Nested Form database is locked: commit transaction

I have 2 models. Category, and Size. I have nested form that isn't working.

PROBLEM. When i submit the form. I get the below error. Does anyone know how to fix this? Any information would be great. Thanks in advance.

ActiveRecord::StatementInvalid (SQLite3::BusyException: database is locked: commit transaction):
  app/controllers/categories_controller.rb:28:in `create'

Category.rb

class Category < ActiveRecord::Base
  has_ancestry

  has_many :products
  has_many :sizes

  validates :name, presence: true, length: { maximum: 20 }, uniqueness: true

  accepts_nested_attributes_for :sizes
end

Size.rb

class Size < ActiveRecord::Base
    validates :title, presence: true, length: { maximum: 15 }
    validates :title, uniqueness: true

  belongs_to :category
end

Here is the form

  <%= simple_form_for(@category) do |f| %>
    <div class="form-inputs">
      <%= f.input :name %>
      <%= f.collection_select :parent_id, Category.order(:name), :id, :name, {prompt: "Select Parrent ID If Applicable"},include_blank: true %>
        <%= f.simple_fields_for :sizes do |s| %>
          <%= s.input :title %>
        <% end %>
      <div class="form-actions"><%= f.button :submit %></div>
    </div>
  <% end %>

Categories Controller

class CategoriesController < ApplicationController

  def create
    @category = Category.new(category_params)
    if @category.save
      redirect_to @category
      flash[:success] = "You have created a new category"
    else
      flash[:danger] = "Your category didn't save"
      render "new"
    end
  end

  def category_params
    binding.pry
    params.require(:category).permit(:name, :parent_id, size_ids: [], sizes_attributes: [:title])
  end
end

Sizes Controller

class SizesController < ApplicationController
  before_action :logged_in_user, only: [:create, :index, :destroy, :update]
  before_action :admin_user, only: [:create, :index, :destroy, :update]

  def create
    @size = Size.create(size_params)
  end

  def index
    @sizes = Size.all
  end

  def destroy
    Size.find(params[:id]).destroy
  end

  def update
    @size.update_attributes(size_params)
  end

  private

  def size_params
    params.require(:size).permit(:title, :category_id)
  end
end