Rails Engine: uninitialized constant User

My first question on stack overflow! I am trying to convert a rails app into an engine so that I can use it in my own Rails app.

I have converted everything to engine modules etc. (I'm still in my engine directory and haven't started implemented it yet in my rails app) but when I run rake db:migrate in the root I get the error:

rake aborted!
NameError: uninitialized constant User
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0    /gems/activesupport-4.2.6/lib/active_support/inflector/methods.rb:261:in `const_get'
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0    /gems/activesupport-4.2.6/lib/active_support/inflector/methods.rb:261:in `block in constantize'
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-4.2.6/lib/active_support/inflector/methods.rb:259:in `each'
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-4.2.6/lib/active_support/inflector/methods.rb:259:in `inject'
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/activesupport-4.2.6/lib/active_support/inflector/methods.rb:259:in `constantize'
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/devise-3.5.6/lib/devise.rb:292:in `get'
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/devise-3.5.6/lib/devise/mapping.rb:81:in `to'
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/devise-3.5.6/lib/devise/mapping.rb:76:in `modules'
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/devise-3.5.6/lib/devise/mapping.rb:93:in `routes'
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/devise-3.5.6/lib/devise/mapping.rb:160:in `default_used_route'
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/devise-3.5.6/lib/devise/mapping.rb:70:in `initialize'
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/devise-3.5.6/lib/devise.rb:326:in `new'
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/devise-3.5.6/lib/devise.rb:326:in `add_mapping'
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/devise-3.5.6/lib/devise/rails/routes.rb:238:in `block in devise_for'
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/devise-3.5.6/lib/devise/rails/routes.rb:237:in `each'
/Users/matthijskolkman/.rbenv/versions/2.3.0/lib/ruby/gems/2.3.0/gems/devise-3.5.6/lib/devise/rails/routes.rb:237:in `devise_for'
/Users/matthijskolkman/Projects/customer_service_gem/brimir/config/routes.rb:2:in `block in <top (required)>'

I think it has to do with the routing, but not sure. Here are my routes:

Brimir::Engine.routes.draw do
  devise_for :users, controllers: { omniauth_callbacks: 'omniauth' }

  resources :users do
    get :tickets, to: 'tickets#index'
  end

  namespace :tickets do
    resource :deleted, only: :destroy, controller: :deleted
    resource :selected, only: :update, controller: :selected
  end

  resources :tickets, except: [:destroy, :edit] do
    resource :lock, only: [:destroy, :create], module: :tickets
  end

  resources :labelings, only: [:destroy, :create]

  resources :rules

  resources :labels, only: [:destroy, :update, :index, :edit]

  resources :replies, only: [:create, :new, :update, :show]

  get '/attachments/:id/:format' => 'attachments#show'
  resources :attachments, only: [:index, :new]

  resources :email_addresses

  resource :settings, only: [:edit, :update]

  root to: 'tickets#index'

  namespace :api do
    namespace :v1 do
      resources :tickets, only: [ :index, :show, :create ]
      resources :sessions, only: [ :create ]
      resources :users, param: :email, only: [ :create, :show ] do
        resources :tickets, only: [ :index ]
      end
    end
  end
end

And the user controller:

module Brimir
  class UsersController < ApplicationController
    include UsersStrongParams
    load_and_authorize_resource :user

    def edit
      @user = User.find(params[:id])
    end

    def update
      @user = User.find(params[:id])

      # if no password was posted, remove from params
      if params[:user][:password] == ''
        params[:user].delete(:password)
        params[:user].delete(:password_confirmation)
      end

      if current_user == @user
        params[:user].delete(:agent) # prevent removing own agent permissions
      end

      if @user.update_attributes(user_params)

        if current_user.agent? && current_user.labelings.count == 0
          redirect_to users_url, notice: I18n.translate(:settings_saved)
        else
          redirect_to tickets_url, notice: I18n.translate(:settings_saved)
        end

      else
        render action: 'edit'
      end
    end

    def index
      @users = User.ordered.paginate(page: params[:page])
      @users = @users.search(params[:q])
      @users = @users.by_agent(params[:agent] == '1') unless params[:agent].blank?
    end

    def new
      @user = User.new
    end

    def create
      @user = User.new(user_params)

      if @user.save
        redirect_to users_url, notice: I18n.translate(:user_added)
      else
        render 'new'
      end
    end

    def destroy
      @user = User.find(params[:id])
      @user.destroy
      redirect_to users_url, notice: I18n.translate(:user_removed)
    end
  end
end

Thanks in advance!