Add join course mutation to backend

Add a mutation which adds a course to the user by the join code.

Closes  ufosc#118

lib/ask_a_gator_web/resolvers/user_resolver.ex

@@ -1,5 +1,6 @@
 defmodule AskAGatorWeb.UserResolver do
   alias AskAGator.Accounts.User
+  alias AskAGator.Courses
   alias AskAGator.Repo
   alias AskAGator.Services.Authenticator
 
@@ -52,4 +53,19 @@ defmodule AskAGatorWeb.UserResolver do
   def signed_in?(_root, _info) do
     {:ok, false}
   end
+
+  def join_course(%{join_code: join_code}, %{context: %{current_user: c}}) do
+    case Courses.get_course_by_join(join_code) do
+      nil ->
+        {:error, "Invalid join code"}
+      course ->
+        User.add_course(c, course)
+        {:ok, course.code}
+    end
+  end
+
+  def join_course(%{join_code: join_code}, _info) do
+    {:error, "Not Signed In"}
+  end
+
 end

lib/ask_a_gator_web/schema.ex

@@ -48,5 +48,10 @@ defmodule AskAGatorWeb.Schema do
         end
       end
     end
+
+    field :join_course, type: :string do
+      arg(:join_code, non_null(:string))
+      resolve(&UserResolver.join_course/2)
+    end
   end
 end