Fix tagging problems for existing instances.

lib/pleroma/user.ex

@@ -838,7 +838,7 @@ def tag(nickname, tags) when is_binary(nickname),
     do: tag(User.get_by_nickname(nickname), tags)
 
   def tag(%User{} = user, tags),
-    do: update_tags(user, Enum.uniq(user.tags ++ normalize_tags(tags)))
+    do: update_tags(user, Enum.uniq((user.tags || []) ++ normalize_tags(tags)))
 
   def untag(user_identifiers, tags) when is_list(user_identifiers) do
     Repo.transaction(fn ->
@@ -849,7 +849,8 @@ def untag(user_identifiers, tags) when is_list(user_identifiers) do
   def untag(nickname, tags) when is_binary(nickname),
     do: untag(User.get_by_nickname(nickname), tags)
 
-  def untag(%User{} = user, tags), do: update_tags(user, user.tags -- normalize_tags(tags))
+  def untag(%User{} = user, tags),
+    do: update_tags(user, (user.tags || []) -- normalize_tags(tags))
 
   defp update_tags(%User{} = user, new_tags) do
     {:ok, updated_user} =

test/user_test.exs

@@ -11,6 +11,23 @@ defmodule Pleroma.UserTest do
     :ok
   end
 
+  describe "when tags are nil" do
+    test "tagging a user" do
+      user = insert(:user, %{tags: nil})
+      user = User.tag(user, ["cool", "dude"])
+
+      assert "cool" in user.tags
+      assert "dude" in user.tags
+    end
+
+    test "untagging a user" do
+      user = insert(:user, %{tags: nil})
+      user = User.untag(user, ["cool", "dude"])
+
+      assert user.tags == []
+    end
+  end
+
   test "ap_id returns the activity pub id for the user" do
     user = UserBuilder.build()