I’m trying to use worker in my Phoenix application but I’m getting the error: undefined function worker/2. I get this error when I up phoenix server (mix phx.server)

My phoenix version is 1.4.0 and my elixir is 1.7.4.

Below I pasted my application.ex, mix.esx and mix.lock
---------------------mix.lock-------------------

%{
  "absinthe": {:hex, :absinthe, "1.4.13", "81eb2ff41f1b62cd6e992955f62c22c042d1079b7936c27f5f7c2c806b8fc436", [:mix], [{:dataloader, "~> 1.0.0", [hex: :dataloader, repo: "hexpm", optional: true]}, {:decimal, "~> 1.0", [hex: :decimal, repo: "hexpm", optional: true]}], "hexpm"},
  "absinthe_plug": {:hex, :absinthe_plug, "1.4.6", "ac5d2d3d02acf52fda0f151b294017ab06e2ed1c6c15334e06aac82c94e36e08", [:mix], [{:absinthe, "~> 1.4.11", [hex: :absinthe, repo: "hexpm", optional: false]}, {:plug, "~> 1.3.2 or ~> 1.4", [hex: :plug, repo: "hexpm", optional: false]}], "hexpm"},
  "connection": {:hex, :connection, "1.0.4", "a1cae72211f0eef17705aaededacac3eb30e6625b04a6117c1b2db6ace7d5976", [:mix], [], "hexpm"},
  "cowboy": {:hex, :cowboy, "2.6.0", "dc1ff5354c89e36a3e3ef8d10433396dcff0dcbb1d4223b58c64c2d51a6d88d9", [:rebar3], [{:cowlib, "~> 2.7.0", [hex: :cowlib, repo: "hexpm", optional: false]}, {:ranch, "~> 1.7.0", [hex: :ranch, repo: "hexpm", optional: false]}], "hexpm"},
  "cowlib": {:hex, :cowlib, "2.7.0", "3ef16e77562f9855a2605900cedb15c1462d76fb1be6a32fc3ae91973ee543d2", [:rebar3], [], "hexpm"},
  "db_connection": {:hex, :db_connection, "1.1.3", "89b30ca1ef0a3b469b1c779579590688561d586694a3ce8792985d4d7e575a61", [:mix], [{:connection, "~> 1.0.2", [hex: :connection, repo: "hexpm", optional: false]}, {:poolboy, "~> 1.5", [hex: :poolboy, repo: "hexpm", optional: true]}, {:sbroker, "~> 1.0", [hex: :sbroker, repo: "hexpm", optional: true]}], "hexpm"},
  "decimal": {:hex, :decimal, "1.6.0", "bfd84d90ff966e1f5d4370bdd3943432d8f65f07d3bab48001aebd7030590dcc", [:mix], [], "hexpm"},
  "ecto": {:hex, :ecto, "3.0.3", "018a3df0956636f84eb3033d807485a7d3dea8474f47b90da5cb8073444c4384", [:mix], [{:decimal, "~> 1.5", [hex: :decimal, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: true]}, {:poison, "~> 2.2 or ~> 3.0", [hex: :poison, repo: "hexpm", optional: true]}], "hexpm"},
  "gettext": {:hex, :gettext, "0.16.1", "e2130b25eebcbe02bb343b119a07ae2c7e28bd4b146c4a154da2ffb2b3507af2", [:mix], [], "hexpm"},
  "jason": {:hex, :jason, "1.1.2", "b03dedea67a99223a2eaf9f1264ce37154564de899fd3d8b9a21b1a6fd64afe7", [:mix], [{:decimal, "~> 1.0", [hex: :decimal, repo: "hexpm", optional: true]}], "hexpm"},
  "mime": {:hex, :mime, "1.3.1", "30ce04ab3175b6ad0bdce0035cba77bba68b813d523d1aac73d9781b4d193cf8", [:mix], [], "hexpm"},
  "mongodb": {:hex, :mongodb, "0.4.7", "7dd4e9f0dd7107fd3f0247f60b7acc21a6789ae0fafc698713b603a79dfee655", [:mix], [{:connection, "~> 1.0", [hex: :connection, repo: "hexpm", optional: false]}, {:db_connection, "~> 1.1", [hex: :db_connection, repo: "hexpm", optional: false]}, {:decimal, "~> 1.0", [hex: :decimal, repo: "hexpm", optional: false]}], "hexpm"},
  "phoenix": {:hex, :phoenix, "1.4.0", "56fe9a809e0e735f3e3b9b31c1b749d4b436e466d8da627b8d82f90eaae714d2", [:mix], [{:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: true]}, {:phoenix_pubsub, "~> 1.1", [hex: :phoenix_pubsub, repo: "hexpm", optional: false]}, {:plug, "~> 1.7", [hex: :plug, repo: "hexpm", optional: false]}, {:plug_cowboy, "~> 1.0 or ~> 2.0", [hex: :plug_cowboy, repo: "hexpm", optional: true]}], "hexpm"},
  "phoenix_ecto": {:hex, :phoenix_ecto, "4.0.0", "c43117a136e7399ea04ecaac73f8f23ee0ffe3e07acfcb8062fe5f4c9f0f6531", [:mix], [{:ecto, "~> 3.0", [hex: :ecto, repo: "hexpm", optional: false]}, {:phoenix_html, "~> 2.9", [hex: :phoenix_html, repo: "hexpm", optional: true]}, {:plug, "~> 1.0", [hex: :plug, repo: "hexpm", optional: false]}], "hexpm"},
  "phoenix_pubsub": {:hex, :phoenix_pubsub, "1.1.1", "6668d787e602981f24f17a5fbb69cc98f8ab085114ebfac6cc36e10a90c8e93c", [:mix], [], "hexpm"},
  "plug": {:hex, :plug, "1.7.1", "8516d565fb84a6a8b2ca722e74e2cd25ca0fc9d64f364ec9dbec09d33eb78ccd", [:mix], [{:mime, "~> 1.0", [hex: :mime, repo: "hexpm", optional: false]}, {:plug_crypto, "~> 1.0", [hex: :plug_crypto, repo: "hexpm", optional: false]}], "hexpm"},
  "plug_cowboy": {:hex, :plug_cowboy, "2.0.0", "ab0c92728f2ba43c544cce85f0f220d8d30fc0c90eaa1e6203683ab039655062", [:mix], [{:cowboy, "~> 2.5", [hex: :cowboy, repo: "hexpm", optional: false]}, {:plug, "~> 1.7", [hex: :plug, repo: "hexpm", optional: false]}], "hexpm"},
  "plug_crypto": {:hex, :plug_crypto, "1.0.0", "18e49317d3fa343f24620ed22795ec29d4a5e602d52d1513ccea0b07d8ea7d4d", [:mix], [], "hexpm"},
  "poolboy": {:hex, :poolboy, "1.5.1", "6b46163901cfd0a1b43d692657ed9d7e599853b3b21b95ae5ae0a777cf9b6ca8", [:rebar], [], "hexpm"},
  "ranch": {:hex, :ranch, "1.7.0", "9583f47160ca62af7f8d5db11454068eaa32b56eeadf984d4f46e61a076df5f2", [:rebar3], [], "hexpm"},
}

-------------------mix.exs---------------------

defmodule ContentProxy.MixProject do
  use Mix.Project

  def project do
    [
      app: :content_proxy,
      version: "0.1.0",
      elixir: "~> 1.5",
      elixirc_paths: elixirc_paths(Mix.env()),
      compilers: [:phoenix, :gettext] ++ Mix.compilers(),
      start_permanent: Mix.env() == :prod,
      aliases: aliases(),
      deps: deps()
    ]
  end

  # Configuration for the OTP application.
  #
  # Type `mix help compile.app` for more information.
  def application do
    [
      mod: {ContentProxy.Application, []},
      extra_applications: [:logger, :runtime_tools]
    ]
  end

  # Specifies which paths to compile per environment.
  defp elixirc_paths(:test), do: ["lib", "test/support"]
  defp elixirc_paths(_), do: ["lib"]

  # Specifies your project dependencies.
  #
  # Type `mix help deps` for examples and options.
  defp deps do
    [
      {:phoenix, "~> 1.4.0"},
      {:phoenix_ecto, "~> 4.0"},
      {:poolboy, ">= 0.0.0"},
      {:mongodb, ">= 0.0.0"},
      {:gettext, "~> 0.11"},
      {:jason, "~> 1.0"},
      {:plug_cowboy, "~> 2.0"},
      {:absinthe, "~> 1.4"},
      {:absinthe_plug, "~> 1.4"}
    ]
  end

  # Aliases are shortcuts or tasks specific to the current project.
  # For example, to create, migrate and run the seeds file at once:
  #
  #     $ mix ecto.setup
  #
  # See the documentation for `Mix` for more info on aliases.
  defp aliases do
    [
      "ecto.setup": ["ecto.create", "ecto.migrate", "run priv/repo/seeds.exs"],
      "ecto.reset": ["ecto.drop", "ecto.setup"],
      test: ["ecto.create --quiet", "ecto.migrate", "test"]
    ]
  end
end

----------------application.ex------------------------

defmodule ContentProxy.Application do
  # See https://hexdocs.pm/elixir/Application.html
  # for more information on OTP Applications
  @moduledoc false

  use Application

  def start(_type, _args) do
    # List all child processes to be supervised
    database_config = Application.get_env(:content_proxy, :db)
    {:ok, _pid} = Mongo.start_link(
      database: database_config[:name],
      seeds: database_config[:hosts]
    )

    children = [
      ContentProxyWeb.Endpoint,
      worker(Mongo, [[name: :mongo, database: database_config[:name], pool: DBConnection.Poolboy]])
    ]

    # See https://hexdocs.pm/elixir/Supervisor.html
    # for other strategies and supported options
    opts = [strategy: :one_for_one, name: ContentProxy.Supervisor]
    Supervisor.start_link(children, opts)
  end

  # Tell Phoenix to update the endpoint configuration
  # whenever the application is updated.
  def config_change(changed, _new, removed) do
    ContentProxyWeb.Endpoint.config_change(changed, removed)
    :ok
  end
end