simplify and correct deep_merge conflict handling and add test for it

Author: mattpolzin

lib/jsonapi/utils/include_tree.ex

@@ -3,6 +3,7 @@ defmodule JSONAPI.Utils.IncludeTree do
   Internal utility for building trees of resource relationships
   """
 
+  @spec deep_merge(Keyword.t(), Keyword.t()) :: Keyword.t()
   def deep_merge(acc, []), do: acc
 
   def deep_merge(acc, [{key, val} | tail]) do
@@ -11,8 +12,8 @@ defmodule JSONAPI.Utils.IncludeTree do
       key,
       val,
       fn
-        [_first | _rest] = old_val -> deep_merge(old_val, val)
-        old_val -> Keyword.put([], old_val, val)
+        [_first | _rest] = old_val when is_list(val) -> deep_merge(old_val, val)
+        _ -> val
       end
     )
     |> deep_merge(tail)

test/utils/include_tree_test.exs

@@ -6,4 +6,11 @@ defmodule JSONAPI.IncludeTreeTest do
     items = [:test, :the, :path]
     assert put_as_tree([], items, :boo) == [test: [the: [path: :boo]]]
   end
+
+  test "deep_merge/2 handles string/keyword conflict by choosing second value" do
+    # one direction
+    assert [other: "thing", hi: [hello: "there"]] = deep_merge([other: "thing", hi: "there"], hi: [hello: "there"])
+    # the other direction
+    assert [hi: "there", other: "thing"] = deep_merge([hi: [hello: "there"]], other: "thing", hi: "there")
+  end
 end