Add test case

test/changeset_parser_test.exs

@@ -43,6 +43,7 @@ defmodule AbsintheErrorPayload.ChangesetParserTest do
       field(:topics, {:array, :string})
       field(:virtual, :string, virtual: true)
       field(:published_at, :naive_datetime)
+      field(:metadata, {:array, :map})
 
       belongs_to(:author, Author)
       has_many(:tags, Tag)
@@ -181,6 +182,23 @@ defmodule AbsintheErrorPayload.ChangesetParserTest do
       assert %ValidationMessage{code: :required, field: "tags.0.name", key: :name} = first
       assert %ValidationMessage{code: :required, field: "tags.1.name", key: :name} = second
     end
+
+    test "nested nil has many fields with errors" do
+      changeset =
+        %{"tags" => nil}
+        |> changeset()
+        |> cast_assoc(:tags,
+          with: fn tag, params ->
+            tag
+            |> cast(params, ~w(name)a)
+            |> validate_required(:name)
+          end
+        )
+
+      result = ChangesetParser.extract_messages(changeset)
+      assert [first] = result
+      assert %ValidationMessage{code: :association, field: :tags, key: :tags, message: "is invalid", options: [%{key: :type, value: "array-map"}]} = first
+    end
   end
 
   describe "construct_message/2" do