Add backend for accepting a group invitation

For #887

lib/graphql/resolvers/invitation.ex

@@ -5,6 +5,7 @@ defmodule Mobilizon.GraphQL.Resolvers.Invitation do
 
   alias Mobilizon.Actors
   alias Mobilizon.Actors.Actor
+  alias Mobilizon.Invitation
   alias Mobilizon.Invitations
   import Mobilizon.Web.Gettext
 
@@ -54,4 +55,33 @@ defmodule Mobilizon.GraphQL.Resolvers.Invitation do
       Invitations.delete_invitation_by_token(group_id, token)
     end
   end
+
+  def accept_invitation_token(
+        _parent,
+        %{group_id: group_id, token: token, actor_id: actor_id_chosen},
+        %{
+          context: %{current_actor: %Actor{id: actor_id_connected}}
+        }
+      ) do
+    with true <-
+           String.to_integer(actor_id_chosen) == actor_id_connected ||
+             {:error, "You can only accept invitations for your own account"},
+         %Invitation{} = _invitation <-
+           Invitations.find_invitation(group_id, token) ||
+             {:error, "Invalid invitation token for this group"},
+         false <-
+           Mobilizon.Actors.member?(actor_id_connected, group_id),
+         {:ok, member} <-
+           Mobilizon.Actors.create_member(%{
+             role: :member,
+             parent_id: group_id,
+             actor_id: actor_id_connected
+           }) do
+      {:ok, member}
+    else
+      {:error, _} = error -> error
+      true -> {:error, "You are already a member of this group"}
+      _ -> {:error, "Unexpected error"}
+    end
+  end
 end