with the post, like "apples,oranges,bananas". I'm eventually going to try and split this string and then create a tag for each string. However, for now I'm just trying to see if I can make one tag with the whole string.

The request is handled by the create method of the posts controller. However, I can't figure out how to create and commit the tag in the posts controller, or at least call the tags controller to delegate the creation. I get the error:

undefined method `new=' for Tag(id: integer, tag_name: string, post_id: integer):Class

This points to the line Tag.new = @post.tags.create(post_params[:tag_name]) of the posts_controller.

Relevant code:

posts_controller

def new
        @post = Post.new
    end

    def create
        @post = current_user.posts.create(post_params)
        Tag.new = @post.tags.create(post_params[:tag_name])
        if @post.save
            redirect_to post_path(@post), :notice => "Post was created successfully."
        else
            render :new
        end
    end

private
    def post_params
        params.require(:post).permit(:title, :content_md, :image, :user_id, :year_created, :medium, :dimension_height, :dimension_width, :measurement, :weight_in_pounds, :price, :quantity)
    end

tags_controller

class TagsController < ApplicationController

  def new
    @tag = Tag.new
  end

  def create
    @tag = tag.create(tag_params)
  end

  private
  def tag_params
    params.require().permit(:tag_name, :user_id)
  end

end

post.rb

class Post < ActiveRecord::Base

  # Relations
  belongs_to :user
  has_many :comments
  has_many :post_votes
  has_many :tags
  accepts_nested_attributes_for :tags
end

tag.rb

class Tag < ActiveRecord::Base
  validates :tag_name, presence: true
  validates :post_id, presence: true
  belongs_to :post
end