Test FlakeID old id compat & Ecto type

lib/pleroma/flake_id.ex

+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2018 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
 defmodule Pleroma.FlakeId do
   @moduledoc """
   Flake is a decentralized, k-ordered id generation service.
@@ -38,6 +42,8 @@ defmodule Pleroma.FlakeId do
     def from_string(unquote(Kernel.to_string(i))), do: <<0::integer-size(128)>>
   end
 
+  def from_string(flake = <<_::integer-size(128)>>), do: flake
+
   def from_string(string) when is_binary(string) and byte_size(string) < 18 do
     case Integer.parse(string) do
       {id, _} -> <<0::integer-size(64), id::integer-size(64)>>

test/flake_id_test.exs

+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2018 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.FlakeIdTest do
+  use Pleroma.DataCase
+  import Kernel, except: [to_string: 1]
+  import Pleroma.FlakeId
+
+  describe "fake flakes (compatibility with older serial integers)" do
+    test "from_string/1" do
+      fake_flake = <<0::integer-size(64), 42::integer-size(64)>>
+      assert from_string("42") == fake_flake
+    end
+
+    test "zero or -1 is a null flake" do
+      fake_flake = <<0::integer-size(128)>>
+      assert from_string("0") == fake_flake
+      assert from_string("-1") == fake_flake
+    end
+
+    test "to_string/1" do
+      fake_flake = <<0::integer-size(64), 42::integer-size(64)>>
+      assert to_string(fake_flake) == "42"
+    end
+  end
+
+  test "ecto type behaviour" do
+    flake = <<0, 0, 1, 104, 80, 229, 2, 235, 140, 22, 69, 201, 53, 210, 0, 0>>
+    flake_s = "9eoozpwTul5mjSEDRI"
+
+    assert cast(flake) == {:ok, flake_s}
+    assert cast(flake_s) == {:ok, flake_s}
+
+    assert load(flake) == {:ok, flake_s}
+    assert load(flake_s) == {:ok, flake_s}
+
+    assert dump(flake_s) == {:ok, flake}
+    assert dump(flake) == {:ok, flake}
+  end
+end